Patent Application
20060123509
The present invention provides a method for purifying recombinant proteins accumulated in recombinat protein bodies-like assemblies (RPBLAs). More specifically, the invention provides for the isolation of recombinant fusion proteins within recombinant protein bodies-like assemblies that permit isolation from other host-cell organelles by a difference in density wherein the desired recombinant protein can be concentrated, separated from other cell components and easily recovered.
Protein bodies (PBs) are subcellular organelles (or large vesicles, about 1-3 microns in diameter, surrounded by a membrane) that specialize in protein accumulation. They are naturally formed in some specific plant tissues, like seeds, and serve as principal source of amino acids for germination and seedling growth.
The storage proteins are co-translationally inserted into the lumen of the endoplasmic reticulum (ER) via a signal peptide to be packaged either in the ER or into the vacuoles (Galili et al., 1993 Trends Cell Biol. 3:437-443) and assembled into multimeric units inside these subcellular compartments, developing specific organelles called (ER)-derived protein bodies (PBs) or protein storage vacuoles (PSV) (Okita and Rogers, 1996 Annu. Rev. Plant Physiol Mol. Biol. 47:327-350; Herman and Larkins, 1999 Plant Cell 11:601-613; Sanderfoot and Raikel, 1999 Plant Cell 11:629-642).
The storage proteins dicotiledoneous plants are primarily soluble proteins such as the 7S globulin or vicilin type, 11S globulins or legumin-type proteins and are sequestered in PSVs together with other proteins (i.e., protease inhibitors, proteolytic enzymes, lectins and the like), sugars and salts.
In contrast to PSVs, PBs (1-3 microns) sequester predominantly prolamins, which are highly hydrophobic storage proteins of cereals (such as zeins of maize and gliadins of wheat), and lack of other auxiliary proteins (Herman and Larkins, 1999 Plant Cell 11:601-613).
At present, no PBs have been found in tissues other than plant seeds, with the exception of the ER bodies. The ER bodies are small in size (0.2-0.4 micrometers) and are formed in Arabidopsis leaves only by wounding and chewing by insects but do not develop under normal conditions (Matsushima et al., 2003 Plant J. 33:493-502).
Genetic engineering approaches have been used to study plant PBs formation, storage protein assembly and targeting. It has been shown that when recombinant proteins, predominantly plant storage proteins are expressed and packaged in Arabidopsis and tobacco, plant tissues that did not contain PBs (as vegetative tissues), develop these organelles “de novo” (Bagga et al., 1997 Plant Cell 9:1683-1696 and Bagga et al., 1995 Plant Physiol. 107:13-23, and U.S. Pat. No. 5,990,384, U.S. Pat. No. 5,215,912, and U.S. Pat. No. 5,589,616; and Geli et al., 1994 Plant Cell 6:1911-1922).
Maize beta-zein when expressed in transgenic tobacco plants was correctly targeted in new formed ER-derived PBs in leaf cells (Bagga et al., 1995 Plant Physiol. 107:13-23). Maize gamma-zein and, truncated gamma-zein cDNAs expressed in Arabidopsis plants also accumulate in a novel ER-derived PBs in leaves (Geli et al., 1994 Plant Cell 6:1911-1922). Lysine-rich gamma-zeins expressed in maize endosperms (Torrent et al. 1997 Plant Mol. Biol. 34(1):139-149) accumulate in maize PBs and co-localized with endogenous zeins. Transgenic tobacco plants expressing alpha-zein gene demonstrated that alpha-zein was not able to form PBs. However, when alpha- and gamma-zein were co-expressed, the stability of alpha-zein increased and both proteins co-localized in ER-derived protein bodies (Coleman et al., 1996 Plant Cell 8:2335-2345). Formation of novel PBs has been also described in transgenic soybean transformed with methionine-rich 10 kDa delta-zein (Bagga et al., 2000 Plant Sci. 150:21-28).
Recombinant storage proteins are also assembled in PBs-like organelles in a non-plant host system such as Xenopus oocytes and in yeast. Rosenberg et al., 1993 Plant Physiol 102:61-69 reported the expression of wheat gamma-gliadin in yeast. The gene expressed correctly and the protein was accumulated in ER-derived PBs. In Xenopus oocytes, Torrent et al., 1994 Planta 192:512-518 demonstrated that gamma zein also accumulates in PB-like organelles when transcripts encoding the protein were microinjected into oocytes. Hurkman et al., 1981 J. Cell Biol. 87:292-299 with alfa-zeins and Altschuler et al., 1993 Plant Cell 5:443-450 with gamma-gliadins had similar results in Xenopus oocytes.
