Patent Application
20250060292
Mass spectrometry is currently used to analyze the abundances, modification states, interaction partners, and even biochemical pathways in which proteins and other biomolecules function. The use of mass spectrometry has also been investigated as a possible tool for the preparation of biological samples for electron microscopy (EM) and other imaging methods. This is partially due to the fact that native mass spectrometry is able to ionize molecule complexes under conditions that frequently preserve biomolecule structure.
However, exposure of biomolecules to vacuum environments is necessary for electron microscopy-based imaging as well as in many other types of imaging methods. Unfortunately, when biomolecules, particularly proteins or protein complexes, are deposited under vacuum conditions, the tertiary and secondary structure of the biomolecule is often destroyed due to lack of hydration and due to interactions of the biomolecule with the sample surface used for the imaging process.
Using mass spectrometry or other ionizing methods to prepare samples for EM imaging, including but not limited to transmission electron microscopy (TEM) and cryogenic electron microscopy (cryo-EM), typically requires the biomolecules be ionized, admitted to a vacuum chamber, and deposited onto an EM grid or similar surface for eventual imaging. For example, to prepare for EM imaging the sample can be removed from vacuum at room temperature and stained with a heavy metal, such as in negative stain TEM, a technique that is often used for rapidly screening samples but typically offers limited structural resolution. Alternatively, another approach is to freeze and then image the sample directly under low electron dose conditions, such as in cryo-EM. Cryo-EM offers much higher resolving power but requires low temperatures and specimens that do not contain crystalline ice.
In either case, if mass spectrometry is used as an upstream biomolecule sorting device, the biomolecules are deposited onto a TEM grid under vacuum. TEM grids come in many varieties but typically have a thin layer, often graphene, upon which the specimens are deposited. Biomolecules landing on these graphene surfaces are largely desolvated when they reach the surface. In addition to being under vacuum and desolvated, these biomolecules also have interactions with the TEM grid surface and over time (i.e., seconds to minutes) can begin to unfold and melt onto the surface. Such behavior causes loss of the very structure that is the object of the intended preparation and eventual TEM imaging.
Accordingly, what is needed is a way to deposit particles, especially ionized particles such as those generated by mass spectrometry devices, onto TEM grids and other surfaces under vacuum while maintaining the structural integrity of the particles.
The present invention provides matrices, devices, and methods of applying such matrices to TEM grids and other surfaces in order to mitigate the deleterious effects of vacuum and other conditions used for sample preparation, thereby preserving the structure of at least a portion of deposited samples and allowing for structural analysis. In particular, the invention provides improved methods for preparing biological samples for structural analysis by electron microscopy (EM) and other imaging systems, with a focus on maintaining protein and other biomolecule structure under challenging conditions and constraints, such as the use of ionization, vacuum or near vacuum conditions, a wide range of temperatures, and interactions with sample grids and surfaces.
In an embodiment, the present invention provides a method for preparing a sample comprising the steps of: a) contacting a substrate surface with a liquid or semiliquid substance thereby forming a matrix layer of the liquid or semiliquid substance on the substrate surface; and b) depositing target molecules having an initial structure onto or within the matrix layer under vacuum, wherein the target molecules are partially embedded in the matrix layer and wherein the initial structure of at least a portion of the target molecules is maintained while within the vacuum.
Target molecules useful with the present invention include, but are not limited to, protein molecules, peptides, glycans, metabolites, drugs, and complexes thereof, multi-protein complexes, protein/nucleic acid complexes, nucleic acid molecules, virus particles, micro-organisms, sub-cellular components (e.g., mitochondria, nucleus, Golgi, etc.), and whole cells. The target molecules are preferably biomolecules, including but not limited to peptides and proteins having secondary, tertiary, and/or quaternary structures, or ions generated from such biomolecules. In an embodiment, the target molecules are proteins having intact secondary and tertiary structure.
In further embodiment, the target molecules are ions generated by ionizing precursor molecules, such as by a mass spectrometer device using electrospray ionization or laser desorption. Optionally, the target molecules are deposited onto or within the matrix layer using a controllable ion beam directed to contact the matrix layer.
As used herein, the term “semiliquid” refers to substances having properties between a solid and a liquid, including but not limited to viscous liquids and sols. In a further embodiment, the liquid or semiliquid is a substance that is a liquid at room temperature. In an embodiment, the liquid or semiliquid forming the matrix layer has a viscosity of at least 800 cP at 20° C., 1000 cP at 20° C., at least 1200 cP at 20° C., or at least 1500 cP at 20° C. In an embodiment, the liquid or semiliquid forming the matrix layer has a boiling point of at least 80° C., at least 100° C., at least 150° C., at least 200° C., or at least 240° C. Suitable liquids and semiliquids for use in the present invention include, but are not limited to glycerol, water, ethylene glycol, poly(ethylene) glycol (PEG), poly(propylene) glycol (PPG), triethanolamine (TEA), TritonX-100, diglycerol, glycose, sucrose, inositol, glycine, proline, trehalose, and combinations thereof. The liquids and semiliquids may contain a mixture of liquids and/or additional components. For example, in an embodiment the liquid comprises a mixture of glycerol with methanol or ethanol. Preferably, the liquid or semiliquid forming the matrix layer is an electrical insulator and prevents electrical interaction between the substrate and the target molecules.
As used herein, the term, “matrix” refers to the components of a sample other than the analyte of interest. Preferably, the matrix layer does not interfere with the structural imaging of the deposited target molecules. Optionally, the matrix layer should have a thickness great enough so that the deposited target molecules are embedded within the matrix layer without being completely covered. In an embodiment, the matrix layer has a thickness between 1 and 100 nm, between 2 and 50 nm, between 5 and 25 nm, or between 10 and 20 nm.
As used in embodiments described herein, “vacuum” refers to a pressure of 10−4 Torr or less, a pressure 10−5 Torr or less, or a pressure 10−6 Torr or less. In certain embodiments, the target molecules are deposited on or within the matrix layer at a pressure equal to or less than 10−4 Torr, 10−5 Torr, or 10−6 Torr.
Preferably, the step of depositing the target molecules on or within the matrix layer is performed a temperature between −90° C. and 50° C., between 0° C. and 40° C., or between 10° C. and 25° C. Optionally, depositing the target molecules is performed at room temperature.
In an embodiment, the target molecules are deposited onto or within the matrix layer using an analyte beam. Preferably, the analyte beam is an ion beam. In certain embodiments, the analyte beam is characterized by an intensity selected from the range of 0.025 to 25 particles per 1 μm2 per second, 0.05 to 10 particles per 1 μm2 per second, or 0.1 to 5 particles per 1 μm2 per second. In certain embodiments, the analyte beam is characterized by a spot size selected from the range of 400 μm2 to 4.0E7 μm2, from the range of 800 μm2 to 3.8E7 μm2, or from the range of 1,000 μm2 to 1,500 μm2.
Optionally, the substrate described in the embodiments provided herein is an electron microscopy (EM) grid as known in the art. The EM grid may comprise a metal, including but not limited to copper, rhodium, nickel, molybdenum, titanium, stainless steel, aluminum, gold, or combinations thereof as known in the art. Additionally, the EM grid may comprise a continuous film or membrane which is positioned across the top or bottom surface of the grid, or within the holes of the grid, so as to provide a solid support for the formation of the matrix layer. Preferably, the EM grid is covered by a thin film or membrane which includes, but is not limited to, films and membranes comprising graphene, graphene oxide, silicon oxide, silicon nitride, carbon, and combinations thereof. In an embodiment, the substrate is an EM grid comprising a graphene or graphene oxide monolayer film or membrane positioned across the surface of the grid.
The structures of the partially embedded target molecules are able to be imaged using imaging techniques as known in the art, including but not limited to transmission electron microscopy (TEM), scanning electron microscopy (SEM), cryogenic electron microscopy (cryo-EM), X-ray imaging, fluorescent labeling, immunolabeling, and combinations thereof. In an embodiment, the method further comprises generating three-dimensional reconstructed images of the target molecules from the imaging results.
In certain embodiments described herein, once the target molecules are deposited on the matrix layer, a staining step is performed so as to facilitate imaging of the deposited target molecules. In an embodiment, the staining step comprises contacting exposed areas of the partially embedded target molecules with metal particles or a metal containing solution. Optionally, the staining step comprises negative staining or rotary shadowing. In an alternative embodiment, the deposited target molecules are reacted so that a portion of the partially embedded target molecules contain a fluorescent tag, epitope tag or antibody tag that are used to detect or analyze the structure or specific portions of the structure of the tagged molecule.