One of the fundamental achievements of the field of the biotechnology (genetic engineering) is the ability to genetically manipulate an organism to produce a protein for therapeutic, nutraceutical or industrial uses. Methods are provided for producing and recovering recombinant proteins from fermentation broth of bacteria, yeast, crop plants and mammalian cell cultures. Different approaches for protein expression in host cells have been described. The essential objectives of these approaches are: protein expression level, protein stability and protein recovery (Menkhaus et al., 2004 Biotechnol. Prog. 20: 1001-1014; Evangelista et al., 1998 Biotechnol. Prog. 14:607-614).
One strategy that can solve a problem with protein recovery is secretion. However, secretion involves some times poor expression levels and product instability. Another strategy is the accumulation of the recombinant protein in the most beneficial location in the cell. This strategy has been extensively used by directing recombinant proteins to the ER by engineering C-terminal extension of a tetrapeptide (HDEL/KDEL) (Conrad and Fiedler, 1998 Plant Mol. Biol. 38:101-109).
Fusion proteins containing a plant storage protein or storage protein domains fused to the heterologous protein have been an alternative approach to direct recombinant proteins to the ER (WO 2004003207). One interesting fusion strategy is the production of recombinant proteins fused to oleosins, constitutive protein of plant oil bodies. The specific characteristics of oil bodies benefit of the easy recovery of proteins using a two-phase system (van Rooijen and Moloney, 1995 Bio/Technology 13:72-77).
Heterologous proteins have been successfully expressed in plant cells (reviews Horn et al., 2004 Plant Cell Rep. 22:711-720; Twyman et al., 2003, Trends in Biotechnology 21:570-578; Ma et al., 1995, Science 268: 716-719; Richter et al., 2000 Nat. Biotechnol. 18:1167-1171), and in some, the expression of the recombinant protein has been directed to ER-derived PB or PSV (PSV). Yang et al., 2003 Planta 216:597-603, expressed human lysozyme in rice seeds using the seed-specific promoters of glutelin and globulin storage proteins. Immunocytochemistry results indicated that the recombinant protein was located in ER-PBs and accumulated with endogenous rice globulins and glutelins. The expression of glycoprotein B of the human cytomegalovirus (hCMV) in transgenic tobacco plants has been carried out using a glutelin promoter of rice. Tackaberry et al., 1999 Vaccine 17:3020-3029. Recently, Arcalis et al., 2004 Plant Physiology 136:1-10 expressed human serum albumin (HSA) with a C-terminal extension (KDEL) in rice seeds. The recombinant HSA accumulated in PSVs with the endogenous rice storage proteins.
One obstacle for the application of plants as biofactories is the need for more research regarding the downstream processing. Protein purification from plants is a difficult task due to the complexity of the plant system. Plant solids of the extract are large, dense and relative elevated (9-20 percent by weight) (see review Menkhaus et al., 2004 Biotechnol. Prog. 20:1001-1014). At present, recombinant protein purification techniques include clarification of the extracts, treatment with solvents to remove lipids and pigments and protein or peptides purification by several ion-exchange and gel-filtration chromatography columns. The existing protocols rely upon the use of specific solvents or aqueous solutions for each plant-host system and recombinant protein. There is a need in the art for efficient and general procedures for recombinant protein recovery from transformed hosts. This need is especially relevant in cases where recombinant proteins produced in plant hosts must to be isolated. The diversity of hosts and proteins and the different physical-chemical traits between them required an efficient method to concentrate and recover recombinant products. The invention disclosed hereinafter provides one means for easing and enhancing the recovery of recombinantly-expressed proteins from non-higher plant organisms such as fungi and mammalian cells.
The present invention provides an efficient and general procedure or method for recombinant protein recovery from transformed hosts. The solution presented herein is based on the discovery that the isolation of the recombinat protein bodies-like assemblies (RPBLAs) from other host-cell proteins can be effected with unexpectedly good yields by a density-based technique, in particular by density cushion or density gradient centrifugation techniques.