Preferably, the present invention is directed to mass spectrometry (MS)-based systems for preparing samples for EM imaging, including but not limited to TEM and cryo-EM, as well as other imaging methods. In an embodiment, a thin layer of a viscous liquid such as glycerol or poly(propylene) glycol (PPG) is applied to a graphene surface of a TEM grid prior to depositing biomolecules onto the surface, thereby preserving the structure of the biomolecules.
In an embodiment, the present invention provides a system for depositing target molecules on a substrate comprising: a) an ion source able to generate ions from a sample of molecules; b) first ion focusing optics in fluid communication with the ion source; c) ion separation optics in fluid communication with the first ion focusing optics, wherein the first ion focusing optics are able to transport ions from the ion source to the ion separation optics, and wherein the ion separation optics are able to separate ions according to the mass-to-charge ratios of the ions; d) a sample chamber in fluid communication with the ion separation optics, wherein the sample chamber contains a substrate able to be removed from the sample chamber. The sample chamber is able to be maintained at a vacuum during operation. Furthermore, the substrate is able to be inserted into and removed from the sample chamber while maintaining the vacuum.
Preferably, the sample chamber has an interior pressure equal to or less than 10−4 Torr, 10−5 Torr, or 10−6 Torr. Additionally, the sample chamber preferably is able to provide an interior temperature between −90° C. and 50° C., between 0° C. and 40° C., or between 10° C. and 25° C. In an embodiment, the substrate is an electron microscopy (EM) grid.
In an embodiment, the system further comprises a controller, operably connected to the first ion focusing optics, the ion separation optics, and the sample chamber, where the controller controls the first ion focusing optics and ion separation optics so as to: transport the ions from the ion source to the ion separation optics; generate a first distribution of precursor ions from the transported ions; isolate a target range of mass-to-charge ratios within the first distribution of precursor ions, thereby generating separated ions; generate an ion beam comprising the separated ions; and contacting the substrate in the sample chamber with the ion beam, thereby depositing separated ions on the substrate under a vacuum. Preferably, a matrix layer of a liquid or semiliquid substance is deposited on the substrate prior to inserting the substrate into the sample chamber and prior to depositing the ions onto the substrate.
In an embodiment, the system further comprises a mass analyzer in fluid communication with the ion separation optics able to detect the separated ions. The controller is able to controller the mass analyzer so as to measure the mass-to-charge ratios of the separated ions and generate mass spectrometry data. Additionally, the system optionally comprises second ion focusing optics in fluid communication with the ion separation optics, the mass analyzer, and sample chamber, and under operational control of the controller so as to be able to transport the separated ions from the ion separation optics to the mass analyzer and/or the sample chamber.
As used throughout the present description, the term “ion optics” is intended to be inclusive of ion optic components of a mass spectrometer system, including, for example, one or more ion guides, ion focusing optics, ion separation optics and combinations thereof. As used throughout the present description, the term “mass analyzer” is intended to be inclusive of detector components of a mass spectrometer system, including, for example, one or more ion detectors.
In an embodiment, the present invention provides a method for preparing a sample for electron microscopy (EM) comprising the steps of: a) contacting a substrate surface with a liquid or semiliquid substance thereby forming a matrix layer of the liquid or semiliquid substance on the substrate surface, wherein the substrate is an electron microscopy (EM) grid; b) generating a first distribution of precursor ions from a sample of target molecules; c) separating a portion of ions from the first distribution of precursor ions according to mass-to-charge ratios of the precursor ions, thereby generating separated ions; d) generating an ion beam containing the separated ions, and e) directing the ion beam to the substrate surface under vacuum; thereby depositing target molecule ions onto or within the matrix layer, wherein the target molecule ions are partially embedded in the matrix layer and wherein the structure of at least a portion of the target molecule ions is retained. Optionally, the substrate surface containing the deposited target molecule ions is directly transferred to a microscope portion of a cryo-electron or transmission electron microscope.
Certain aspects of the invention further include the use of mass spectrometry to purify target molecule ions, including but not limited to desired proteins, protein complexes, and other biomolecules, in the gas-phase for subsequent imaging. One implementation of this method utilizes a modified mass spectrometer that allows for gas-phase purification of target molecule ions. In an embodiment, the step of generating a first distribution of precursor ions and the separating step are performed by a modified mass spectrometer.
In an embodiment, the target molecule ions are purified or isolated, such as by a mass spectrometer device, before being deposited onto the matrix layer. Preferably, the target molecule ions are characterized by a purity of at least 50%, 60%, 75%, 85%, 90%, 95%, or 99%. For target molecules, such as proteins, which may have significant conformational structures, it is desirable that the target molecules are characterized by a conformation purity of at least 50%, 60%, 75%, 85%, 90%, 95% or 99%. For example, it may be desirable to analyze the structure of a particular protein as expressed in a cell. Accordingly, it is necessary to provide an EM sample where all or most of the protein target molecules retain the same conformational structure.
In certain embodiments, the substrate is exposed to the ion beam for a time between 10 to 1,000 seconds, for a time between 30 to 800 seconds, for a time between 60 to 600 seconds, for a time between 30 to 300 seconds, or for a time between 60 to 300 seconds.
As used herein, the term “ion source” refers to a device component which produces ions from a sample. Examples of ion sources include, but are not limited to, electrospray ionization sources and matrix assisted laser desorption/ionization (MALDI) sources.
As used herein, the term “ion optic” refers to a device component which assists in the transport and manipulation of charged particles, for example ions, by the application of electric and/or magnetic fields. The electric or magnetic field can be static, alternating, or can contain both static and alternating components. Ion optical device components include, but are not limited to, ion deflectors which deflect ions, ion focusing optics and lenses which focus ions, and multipoles and flatapoles (such as quadruples) which confine ions to a specific space or trajectory. Ion optics include multipole RF device components which comprise multiple rods having both static and alternating electric and/or magnetic fields.
As used herein, the term “controller” refers to a device component which can be programmed to control a device or system, as is well known in the art. Controllers can, for example, be programmed to control mass spectrometer systems as described herein. Controllers can be programmed, for example, to carry out ion manipulation and sample analysis methods as described herein on systems and devices as described herein.
As used herein, the term “mass spectrometer” refers to a device which creates ions from a sample, separates the ions according to mass, and detects the mass and abundance of the ions. Mass spectrometers include multistage mass spectrometers which fragment the mass-separated ions and separate the product ions by mass one or more times. Multistage mass spectrometers include tandem mass spectrometers which fragment the mass-separated ions and separate the product ions by mass.
As used herein, the term “precursor ion” is used herein to refer to an ion which is produced during ionization by a mass spectrometer device analysis, such as during MS1 ionization during MS analysis.
As used herein, the term “mass-to-charge ratio” refers to the ratio of the mass of a species to the charge state of a species. The term “m/z unit” refers to a measure of the mass to charge ratio. The Thomson unit (abbreviated as Th) is an example of an m/z unit and is defined as the absolute value of the ratio of the mass of an ion (in Daltons) to the charge of the ion (with respect to the elemental charge).
The terms “peptide” and “polypeptide” are used synonymously in the present disclosure, and refer to a class of compounds composed of amino acid residues chemically bonded together by amide bonds (or peptide bonds). Peptides are polymeric compounds comprising at least two amino acid residues or modified amino acid residues. Peptides include compositions comprising a few amino acids and include compositions comprising intact proteins or modified proteins. Modifications can be naturally occurring or non-naturally occurring, such as modifications generated by chemical synthesis. Modifications to amino acids in polypeptides include, but are not limited to, phosphorylation, glycosylation, lipidation, prenylation, sulfonation, hydroxylation, acetylation, methionine oxidation, alkylation, acylation, carbamylation, iodination and the addition of cofactors. Peptides include proteins and further include compositions generated by degradation of proteins, for example by proteolytic digestion. Peptides and polypeptides may be generated by substantially complete digestion or by partial digestion of proteins. Identifying or sequencing a peptide refers to determination of is composition, particularly its amino acid sequence, and characterization of any modifications of one or more amino acids comprising the peptide or polypeptide.
“Protein” refers to a class of compounds comprising one or more polypeptide chains and/or modified polypeptide chains. Proteins may be modified by naturally occurring processes such as post-translational modifications or co-translational modifications. Exemplary post-translational modifications or co-translational modifications include, but are not limited to, phosphorylation, glycosylation, lipidation, prenylation, sulfonation, hydroxylation, acetylation, methionine oxidation, the addition of cofactors, proteolysis, and assembly of proteins into macromolecular complexes. Modification of proteins may also include non-naturally occurring derivatives, analogues and functional mimetics generated by chemical synthesis. Exemplary derivatives include chemical modifications such as alkylation, acylation, carbamylation, iodination or any modification that derivatizes the protein. Proteins of the present invention may be derived from sources, which include but are not limited to cells, cell or tissue lysates, cell culture medium after cell growth, whole organisms or organism lysates or any excreted fluid or solid from a cell or organism. Proteins may be characterized and/or analyzed by their secondary structure, tertiary structure, and quaternary structure. Accordingly, it is beneficial if the protein structure can be maintained during preparation of samples for analysis.
“Native mass spectrometry” refers to the analysis and characterization of macromolecules, predominantly intact proteins and protein complexes, where as much as possible the native structural features of the molecules are retained
Given the demands of cryo-EM (e.g., sample preparation time, required conditions, expense of necessary equipment, etc.), researchers commonly use EM techniques able to be performed at room temperature, such as TEM. A commonly used method for TEM is negative staining, which requires removal of the sample from vacuum followed by staining with a heavy metal. Negative staining has several advantages, including rapid preparation time and analysis as well as compatibility with room temperature preparation. However, negative staining suffers from limited structural resolution. As a result, TEM imaging is often supplemented with higher resolution cryo-EM for structural analysis. In either instance, the biomolecules must be landed on a sample grid under vacuum, where the sample grid is generally graphene based or coated with graphene, which introduces opportunities for the sample to lose its characteristic structure.
Accordingly, it is desirable to minimize changes in sample structure by modifying the sample grid and providing a protective material around the deposited samples. Such an improvement could expand the capabilities of MS-based sample preparation system to include negative staining TEM and other imaging systems performed at room temperature.
In one aspect of the invention, a thin layer of a viscous liquid (e.g., glycerol, poly(ethylene) glycol, or poly(propylene) glycol) is deposited on the TEM grid prior to the deposition of the sample. This approach is compatible with MS-based sample preparation, requiring only a relatively simple addition to the TEM grid. The thin layer of viscous liquid preserves the landed biomolecules, protecting them from the negative effects of the vacuum and interactions with the grid surface itself.
Without being bound by theory, it is believed the liquid layer is providing protection to the sample, not just from the vacuum conditions, but also from interactions with the TEM grid itself. This protection could be a direct or indirect interaction with the protective liquid, such as via hydrogen bonding to residual water on the sample to form a “shell”. However, because the protective liquid can evaporate, there are timing constraints in that the landed sample must be negative stained before the liquid fully evaporates. Further, if the liquid layer is too thick, it may not be possible to perform staining. Accordingly, the process should be optimized to arrive at the most advantageous timing, temperature, exposure to vacuum, and amount of sample to be landed. Previous experiments have used mass spectrometry to generate ionized molecules that are deposited into an untreated TEM grid at room temperature for negative staining. However, proteins deposited under these untreated conditions did not retain any identifying structural features.
For example,
However, using the matrices of the present invention, the biomolecular structure of these protein complexes are able to be preserved.
Introduction. Native mass spectrometry (MS) is an emerging technology that can provide complementary data to electron microscopy (EM) for protein structure characterization. Beyond the ability to provide mass measurements of gas-phase biomolecular ions, MS instruments offer the ability to purify, select, and precisely control the spatial location of these ions. This example presents a modified Orbitrap MS system capable of depositing a native MS ion beam onto EM grids. Further described is the use of a chemical landing matrix that both preserves and protects the structural integrity of the deposited particles. With this system, the first 3D reconstructed structure of gas-phase, deposited biomolecular ions, the ˜800 KDa protein complex GroEL, was obtained. These data provide direct evidence that non-covalent protein complexes can indeed retain their condensed-phase structures following ionization and vaporization. Developments of this technology can further pave the way to an integrated MS-EM technology able to provide improved cryo-EM sample preparation over conventional plunge-freezing techniques.
Description Electrospray ionization (ESI) coupled with mass spectrometry (MS) has transformed the ability to characterize proteins on a terrific scale.1 Perhaps no area of protein mass spectrometry better exemplifies this than native mass spectrometry, where entire protein complexes are gently ionized, vaporized, and mass analyzed all while remaining intact.2 These mass measurements can provide invaluable information on sub-unit stoichiometry, connectivity, and even the presence of non-covalently bound ligands.
Whether ionized protein complexes truly retain their structure in the gas-phase has been debated for decades.3 Collisional cross-sections of numerous gas-phase protein complexes have been experimentally determined by ion mobility mass spectrometry.4-6 In general, these collective data indicate that under optimal conditions the measured cross-sections are consistent with condensed-phase structures. Seeking a direct measurement, Robinson and co-workers configured a quadrupole time-of-flight (q-ToF) mass spectrometry with a transmission electron microscopy (TEM) grid holder and deposited ions GroEL and ferritin.7,8 These studies imaged particles of roughly the correct size and shape via negative and positive staining TEM. The resultant images, however, lacked the higher resolution features typical of a conventional staining experiment. This lack of detail opens the possibility of structural damage occurring during the experiment and prevents solving the 3D structure from the images. More recently, Longchamp et al. imaged soft-landed small proteins using low-energy electron holography followed by numerical reconstruction-again, confirming the ability to soft land onto a surface. However, given the small size of the molecules imaged and lack of details in the reconstructed images it is difficult to tell whether the landed proteins are damaged or not.9
Encouraged by the unrealized potential of native mass spectrometry to couple directly with electron microscopy, the present examples explored new configurations and approaches for depositing gas-phase protein complexes onto surfaces for direct EM imaging. First, a quadrupole Orbitrap hybrid system (Ultra-High Mass Range Q-Exactive10) was modified by removing the collision cell and modifying the end vacuum cap to allow for an insertion probe to hold a TEM grid to the rear of the C-trap exit lens (
To test the apparatus, the bacterial chaperonin GroEL, an ˜800 kDa homo-oligomer having 14 identical subunits, was analyzed as it has been extremely well-characterized both by native mass spectrometry and microscopy.11 Using nanoESI, charge states were observed ranging from +71 to +62 across the m/z range of 11,000 to 13,000 and having the calculated molecular weight of 802,500 Da+300 Da (
After deposition the TEM grids were removed from vacuum, stained with uranyl acetate, and immediately viewed using TEM (
From these data it was supposed that the GroEL ions had lost their condensed-phase structure via: (1) the process of ionization, vaporization, and transport through mass spectrometry, (2) lengthy exposure to high vacuum without solvent (i.e., up to 600 seconds on surface), (3) dissociation upon collision with grid surface during landing, and/or (4) interactions with the TEM grid surface. The most central of these possibilities being the effect of vacuum exposure—about ten minutes for the landing durations used in
To further probe this issue, the TEM grid surface was altered by addition of a chemical matrix. Cryoprotective compounds (e.g., glycerol, trehalose, glucose, ionic liquids, etc.) can promote preservation of protein structure, even when dehydrated and/or in vacuum environments; further, several studies have shown the benefits of direct TEM imaging from sugar-fixed particles.12-15 Additionally, two MS studies reported using glycerol-coated deposition surfaces to collect and, ultimately, show biological viability of soft-landed single proteins and intact viruses.16,17
Following these leads, a uniform thin film of glycerol matrix was produced by depositing a small volume (˜3 μL) of glycerol onto the carbon TEM grid surface followed by manual blotting, insertion into the modified Orbitrap, and GroEL cation beam deposition. After a deposition period of up to 600 seconds, the TEM grids were removed and negatively stained. The coated grid was then spotted with GroEL, placed in a vacuum (10-60 minutes, 2×10−5 Torr), and imaged using negative stain TEM.
The resultant images revealed structurally intact GroEL (
To test whether this phenomenon was unique to GroEL, the deposition behavior of two other well-studied protein complexes were explored. The first, alcohol oxidase (AOX, a homo-octamer) 18 was similarly subjected to nano-electrospray and measured at a mass of 598,400±400 Da (
Following the initial supposition, these data provide strong evidence that either exposure to vacuum and/or interactions with the TEM grid surface are likely the key factors governing the structural preservation of soft-landed particles. At present, it is believed both phenomena are likely at play and, to some extent, are mitigated by the matrix. First, few quality particles are observed following extended vacuum exposures (>30 minutes) of a matrix-landed protein complex following negative staining, underscoring the importance of a high flux ion beam to reduce the time required for sample preparation. Second, because glycerol is a dielectric, it is hypothesized that the matrix serves to prevent neutralization of the ionic protein complex by insulating it from the conducting carbon surface. When cationic proteins are reduced on surfaces, electron-based dissociation can occur.20 Charge neutralization without the stabilizing forces of water could also result in structural deformation. Supporting this idea, attempts to land GroEL cations into an ionic liquid matrix resulted in no observable particles. However, this same solution protects and preserves neutral protein complexes from the deleterious effects of vacuum.