More particularly, the present invention contemplates a method for purifying a recombinant fusion protein that is expressed as RPBLAs in host cells. In accordance with a contemplated method, a aqueous homogenate of transformed host cells that express a fusion protein as RPBLAs is provided. Those RPBLAs have a predetermined density that can differ among different fusion proteins, but is known for a particular fusion protein to be separated. That predetermined density of the RPBLAs is typically greater than that of substantially all of the endogenous host cell proteins present in the homogenate, and is typically about 1.1 to about 1.35 g/ml. Regions of different density are formed in the homogenate to provide a region that contains a relatively enhanced concentration of the RPBLAs and a region that contains a relatively depleted concentration of the RPBLAs. The RPBLAs-depleted region is separated from the region of relatively enhanced concentration of RPBLAs, thereby purifying said fusion protein. The region of relatively enhanced concentration of RPBLAs can thereafter be collected or can be treated with one or more reagents or subjected to one or more procedures prior to isolation of the RPBLAs or the fusion protein therein.
In preferred practice, the RPBLAs contains two polypeptide sequences linked together in which one sequence is that of a protein body-inducing sequence (PBIS) whereas the other is the sequence of a product of interest such as a drug molecule, and enzyme or the like. Preferred protein body-inducing sequences are those of prolamin compounds such as gamma-zein, alpha-zein or rice prolamin.
The host cells here are eukaryotic cells such as those of higher plants, yeasts and fungi, animal cells such as mammalian cells and algal cells. Those cells can be fresh as are obtained directly from fresh biomass, or can be dried as are obtained from dried biomass. Biomass is the mass obtained from living organisms or cells, such as a culture medium or a living organism as, for instance, a plant leaf.
In the drawings forming a part of this disclosure,
The present invention has several benefits and advantages.
One benefit is that its use enables relatively simple and rapid purification of expressed proteins based on differences in density of the expressed product from the remainder of the soluble cellular materials.
Thus, an advantage of the invention is that it provides a method to eliminate endogenous compounds (or non-recombinant products) from the host-organism and cell cultures.
Another benefit of the invention is that it provides a reliable and reproducible way to purify recombinant peptides or proteins from fresh or dried biomass (host-organism).
The present invention relates generally to a downstream process for isolating and purifying recombinant proteins and peptides of interest from transformed organisms or cell cultures. More particularly, the present invention contemplates a method for purifying a recombinant fusion protein that is expressed and accumulates as recombinant protein body-like assemblies (RPBLAs) in host cells. The RPBLAs are recombinant fusion protein assemblies induced by storage protein domains that form high density deposits inside the cells. These dense deposits can accumulate in the cytosol in the endomembrane system organelles, mitochondria, plastids or can be secreted. In accordance with a contemplated method, an aqueous homogenate of transformed host cells that express a fusion protein as RPBLAs is provided. The homogenate is preferably clarified for use. Regions of different density are formed in the homogenate to provide a region that contains a relatively enhanced concentration of the RPBLAs and a region that contains a relatively depleted concentration of the RPBLAs. The RPBLA-depleted region is separated from the region of relatively enhanced concentration of RPBLAs, thereby purifying the fusion protein. The region of relatively enhanced concentration of RPBLAs can thereafter be collected or can be treated with one or more reagents or subjected to one or more procedures prior to isolation of the RPBLAs or the fusion protein therein.
In preferred practice, the fusion protein contains two polypeptide sequences linked together in which one sequence is that of a protein body-inducing sequence (PBIS), whereas the other is the sequence of a polypeptide product of interest such as a drug molecule, and enzyme or the like. Preferred PBIS are those of prolamin compounds such as gamma-zein, alpha-zein or rice prolamin.
One aspect of the present method comprises the provision of an aqueous homogenate or other appropriate extract (collectively referred to herein as a homogenate) of a host-organism or cell culture that expresses and accumulates the desired fusion protein as recombinant protein body-like assemblies (RPBLAs). The homogenate is typically pre-clarified (clarified) prior to use to remove cellular debris as by filtration. The homogenate containing fusion protein-containing protein body-like structures (RPBLAs), lipids, soluble proteins, cell organelles, sugars, pigments and alkaloids is directly loaded on a step density gradient and the homogenate is separated on the basis of the density of its constituents, as by centrifugation. Regions of different density are formed in the homogenate during the centrifugation to provide a region that contains a relatively enhanced concentration of the RPBLA and a region that contains a relatively depleted concentration of the RPBLA. The desired fusion protein-containing RPBLAs can be collected at a specific density interphase. This procedure has permitted the recovery of more than about 90 percent of the expressed recombinant fusion protein at a more than about 80 percent purity.