Having established a method to deposit and preserve protein complexes from a gaseous ion beam, the question of whether gas-phase biomolecular ions retain their condensed-phase structures was examined. A dataset of 340 images was collected from a GroEL matrix-landed grid using an L120C microscope equipped with a Ceta camera. It was expected that with these matrix-landed molecular images a medium resolution (˜20 Å) negative stain 3D reconstruction could be generated. From the resultant images, having a pixel size of 3.2 Å, 3,500 particles were picked and processed using cisTEM.21 2D classification was performed and ˜2,700 particles contained in the high-quality class averages (
This example describes a technique—matrix-landing—that promotes the preservation of structure in non-covalent protein complexes that have traversed a mass spectrometer and been deposited onto a TEM grid. Negative staining provides a simple and robust method to image these structures and further data processing of the negatively stained images can offer detailed structural information. It is further envisioned matrix-landing as being applied to mixtures of protein complexes where the MS filtering capabilities enable gas-phase purification of the desired particles. Further, a wide variety of native MS technologies exist—e.g., ion mobility22, surface-induced dissociation23, collisions24, and photo-activation25—and may be utilized to characterize protein complexes and their interactions beyond intact mass measurement. The ability to image the products of these processes could prove invaluable both for structural biology and to the study of gas-phase chemistry.
Detailed evaluations of several relevant native MS conditions (e.g., buffer, ion source, desolvation energies, time-in-transit, etc.) should also yield further insights and improvements. Exploration of other chemical matrices and methods for generating the highest performing matrix films are similarly anticipated. Landing conditions likewise play an essential role mandating thorough characterization of ion beam energetics, landing pressures, and effects of vacuum exposure. Precise control of the TEM grid surface temperature may also be key for maintenance of the ideal matrix surface and for high-resolution structural preservation. All the depositions described in this example were conducted at room temperature; although reduced TEM grid temperatures is also anticipated in order to extend the protective effects of glycerol and to aid in retention of any water/solvent associated with the protein complex ion.
An extension of this reduced-landing temperature concept is to deposit partially hydrated and mass selected samples directly onto cryogenically-cooled (<180° C.) TEM grids. An expected thin coating of amorphous ice would provide protection from the deleterious effects of both vacuum and the TEM grid surface. Directly coupling cryo-EM grid preparation to MS could provide a host of advantages over conventional grid preparation including improved signal and decreased beam-induced motion (due to lower ice background and lack of in-built ice strain26, respectively). Aside from the aforementioned benefits derived from MS, the boosted signal could increase resolution and enable imaging of smaller particles. The ability to deposit and preserve protein complexes within a vacuum environment may allow for improved TEM sample preparation capabilities and facilitate integration of two disparate fields into one unified technology.
Materials. Water (Optima LC/MS grade, W6-4) and methanol (Optima for HPLC, A454SK-4) were purchased from Fisher Chemical. Ammonium Acetate (431311-50G), Glycerol (for molecular biology, G5516-100 ml), Amicon Ultra-0.5 centrifugal filter (Ultracel-100 regenerated cellulose membrane, UFC510024). Alcohol Oxidase (Pichia pastoris buffered aqueous solution, 55 mg protein/mL, A2404-1KU), GroEl (Chapernion 60 from Escherichia, C7688-1 MG), and β-Galactosidase (from Escherichia coli, G3153-5 MG) were purchased from Sigma Aldrich. Uranyl Acetate (1% solution, 22400-1) and TEM grids (Carbon support film on 400 mesh copper, CF400-CU) were purchased from Electron Microscopy Sciences.
Sample Preparation. Chaperonin 60 from Escherichia coli (GroEL, Sigma Aldrich) was prepared at 1 mg/mL in 100 mM ammonium acetate. 200 μL of acetone was added to 100 μL of buffered protein solution and allowed to sit for 5 minutes to precipitate the protein. The sample was centrifuged (Fisherbrand Gusto Mini tabletop centrifuge) and the remaining solvent was removed from the pellet. Following this, 400 μL of buffer was added to redissolve the protein and placed in an Amicon centrifugal filter. The sample was centrifuged (Thermo Scientific Sorvall Legend Micro 21R) at 10,000 g for 10 minutes at 4° C. Buffer which passed through the filter was discarded and an additional 400 μL of buffer was added to the sample for another round of washing using the same centrifugal settings. To obtain the sample, the filter was inverted and centrifuged at 2000 g for 1 minute and diluted with 80 μL of buffer.
Alcohol Oxidase was thawed on ice and 10 μL was taken and diluted in 390 μL of buffer. This solution was buffer exchanged by transfer to an Amicon spin filter and centrifuged at 10,000 g for 10 minutes at 4° C. This cycle was repeated three times replenishing with 400 μL of 100 mM ammonium acetate after each spin. To obtain the final sample the solution remaining above the filter was removed via pipette.
β-Galactosidase was prepared at 1 mg/mL in 100 mM ammonium acetate and buffered exchanged as per Alcohol Oxidase. To obtain the final sample, the filter was inverted and centrifuged at 2000 g for 1 minute and diluted with 200 μL of buffer.
Mass Spectrometry. All mass spectrometry experiments in this example were performed on a modified Thermo Scientific Q-Exactive UHMR Hybrid Quadrupole-Orbitrap mass spectrometer.10 As illustrated in
For this experiment, modifications of the Quadrupole-Orbitrap mass spectrometer included the removal of the HCD cell ion optics along with the HCD cell vacuum chamber rear cover plate. The cover was replaced with a modified plate containing a ball valve assembly which could be evacuated by the roughing pump of the UHMR system (see
For landing experiments, the Obitrap mass analyzer was not employed, and the ions were not stopped within the C-trap. To prevent trapping, the trapping gas pressure was set to a value of 0.1. A decreasing DC gradient was placed on all the ion optics from the inlet of the mass spectrometer to the TEM grid. Specifically, lens voltages of to 20V, 19V, 18V, 17V, 16V, 15V, and 0V were employed on the injection flatapole, inter flatapole lens, bent flatapole, transfer multipole, C-trap entrance lens, and TEM grid respectively. All voltages are adjustable through the user interface with exception of the TEM grid which is tied to ground. No insource trapping was employed, the inlet capillary temperature set to 30° C., and a wide mass filter isolation of 10,000-20,000 m/z for GroEL and 8,000-18,000 m/z for Alcohol Oxidase and β-Galactosidase. All protein complex solutions were sprayed at a concentration of approximately 0.1 to 0.3 mg/μL.
Matrix-Landing. Plasma treated carbon film TEM grids were coated with 3 μl of the glycerol/methanol mix (50-50 by volume) and allowed to sit for 30 seconds. Excess solution was removed by touching the edge of the grid to a piece of filter paper. The remaining solution was allowed to equilibrate for 10 minutes. The grid was then placed within the mass spectrometer using the insertion probe and exposed to the ion beam for up to 10 minutes. Upon removal, the grid was negatively stained with 1% uranyl acetate.
Transmission electron microscopy. TEM studies of for the reconstruction of negatively stained GroEL were performed at ambient temperature on a Talos L120C (Thermo Fisher Scientific) operated at 120 kV. Electron micrographs were recorded on a Ceta 16 Mpix camera (Thermo Fisher Scientific). All other TEM studies were performed at ambient temperature on a Techai G2 Spirit BioTwin (Thermo Fisher Scientific) fitted with a NanoSprint15 MK-II 15 Mpix camera (AMT Imaging), also operated at 120 kV. For the 3D reconstruction a defocused image series ranging from 0.1 μm to 2 μm in 0.1 μm steps were collected using the SerialEM software package (https://bio3d<dot>coloradu<dot>edu/SerialEM/). Particle picking, classification, reconstruction and refinement of 3D maps were performed in cisTEM.
Introduction. Cryo-EM has become the choice technique for structural biology. Obtaining a high-resolution structure, however, requires that all particles be of the same structural conformation but randomly oriented within amorphous ice. Current methods provide non-uniform ice thickness, preferred particle orientations, and undesirable structural deformation at the air-water interface. The ideal cryo-EM sample has a high density of particles situated in random orientations and covered with a few nanometers of ice. A modified Orbitrap MS system is described that directs an ion beam to a cryogenically cooled TEM grid. Once landed, particles are coated with a thin jacket of amorphous ice using a molecular beam doser. This technology provides a direct method by which to visualize MS analyzed particles using cryoEM.