Another aspect of the invention contemplates a method for RPBLA isolation from a preferably clarified homogenate by one-step density cushion. Here, a preferably clarified homogenate is loaded on a specific density cushion so that endogenous-contaminant compounds do not cross the density cushion and centrifugally separated so that the dense RPBLAs cross the cushion and can be collected. The before-discussed regions of different density are formed in the homogenate to provide a region that contains a relatively enhanced concentration of the RPBLA (the region below the cushion) and a region that contains a relatively depleted concentration of the RPBLA (the region above the cushion). Thus, the density of the recombinant protein body-like assemblies is greater than that of the cushion.
In yet another embodiment, the preferably clarified homogenate is centrifugally separated directly and in the absence of a sucrose or other added density-providing solute. Again, the centrifugation provides regions of different density are formed in the homogenate to provide a region that contains a relatively enhanced concentration of the RPBLA (pellet) and a region that contains a relatively depleted concentration of the RPBLA (supernatant). The RPBLAs can thereafter be separated, to provide purification of the RPBLAs and thereby the fusion protein.
The invention provides a method for recovery of recombinant peptides or protein expressed within RPBLAs, organelles formed in transformed host cells. The host cells here are eukaryotic cells such as those of higher plants, yeasts and fungi, animal cells such as cultured mammalian cells, cells from transgenic animals, animal eggs and the like, and algal cells. Those cells can be fresh as are obtained directly from a culture medium or living organism such as a plant leaf or animal, or can be dried.
The recombinant protein body-like assemblies have a predetermined density that can differ among different fusion proteins, but is known for a particular fusion protein to be separated. That predetermined density of the RPBLAs is typically greater than that of substantially all of the endogenous host cell proteins present in the homogenate, and is typically about 1.1 to about 1.35 g/ml. The high density of novel RPBLAs is due to the general ability of the recombinant fusion proteins to assemble as multimers and accumulate.
The contemplated RPBLAs are expressed in eukaryotes and are typically characterized by their densities as noted above. When expressed in higher plant and animal cells, the RPBLAs are typically spherical in shape, have diameters of about 1 micron (μ) and have a surrounding membrane.
The fusion proteins are separated by their densities, which tend to be greater than that of any other protein present in a transfected cell. That separation by density is typically carried out by use of a centrifuge as is commonly found in biochemistry laboratories through out the world. An illustrative commercially available centrifuge is a Beckman Coulter Avanti™ model J-25 that is used hereinafter for one cushion runs and direct centrifugation. The Beckman Coulter Optima™ XL-100K ultracentrifuge (rotor SW41Ti) was used for the gradient studies. The centrifugation is frequently carried out in the presence of an added differential density-providing solute such as a salt like cesium chloride or a sugar such as sucrose. Combining the homogenate and differential density-providing solute forms a homogenate-solute admixture.
In a particular embodiment, the recombinant fusion proteins comprise, or are preferably made of protein body-inducing sequences (PBIS) linked by a peptide bond to products (e.g., peptides or proteins) of interest (targets). PBIS are protein or amino acid sequences that mediate protein entry and/or accumulation in RPBLAs. Illustrative, non-limiting examples of PBIS include storage proteins or modified storage proteins, as for instance, prolamins or modified prolamins, prolamin domains or modified prolamin domains. Prolamins are reviewed in Shewry et al., 2002 J. Exp. Bot. 53(370):947-958. gamma-Zein, a maize storage protein whose DNA and amino acid residue sequences are shown hereinafter, is one of the four maize prolamins and represents 10-15 percent of the total protein in the maize endosperm. As other cereal prolamins, alpha- and gamma-zeins are biosynthesized in membrane-bound polysomes at the cytoplasmic side of the rough ER, assembled within the lumen and then sequestered into ER-derived PB (Herman et al., 1999 Plant Cell 11:601-613; Ludevid et al., 1984 Plant Mol. Biol. 3:277-234; Torrent et al., 1986 Plant Mol. Biol. 7:93-403).
gamma-Zein is composed of four characteristic domains i) a peptide signal of 19 amino acids, ii) the repeat domain containing eight units of the hexapeptide PPPVHL (SEQ ID NO: 1) (53 aa), iii) the ProX domain where proline residues alternate with other amino acids (29 aa) and iv) the hydrophobic cysteine rich C-terminal domain (111 aa).