Methods. A Thermo Fisher Q Exactive UHMR Hybrid Quadrupole-Orbitrap mass spectrometer, mass range m/z 350-80,000, was modified to deposit charged analyte particles onto a graphene oxide surface of a TEM grid. For these experiments the HCD cell was removed and the instrument modified to accept a direct insertion probe holding a single grid. The grid surface was cleaned in advance by air-glow discharge for 10 seconds before loading into the insertion probe. After ion exposure the grid is recovered and negatively stained in 2% uranyl acetate. Images were collected on a FEI Tecnai 120 kV TEM equipped with an AMT camera. Image analysis was performed using ImageJ and EMAN 2.31.
Preliminary Data. To examine the effects of the MS environment on protein structure a variety of analytes was selected, including GroEL, apoferritin, and the flock-house virus. The structures of all these macromolecules are well-characterized by electron microscopy and all amenable to native mass spectrometry. First, experiments were designed aimed at preserving protein complexes when immobilized on a surface in vacuum. For example, when apoferritin in buffer was simply placed on a TEM grid and then exposed to high vacuum, followed by negative stain TEM, almost no intact particles were observed. However, protein complexes suspended in viscous liquids prior to vacuum exposure maintained their structure and were clearly observed via TEM. From these data it was concluded that protein complexes that are entirely desolvated are disassembled and/or lose structural integrity when residing on a TEM surface in vacuo.
In parallel, these particles were ionized under native conditions, injected into the Orbitrap MS system, and either mass analyzed or soft landed onto TEM grids. Following deposition, typically ten to thirty minutes, the grid is removed from the vacuum chamber, negatively stained, and placed directly into a TEM for visualization. From the resulting data, individual particle images are selected and processed. The soft-landed particles had structural distortions and deformations that resembled those which had been pipetted onto the TEM grid and exposed to vacuum.
To date, the data supports the current theory that the conditions required for excellent native spray mass spectrometry are counter to those required to preserve condensed-phase structure. These experiments aim to fully understand the implications of ionizing and mass analysis on structure and, finally, whether these structural distorting processes can be mitigated resulting in the modification of commercial instrumentation for landing of isolated and/or activated biomolecular ions.
Introduction. Native mass spectrometry has proven valuable for understanding the structural organization of proteins and macromolecular complexes. That said, other than collisional cross-section measurements obtained through ion mobility, little direct data exists to confirm that these macromolecular complexes retain their condensed-phase configurations when desolvated and placed in vacuum. Addressing this question is central to the field of structural biology and especially native mass spectrometry. Using native mass spectrometry and EM, how exposure to vacuum impacts the structure of protein complexes was investigated. By either spotting protein complex containing solution onto a TEM grid and placing under vacuum or through soft-landing protein complexes using a modified mass spectrometer, hundreds of TEM grids prepared in these ways have been analyzed, providing insight into gas-phase protein structure.
Methods. A Thermo Fisher Q Exactive UHMR Hybrid Quadrupole-Orbitrap mass spectrometer, mass range m/z 350-80,000, was modified to deposit charged analyte particles onto a graphene oxide surface of a TEM grid. For these experiments the HCD cell was removed and the instrument modified to accept a direct insertion probe holding a single grid. The grid surface was cleaned in advance by air-glow discharge for 10 seconds before loading into the insertion probe. After ion exposure the grid is recovered and negatively stained in 2% uranyl acetate. Images were collected on a FEI Tecnai 120 KV TEM equipped with an AMT camera. Image analysis was performed using ImageJ and EMAN 2.31.
Preliminary Data. To examine the effects of the MS environment on protein structure, a variety of analytes were selected including the chaperonin GroEL, apoferritin, and the flock-house virus. The structures of all these macromolecules are well-characterized by electron microscopy and all amenable to native mass spectrometry. First, experiments were designed aimed at preserving protein complexes when immobilized on a surface in vacuum. For example, when apoferritin in buffer was simply placed on a TEM grid and then exposed to high vacuum, followed by negative stain TEM, almost no intact structures were observed. However, protein complexes suspended in viscous liquids prior to vacuum exposure maintained their structure and were clearly observed via TEM. From these data it was concluded that protein complexes that are entirely desolvated are disassembled and/or lose structural integrity when residing on a TEM surface in vacuo.
In a parallel line of research, these particles were ionized using nanoelectrospray under native conditions, injected into the Orbitrap MS system, and either mass analyzed or soft landed onto TEM grids placed to the rear of the C-trap. Following deposition, typically ten to thirty minutes, the grid is removed from the vacuum chamber, negatively stained, and placed directly into a TEM for visualization. From the resulting data, individual particle images are selected and processed. The soft-landed particles had structural distortions and deformations that resembled those which had been pipetted onto the TEM grid and exposed to vacuum.
This data further supports the current theory that the conditions required for excellent native spray mass spectrometry are counter to those required to preserve condensed-phase structure. These experiments aim to fully understand the implications of ionizing and mass analysis on structure and, finally, whether these structural distorting processes can be mitigated resulting in the modification of commercial instrumentation for landing of isolated and/or activated biomolecular ions
This example describes a modified Orbitrap mass spectrometer system capable of depositing a native MS ion beam onto the surface of TEM grids. With this system and the use of a chemical landing matrix, it was demonstrated that non-covalent gaseous ions of protein-protein complexes can retain their condensed-phase structures by obtaining a 3D reconstruction of landed particles. Thus, this system is able to provide an integrated mass spectrometry-electron microscopy methodology.
A quadrupole Orbitrap hybrid system (Ultra-High Mass Range Q-Exactive10) was modified similar to Examples 1 and 2 by removing the collision cell and modifying the end vacuum cap to allow for an insertion probe to hold a TEM grid to the rear of the c-trap exit lens (see
Results. Having established a method to deposit and preserve protein complexes from a gaseous ion beam, the decades old question of whether gas-phase biomolecular ions retain their condensed-phase structures was examined. From a GroEL matrix-landed grid, a dataset of ˜600 images was collected on a Technai G2 Spirit BioTwin microscope equipped with a NanoSprint15 MK-II 15 Mpix camera. It was expected that with these matrix-landed molecular images, medium resolution (˜20 Å) negative stain 3D reconstruction could be generated. Approximately 50 of the highest quality images were selected, having a pixel size of 3.4 Å, and ˜15,000 particles were processed using cisTEM 2D.21 Classification was performed and ˜7,000 (47%) particles contained in the high-quality class averages (
Ab-initio reconstruction and auto-refinement, assuming D7 symmetry, resulted in the 3D reconstruction shown in panels B and C of
The correlation co-efficient between the landed map and a density map simulated from the model with a resolution cut-off of 15 Å was measured as 0.845 using UCSF Chimera.28 As a further control, a reconstruction was obtained of the same sample prepared conventionally. From this grid a dataset of ˜40 images was collected and analyzed in an identical manner as the matrix landed sample, with ˜9,000 particles being picked and ˜7,000 (75%) being carried forward after 2D classification (
Discussion. This example further characterizes a technique, matrix-landing, that promotes the preservation of structure in non-covalent protein complexes that have traversed a mass spectrometer and been deposited onto a TEM grid. This method confirms—at the highest resolution to date—that (1) the process of ionization, vaporization, and transport through the MS, which is on the order of ˜10 ms in this system, can be accomplished without loss of particle structural integrity, and (2) lengthy exposure of particles to high vacuum without solvent/matrix is problematic. Presently it is believed that protection from dehydration is the key factor governing the structural preservation of landed particles. However, the possibility that the matrix may provide additional channels for energy dissipation upon particle impact, nor the potential for the matrix to inhibit undesirable surface interactions of the particle with the TEM grid itself, cannot be ruled out. Detailed evaluations of several relevant native MS conditions (e.g., buffer, ion source, desolvation energies, time-in-transit, etc.) should also yield further insights and improvements. Exploration of other landing matrices and methods for generating the highest performing matrix films are similarly critical. Landing conditions likewise play an essential role mandating thorough characterization of ion beam energetics, landing pressures, and effects of vacuum exposure.
Materials. Water (Optima LC/MS grade, W6-4) and methanol (Optima for HPLC, A454SK-4) were purchased from Fisher Chemical. Ammonium Acetate (431311-50G), Glycerol (for molecular biology, G5516-100 ml), Amicon Ultra-0.5 centrifugal filter (Ultracel-100 regenerated cellulose membrane, UFC510024). Alcohol Oxidase (Pichia pastoris buffered aqueous solution, 55 mg protein/mL, A2404-1KU), GroEl (Chapernion 60 from Escherichia, C7688-1 MG), and β-Galactosidase (from Escherichia coli, G3153-5 MG) were purchased from Sigma Aldrich. Uranyl Acetate (1% solution, 22400-1) and TEM grids (Carbon support film on 400 mesh copper, CF400-CU) were purchased from Electron Microscopy Sciences.