The ability of gamma-zein to assemble in ER-derived protein bodies (PBs) is not restricted to seeds. In fact, when gamma-zein-gene was constitutively expressed in transgenic Arabidopsis plants, the storage protein accumulated within ER-derived recombinant PBs in leaf mesophyl cells (Geli et al., 1994 Plant Cell 6:1911-1922). Looking for a signal responsible for the gamma-zein deposition into the ER-derived PB (prolamins do not have KDEL signal), it has been demonstrated that the proline-rich N-terminal domain including the tandem repeat domain was necessary for ER retention and that the C-terminal domain was involved in PB formation. However, the mechanisms by which these domains promote the PB assembly are still unknown. Inasmuch as protein bodies are appropriately so-named only in seeds, similar structures produced in other plant organs and in non-higher plants are referred to generally as recombinant protein body-like assemblies (RPBLAs).
Illustrative other useful prolamin-type sequences are shown in the Table below along with their GenBank identifiers.
Further useful sequences are obtained by carrying out a BLAST search in the all non-redundant GenBank CDS translations+PDB+SwissProt+PIR+PRF (excluding environmental samples) data base as described in Altschul et al., 1997 Nucleic Acids Res. 25:3389-3402 using a query such as those shown below:
An illustrative modified prolamin includes (a) a signal peptide sequence, (b) a sequence of one or more copies of the repeat domain hexapeptide PPPVHL (SEQ ID NO: 1) of the protein gamma-zein, the entire domain containing eight hexapeptide units; and (c) a sequence of all or part of the ProX domain of gamma-zein. Illustrative specific modified prolamins include the polypeptides identified below as R3, RX3 and P4 whose DNA and amino acid residue sequences are also shown below.
Particularly preferred prolamins include gamma-zein and its component portions as disclosed in published application WO2004003207, the rice rP13 protein and the 22 kDa N-terminal fragment of the maize alpha-zein. The DNA and amino acid residue sequences of the gamma-zein, rice and alpha-zein proteins are shown below.
Gamma-zein of 27 kD
DNA Sequence:
Protein Sequence:
RX3
DNA Sequence:
Protein Sequence:
R3
DNA Sequence:
Protein Sequence:
P4
DNA Sequence:
Protein Sequence:
X10
DNA Sequence:
Protein Sequence:
rP13—rice prolamin of 13 kD homologous to the clone—(GenBank AB016504) Sha et al., 1996 Biosci. Biotechnol. Biochem. 60(2):335-337; Wen et al., 1993 Plant Physiol. 101(3):1115-1116; Kawagoe et al., 2005 Plant Cell 17(4):1141-1153; Mullins et al., 2004 J. Agric. Food Chem. 52(8):2242-2246; Mitsukawa et al., 1999 Biosci. Biotechnol. Biochem. 63(11):1851-1858
Protein Sequence:
DNA Sequence:
22aZt N-terminal fragment of the maize alpha-zein of 22 kD—(GenBank V01475) Kim et al., 2002 Plant Cell 14(3):655-672; Woo et al., 2001 Plant Cell 13(10):2297-2317; Matsushima et al., 1997 Biochim. Biophys. Acta 1339(1):14-22; Thompson et al., 1992 Plant Mol. Biol. 18(4):827-833.
Protein Sequence (Full Length):
DNA Sequence (Full Length):
Examples of proteins of interest include any protein having therapeutic, nutraceutical, biocontrol, or industrial uses, such as, for example monoclonal antibodies (mAbs such as IgG, IgM, IgA, etc.) and fragments thereof, antigens for vaccines (human immunodeficiency virus, HIV; hepatitis B pre-surface, surface and core antigens, gastroenteritis corona virus, etc.), hormones (calcitonin, growth hormone, etc.), protease inhibitors, antibiotics, collagen, human lactoferrin, cytokines, industrial enzymes (hydrolases, glycosidases, oxido-reductases, and the like). Illustrative DNA and amino acid residue sequences for illustrative proteins of interest are provided below.