Sample Preparation. GroEL was prepared at 1 mg/ml in 100 mM ammonium acetate. 200 μL of acetone was added to 100 μL of buffered protein solution and allowed to sit for five minutes to precipitate the protein. The sample was centrifuged (Fisherbrand Gusto Mini tabletop centrifuge) and the remaining solvent was removed from the pellet. Following this, 400 μL of buffer was added to redissolve the protein and placed in an Amicon centrifugal filter. The sample was centrifuged (Thermo Scientific Sorvall Legend Micro 21R) at 10,000 g for 10 minutes at 4° C. Buffer which passed through the filter was discarded and an additional 400 μL of buffer was added to the sample for another round of washing using the same centrifugal settings. To obtain the sample, the filter was inverted and centrifuged at 2000 g for 1 minute and diluted with 80 μL of buffer.
Alcohol Oxidase was thawed on ice and 10 μL was taken and diluted in 390 μL of buffer. This solution was buffer exchanged by transfer to an Amicon spin filter and centrifuged at 10,000 g for 10 minutes at 4° C. This cycle was repeated three times replenishing with 400 μL of 100 mM ammonium acetate after each spin. To obtain the final sample the solution remaining above the filter was removed via pipette. β-Galactosidase was prepared at 1 mg/mL in 100 mM ammonium acetate and buffered exchanged as per Alcohol Oxidase. To obtain the final sample, the filter was inverted and centrifuged at 2000 g for 1 minute and diluted with 200 μL of buffer.
Mass Spectrometry. All mass spectrometry experiments were performed on a modified Thermo Scientific Q-Exactive UHMR Hybrid Quadrupole-Orbitrap mass spectrometer running Q-Exactive Tune 2.11QF2 control software.10 Modifications included the removal of the HCD cell ion optics along with fabrication of a new the HCD cell vacuum chamber rear cover plate. Note, without the HCD cell, the system is no longer able to perform collisional activation of mass selected precursors. Otherwise, the system works as usual and injection of ions into the Orbitrap is unaffected.
Using readily available components, a simple device was implemented to insert the TEM grid into the vacuum environment of the UHMR mass spectrometer. To insert and remove TEM grids for landing, the vacuum interlock and probe components of a retired Thermo Fisher Scientific ETD module were adapted. The ion volume insertion and removal tool is used to hold the TEM grid. Incorporation of the inlet valve and guide bar assembly required fabrication of a new endplate for the vacuum chamber housing of the UHMR HCD cell (
The new cover plate (shown in
All full scan MS1 experiments were conducted with an ESI voltage of 1.1 kV to 1.5 kV, mass resolving power of 6250 at m/z 400, inlet capillary temperature of 250° C., and in-source trapping with −100V offset. For landing experiments, the Obitrap mass analyzer was not employed, and the ions were not stopped within the c-trap. To prevent trapping, the trapping gas pressure was set to a value of 0.1. A decreasing DC gradient was placed on all the ion optics from the inlet of the mass spectrometer to the TEM grid. Specifically, lens voltages of 20V, 19V, 18V, 17V, 16V, 15V, and 0V were employed on the injection flatapole, inter flatapole lens, bent flatapole, transfer multipole, C-trap entrance lens, and TEM grid respectively. All voltages are adjustable through the user interface with exception of the TEM grid which is tied to ground. No insource trapping was employed, the inlet capillary temperature set to 30° C., and a wide mass filter isolation of 10,000-20,000 m/z for GroEL and 8,000-18,000 m/z for Alcohol Oxidase and β-Galactosidase. All protein complex solutions were sprayed at a concentration of approximately 0.1 to 0.3 mg/μL.
Matrix-Landing. Plasma treated carbon film TEM grids were coated with 3 μl of the glycerol/methanol mix (50-50 by volume) and allowed to sit for 30 seconds. Excess solution was removed by touching the edge of the grid to a piece of filter paper. The remaining solution was allowed to equilibrate for 10 minutes. The grid was then placed within the mass spectrometer using the insertion probe and exposed to the ion beam for up to 10 minutes. Upon removal, the grid was negative stained with 75 μL of 1% uranyl acetate by edge blotting. It was estimated that the flow rate of the nano electrospray emitter to be in the range of 20 to 40 nL/min. A 10 minute deposition experiment would therefore consume ˜300 nL of GroEL solution. At ˜0.3 mg/mL; ˜90 ng GroEL would be consumed.
Transmission electron microscopy. TEM studies of for the reconstruction of negatively stained GroEL were performed at ambient temperature on a Technai G2 Spirit BioTwin (Thermo Fisher Scientific) fitted with a NanoSprint15 MK-II 15 Mpix camera (AMT Imaging), operated at 120 kV. Microscope and camera control software included Tecnai version 3.1.3 and AMT Capture Engine version 7.00 respectively. For the 3D reconstruction a defocused image series ranging from 0.1 μm to 2 μm in 0.1 μm steps were collected using the SerialEM software package version 3.8.7 (https://bio3d.colorado.edu/SerialEM/). Particle picking, classification, reconstruction and refinement of 3D maps were performed in cisTEM version 1 (https://cistem.org/). Model fitting, imaging and figure creation were done using UCSF Chimera version 1.16 (https://www.cgl.ucsf.edu/chimera/).
Data Collection and Processing. The conditions for collecting cryo-EM data for this experiment is provided below in Table 1.
/Å2)
indicates data missing or illegible when filed
The electron microscopy data processing workflow is illustrated in the flow chart of
Native spray mass spectrometry (MS) and Electron Microscopy (EM) have increasingly been used in parallel to increase the amount of information that can be obtained from a single protein complex analysis. The benefits of mass spectrometry, including the ability to perform isolation, determine subunit stoichiometry, and induce fragmentation paired with the direct imaging of EM makes for a strong coupling of technologies. Utilizing a modified Orbitrap MS system, native spray MS of protein complexes allowed for the deposition of an ion beam directly onto an EM grid. This example presents the use of thin film chemical matrices enabling the preservation of deposited protein complexes. Applying a thin film matrix to a TEM grid allowed for an increase in particle density on grid as well as obtaining 3D reconstructions of the 801 kDa chaperonin GroEL, 674 kDa 20S proteasome core, and 467 kDa β-Galactosidase from gas phase deposited ions.
In particular, the ability of five chemical compounds to preserve soft-landed protein complexes in a vacuum environment were examined. The five compounds tested were polyethylene glycol (PEG), triethanolamine (TEA), polypropylene glycol (PPG), Triton X-100 (2-[4-(2,4,4-trimethylpentan-2-yl)phenoxy]ethanol), and diglycerol. All five compounds provided protection from vacuum in a pipetted sample test, although only PPG, TritonX, and diglycerol presented positive results in an actual soft-landing test. Although the performance of TEA and PEG was suboptimal in the initial soft-landing test, these compounds are still believed to be viable options given that the proper conditions under which to perform soft landings can be found.
It was attempted to optimize the experimental conditions for PPG, as this compound was the best performing matrix in the initial landing experiments, meaning that it provided the greatest probability of being able to find an area on a soft-landed TEM grid which is imageable. In the optimization of PPG it was also discovered that rinsing the TEM grid with 1M ammonium acetate buffer after soft-landing and prior to staining could be beneficial. This rinsing procedure enabled β-Galactosidase to be landed and imaged. Prior to employing the rinsing procedure, β-Galactosidase could be landed but the particles could not be stained and imaged properly. The same rinsing procedure has been tested on larger molecules, such as GroEL or the 20S Proteasome, and was not found to be detrimental to the outcome.
Overall this example provides further evidence that the noncovalent structure inherent to protein complexes is retained through the ionization, desolvation, and the landing process of the mass spectrometer as well as highlighting the benefits of using dilute chemical matrices to form thin films for preservation.
Introduction. Electrospray ionization mass spectrometry (ESI-MS) as a technique has played a pivotal role in allowing the correlation of gas phase biomolecular structure to condensed phase structure.16 ESI-MS allows for the investigation of native protein structure, multiprotein complexes, and protein-ligand complexes due to the soft ionization that occurs allowing a gentle transition to the gas phase.32 The gentle transition from solution phase to gas phase, and the relative effects on structure from ionization, desolvation, and mass analysis has motivated research investigating overall structure of complexes that have traversed a mass spectrometer.6 Recently it was shown that condensed phase structure of the 801 kDa chaperonin GroEL is retained after ionization, vaporization, and sequential deposition onto a TEM grid with a layer of glycerol using a modified Orbitrap MS system followed by direct imaging using electron microscopy.33 The use of excipients and cryoprotectants, such as glycerol, is ubiquitous in the preservation of protein complexes.34-37 Capelle et al. shows a variety of chemical compounds that help in the preservation of proteins which includes buffers, amino acids, surfactants, antioxidants, polymers, sugars, and polyols.35
This example investigates the use of chemical matrices on TEM grids to preserve landed protein complexes from the deleterious effects of vacuum. Whereas previous experiments relied on excess glycerol to preserve protein structure, these results demonstrate that thin films (˜180 nm) of certain chemicals provide equally effective protection while increasing landing efficiency. This approach results in more reproducible distributions and higher densities of particles, thus allowing for the preparation of TEM grids comparable in particle distribution and density to conventionally prepared TEM samples. Additionally, this examples provides further evidence that the condensed phase structure of multiple landed protein complexes is retained when soft-landing into a thin film matrix.
When choosing the matrices to analyze, a few different properties were sought after including (1) high viscosity (2) high boiling point as to retain the matrix under vacuum (3) similarity of structure to glycerol, and (4) compatibility with protein complexes. It was suspected, as with the initial glycerol soft-landings experiments, that the protection from dehydration is the main contributor to the overall protection of the landed ions.
Methods and Materials. Water (Optima LC/MS grade, W6-4) and methanol (Optima for HPLC, A454SK-4) were purchased from Fisher Chemical. Ammonium Acetate (431311-50G), Amicon Ultra-0.5 centrifugal filter (Ultracel-100 regenerated cellulose membrane, UFC510024), Glycerol (for molecular biology, G5516-100 mL), Triethanolamine (90279-100 mL), Poly(ethylene glycol) average MW 200 (P3015-250G), Poly(propylene glycol) average MW 2700 (202347-5G), Triton X-100 (×100-5 mL), Diglycerol (TCI-T0119-25G). GroEl (Chapernion 60 from Escherichia, C7688-1 MG) and β-Galactosidase (from Escherichia coli, G3153-5 MG) were purchased from Sigma Aldrich. 20S Proteasome core particles (Thermoplasma acidophilium expressed in E. coli BL21 (DE3)) provided by non-commercial source. Uranyl Acetate (1% solution, 22400-1) and TEM grids (Carbon support film on 400 mesh copper, CF400-CU) were purchased from Electron Microscopy Sciences.
Sample preparation. GroEL was reconstituted in 100 mM ammonium acetate at 1 mg/mL. On ice, 500 μL of GroEL solution was mixed with 1000 μL of acetone and was allowed to precipitate for 5 minutes. The precipitated protein was centrifuged (Fisherbrand Gusto Mini tabletop centrifuge) and the remaining solvent was removed. 1000 μL of 100 mM ammonium acetate was added to the pellet, redissolved, and then split between two Amicon centrifugal filter. The sample was centrifuged (Thermo Scientific Sorvall Legend Micro 21R) at 10,000 g for 10 minutes at 4° C. Flow through was discarded and an additional 400 μL of ammonium acetate was added followed by another round of centrifugation using the same settings as above. Samples were obtained by inverted the tubes and centrifuging at 2,000 g for 1 minutes at 4° C. The two samples were combined and diluted in 350 μL buffer.
20S Proteasome was thawed on ice and 100 μL was taken and transferred to an Amicon spin filter and centrifuged at 10,000 g for 10 minutes at 4° C. Three rounds of purification were done with 400 μL each of 100 mM ammonium acetate followed by centrifugation at 10,000 g for 10 minutes at 4° C. with flow through being discarded. Samples were obtained as per GroEL and diluted in 100 μL buffer.
β-Galactosidase was reconstituted in 100 mM ammonium acetate at 1 mg/mL and buffered exchanged as per 20S Proteasome. The final sample was obtained by inverting the tube and centrifuged at 2000 g for 1 minute and diluted with 100 μL of buffer.
Mass Spectrometry. All mass spectrometry experiments were performed on a modified Thermo Scientific Q-Exactive UHMR Hybrid Quadrupole-Orbitrap mass spectrometer. Modifications and full scan MS1 experiments are previously described33 with no changes required to the mass spectrometer electronics or software. In-house emitters with an inner diameter of 1-5 microns were pulled with borosilicate glass capillaries using a model P-2000 laser-based micropipette puller (Sutter Instrument, CA). Continuity between the ESI-MS power supply and the solution being sprayed was achieved by placing a platinum wire in the backside of the emitter. Landing experiments were conducted without the Orbitrap mass analyzer and the trapping gas pressure was set to a value of 0.1 to prevent trapping. A linear decrease in voltage was applied to the injection flatapole, inter flatapole lens, bent flatapole, transfer multipole, and C-trap entrance lens with values of 20V, 19V, 18V, 17V, 16V, 15V, respectively, followed by the TEM grid at ground (0V). No mass filter isolation or insource trapping was used and the inlet capillary temperature was set to 80° C. with all protein complex solutions being sprayed at 0.1 to 0.3 mg/mL.
Matrix Negative Stain. The matrices PEG, PPG, Glycerol, Triethanolamine, and 100 mM ammonium acetate buffer (control) were mixed in a ratio (by volume) of 1:1:1 of methanol, matrix, and GroEL at a concentration of 0.2 mg/mL, respectively, with TritonX-100 and Diglycerol having a ratio of 2:1:2. Each matrix solution with GroEL was pipetted on a plasma treated carbon film TEM grid and allowed to sit for 1 minute. The grid was negative stained with 75 μL of 1% uranyl acetate and edge blotted to remove excess stain.
Excess Matrix Vacuum Negative Stain. The matrices PEG, PPG, Glycerol, and 100 mM ammonium acetate buffer (control) were mixed in a ratio (by volume) of 1:1:1 of methanol, matrix, and GroEL at a concentration of 0.02 mg/mL respectively with triethanolamine, TritonX-100, and diglycerol having a ratio of 3:1:3. The solutions were pipetted on a plasma treated carbon film TEM grid and allowed to sit for 1 minute followed by edge blotting to remove excess solution. Grids were then placed under vacuum for 15 min at 2×10−5 Torr followed by negative stain as previously described.
Minimal Matrix Vacuum Negative Stain. 1% solutions by volume of each matrix in methanol were prepared. Separate plasma treated carbon film TEM grids were coated with 0.5 μL of matrix/methanol mix and allowed to sit for 30 seconds. To this, 0.5 μL of GroEL at 0.02 mg/mL was pipetted and grids were then placed under vacuum for 15 min. Upon removal, the grid was rinsed with 150 μL of 1M ammonium acetate and immediately negative stained with 75 μL of 1% uranyl acetate and edge blotted to remove excess stain.
Matrix-Landing. Plasma treated carbon film TEM grids were coated with 0.5 μL of matrix/methanol mix (0.25% by volume with matrices being PPG, TritonX-100, or diglycerol) and allowed to sit for 30 seconds. The grid was inserted into the mass spectrometer using the insertion probe and deposition occurred for up to 10 minutes. The grid was then rinsed with 1M ammonium acetate buffer and sequentially stained as previously described.
Transmission electron microscopy (TEM). TEM studies of the reconstruction of negatively stained GroEL, β-Galactosidase, and 20S Proteasome were performed on a Technai G2 Spirit BioTwin (Thermo Fisher Scientific) at ambient temperature fitted with a NanoSprint15 MK-II 15 Mpix camera (AMT Imaging), operated at 120 kV. Defocused image series for 3D reconstruction ranging from 0.1 μm to 2 μm in 0.1 μm steps were collected using the SerialEM software package (https: bio3d.colorado.edu/SerialEM/). Particle picking, classification, reconstruction and refinement of 3D maps were performed in cisTEM (https:cistem.org/). Model fitting, imaging and figure creation were done using UCSF Chimera (https:www.cgl.ucsf.edu/chimera/).
Results. With the success of glycerol to preserve protein complexes on a TEM grid, allowing for the first-ever 3D reconstruction of a soft-landed protein complex,33 other chemical matrices were investigated that may harbor comparable properties to glycerol. A few of these properties include being polar, having a high viscosity (1200 cP at 20° C.), being a liquid at room temperature, and having favorable interactions with complexes as to not induce unfolding.38 Using these properties as a preliminary guide, a total of five different matrices were chosen for investigation which includes triethanolamine (TEA), poly(ethylene) glycol (PEG), poly(propylene) glycol (PPG), TritonX-100, and diglycerol, with structures of each molecule, as well as glycerol, shown in
For initial matrix-protein compatibility investigations GroEL, the matrix of interest, and methanol were mixed together. A small volume of this solution (3 μL) was deposited on a TEM grid, edge blotted to remove excess protein solution, and then negative stained for TEM imaging. In all instances, the resulting images of GroEL in each matrix (
Next, to determine how vacuum exposure in each matrix impacted the structure of GroEL, a similar mixture of GroEL, matrix, and methanol was combined, pipetted onto a TEM grid, edge blotted to remove excess solution, and then placed in a vacuum (2×10−5 Torr) for 15 minutes. Upon removal from vacuum the grid was negatively stained and imaged. Most notably in
To further probe the interference of the matrix when staining and importantly determine which matrices could be used at extremely low volumes for soft-landing, a final vacuum experiment was conducted using a thin film of matrix. Previous experiments with glycerol for soft-landing were prepared in excess by pipetting 3 μL of 1:1 glycerol/methanol followed by edge blotting leaving ˜0.5 μL of matrix on grid. It was suspected that the use of extremely thin layers of matrix would enable more particles to be retained and make the layer of matrix more reproducible compared to the previous edge blotting method which removed an arbitrary amount of matrix. This thin film matrix would reduce the thickness of liquid that the protein complex would need to traverse to interact with the grid and increase the concentration of protein in the matrix, which would increase the probability of particle-surface interaction.
To a TEM grid, 0.5 μL of a 1% solution of matrix by volume in methanol was deposited and allowed to dry. The matrix-covered grid was then spotted with 0.5 μL GroEL to more closely mimic soft-landing, placed in a vacuum for 15 minutes, removed and rinsed with 150 μL of buffer, and then immediately stained. In this instance both glycerol (
Matrix-landing and TEM imaging of GroEL, 20S Proteasome, and β-Galactosidase. With the three new matrices PPG, TritonX-100, and Diglycerol allowing for the preservation of GroEL under vacuum using thin films, it was determined if the three matrices would be able to preserve landed protein complexes. To a TEM grid, 0.5 μL of a 0.25% matrix solution in methanol was deposited and allowed to dry. Next, the matrix-covered grid was inserted into the modified Orbitrap MS system (
First, the chaperonin GroEL, with a mass of ˜800 kDa and average charge state of +67 (
To further investigate the capabilities of PPG as a thin film matrix to protect soft-landed ions, the well-studied protein complex β-Galactosidase was examined. β-Galactosidase is a tetramer with 4 identical subunits and a mass of ˜467 kDa and average charge state of +45.19 Using the same workflow as previously described with GroEL, β-Galactosidase was soft-landed into a thin film of PPG, washed with buffer, and negative stained. Interestingly, the density of the soft-landed β-Galactosidase consistently yields less overall protein complex on grid compared to GroEL. This could be due to a couple of aspects including potentially having lower ionization efficiency and/or matrix interference with staining due to its smaller structure.
Finally, the 20S proteasome core particle (CP), which is a barrel-shaped protein complex composed of four stacked homoheptameric rings,40 was obtained and analyzed. Unlike with GroEL and β-Galactosidase, there was no prior experience with the 20S proteasome CP. Using identical methodology as the previous two complexes, the 20S proteasome CP was able to be purified, electrosprayed, soft-landed, and reconstructions obtained in less than half a day. The
Discussion/Conclusion. This example demonstrates the use of thin films to preserve three different protein-protein complexes which have been ionized into the gas phase, traveled through a mass spectrometer, and landed on a TEM grid showing excellent agreement to published crystal structures and pipetted reconstructions. Through this, it is shown not only that a variety of matrices at higher concentrations have the capability to preserve protein complexes from the detrimental effect of vacuum but that chemical matrices such as PPG, TritonX-100, and diglycerol can protect protein complexes at exceptionally low amounts. These thin films provide equally effective protection while increasing landing efficiency.
While the exact mechanism of protection is currently unknown for soft-landed protein complexes in this example, previous research looking at the interaction of solution phase protein-matrix interactions may provide insight into a potential mechanism. Gekko and Timasheff41 investigated the effect of glycerol in solution with a variety of proteins showing that glycerol is preferentially excluded from the domain of the protein. The exclusion of glycerol is suggested to stabilize the proteins and favor folded native states. In a similar line of work with PEG, Lee and Lee42 investigated the preferential solvent interactions between proteins and polyethylene glycol suggesting an identical mechanism as glycerol; PEGs are excluded from the protein domain with an increase in polymer size causing an increase in exclusion thereby stabilizing the protein. More recent studies by Li et al.43 analyzing sugars and polyols (e.g. glycerol) interactions further shows preferential exclusion from the protein surface thereby stabilizing proteins. The combination of these works and others44-48 suggests that the matrices used may have a similar mechanism in order to protect/preserve the soft-landed proteins of interest with low amounts of PPG, TritonX-100, and diglycerol showing the most extreme protection capabilities of the matrices examined.
It is suspected that by decreasing the distance the protein complexes need to traverse to interact with the grid while increasing the concentration of protein in the matrix allows for increased probability of particle-surface interaction. This approach results in more reproducible distributions and higher densities of particles, thus allowing for the preparation of TEM grids comparable in particle distribution and density to conventionally prepared TEM samples. With the reproducibility and particle density afforded by PPG, it is possible to probe aspects of the mass spectrometer that were difficult to investigate with glycerol previously. This could allow for insight into the effects of desolvation, isolation capabilities, in-source fragmentation, and charge reduction on the structure of protein complexes.
Having now fully described the present invention in some detail by way of illustration and examples for purposes of clarity of understanding, it will be obvious to one of ordinary skill in the art that the same can be performed by modifying or changing the invention within a wide and equivalent range of conditions, formulations and other parameters without affecting the scope of the invention or any specific embodiment thereof, and that such modifications or changes are intended to be encompassed within the scope of the appended claims.
When a group of materials, compositions, components or compounds is disclosed herein, it is understood that all individual members of those groups and all subgroups thereof are disclosed separately. Every formulation or combination of components described or exemplified herein can be used to practice the invention, unless otherwise stated. Whenever a range is given in the specification, for example, a temperature range, a time range, or a composition range, all intermediate ranges and subranges, as well as all individual values included in the ranges given are intended to be included in the disclosure. Additionally, the end points in a given range are to be included within the range. In the disclosure and the claims, “and/or” means additionally or alternatively. Moreover, any use of a term in the singular also encompasses plural forms.
As used herein, “comprising” is synonymous with “including,” “containing,” or “characterized by,” and is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. As used herein, “consisting of” excludes any element, step, or ingredient not specified in the claim element. As used herein, “consisting essentially of” does not exclude materials or steps that do not materially affect the basic and novel characteristics of the claim. Any recitation herein of the term “comprising”, particularly in a description of components of a composition or in a description of elements of a device, is understood to encompass those compositions and methods consisting essentially of and consisting of the recited components or elements.
One of ordinary skill in the art will appreciate that starting materials, device elements, analytical methods, mixtures and combinations of components other than those specifically exemplified can be employed in the practice of the invention without resort to undue experimentation. All art-known functional equivalents, of any such materials and methods are intended to be included in this invention. The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention that in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. The invention illustratively described herein suitably may be practiced in the absence of any element or elements, limitation or limitations which is not specifically disclosed herein. Headings are used herein for convenience only.
All publications referred to herein are incorporated herein to the extent not inconsistent herewith. Some references provided herein are incorporated by reference to provide details of additional uses of the invention. All patents and publications mentioned in the specification are indicative of the levels of skill of those skilled in the art to which the invention pertains. References cited herein are incorporated by reference herein in their entirety to indicate the state of the art as of their filing date and it is intended that this information can be employed herein, if needed, to exclude specific embodiments that are in the prior art.
This application claims priority from U.S. Provisional Patent Application No. 63/247,705, filed Sep. 23, 2021, U.S. Provisional Patent Application No. 63/331,537, filed Apr. 15, 2022, and U.S. Provisional Patent Application No. 63/348,878, filed Jun. 3, 2022, which are incorporated by reference herein to the extent that there is no inconsistency with the present disclosure
This invention was made with government support under GM118110 awarded by the National Institutes of Health. The government has certain rights in the invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2022/044560 | 9/23/2022 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63247705 | Sep 2021 | US | |
| 63331537 | Apr 2022 | US | |
| 63348878 | Jun 2022 | US |