Salmon Calcitonin Genbank BAC57417
Protein Sequence:
DNA Sequence:
hEGF—Construction based on the GenBank AAF85790 without the signal peptide
Protein Sequence:
DNA Sequence:
hGH—Construction based in the P01241 without the signal peptide
Protein Sequence:
DNA Sequence:
Using Plant-Preferred Codons
Using Native Codons
In another embodiment, the recombinant fusion protein further comprises in addition to the sequences of the PBIS and product of interest, a spacer amino acid sequence. The spacer amino acid sequence can be an amino acid sequence cleavable by enzymatic or chemical means or not cleavable. In a particular embodiment, the spacer amino acid sequence is placed between said PBIS and product of interest. An illustrative an amino acid sequence is cleavable by a protease such as an enterokinase, Arg—C endoprotease, Glu—C endoprotease, Lys—C endoprotease, Factor Xa and the like. Alternatively, an amino acid sequence is encoded that is specifically cleavable by a chemical reagent, such as, for example, cyanogen bromide that cleaves at methionine residues.
In further embodiment, the nucleic acid sequence used for transformation purposes is as disclosed according to co-assigned patent application WO 2004003207 that includes a cleavable amino acid residue sequence between the PBIS and the polypeptide of interest. Further, in yet another embodiment, the nucleic acid sequence is as disclosed according to patent application WO 2004003207, but the nucleic acid sequence coding for the cleavable amino acid sequence is absent.
In a preferred embodiment, the fusion proteins are prepared according to a method that comprises transforming the host cell system such as an animal, animal cell culture, plant, plant cell culture, fungi or algae with a nucleic acid sequence comprising (i) a first nucleic acid coding for a PBIS that is operatively linked in frame to (ii) a second nucleic acid sequence comprising the nucleotide sequence coding for a product of interest; that is, the nucleic acid sequence that encodes the PBIS is chemically bonded to the sequence that encodes the polypeptide of interest such that both polypeptides are expressed from their proper reading frames. Upon expression, the resulting fusion protein accumulates in the transformed host-system as high density recombinant protein body-like assemblies. In one embodiment, the 3′ end of the first nucleic acid sequence (i) is linked (bonded) to the 5′ end of the second nucleic acid sequence (ii). In another embodiment, the 5′ end of the first nucleic acid sequence (i) is linked (bonded) to the 3′ end of the second nucleic acid sequence (ii). In another embodiment, the PBIS comprises a storage protein or a modified storage protein, a fragment or a modified fragment thereof.
In another particular embodiment, a fusion protein is prepared according to a method that comprises transforming the host cell system such as an animal, animal cell culture, plant, plant cell culture, fungi or algae with a nucleic acid sequence comprising, in addition to the nucleic acid sequences (i) and (ii) previously mentioned, an in frame nucleic acid sequence (iii) that codes for a spacer amino acid sequence. The spacer amino acid sequence can be an amino acid sequence cleavable by enzymatic or chemical means or not cleavable, as noted before. In one particular embodiment, the nucleic acid sequence (iii) is placed between said nucleic acid sequences (i) and (ii), e.g., the 3′ end of the third nucleic acid sequence (iii) is linked to the 5′ end of the second nucleic acid sequence (ii). In another embodiment, the 5′ end of the third nucleic acid sequence (iii) is linked to the 3′ end of the second nucleic acid sequence (ii).
As used herein, the term plant host cell comprises plants, including both monocots and dicots, and, specifically, cereals (e.g., maize, rice, oats, and the like), legumes (e.g., soy, and the like), a cruciferous plant (e.g., Arabidopsis thaliana, colza, and the like) and a solanaceous plant (e.g., potato, tomato, tobacco, and the like).
A plant host system also encompasses plant cells. Plant cells include suspension cultures, embryos, meristematic regions, callus tissue, leaves, roots, shoots, gametophytes, sporophytes, pollen, seeds and microspores. A plant host cell system can be at various stages of maturity and can be grown in liquid or solid culture, or in soil or suitable medium in pots, greenhouses or fields. Expression in plant host cell systems can be transient or permanent. Plant host cell system also refers to any clone of such plant, seed, selfed or hybrid progeny, propagule whether generated sexually or asexually, and descendants of any of these, such as cuttings or seeds.
Transformation of plant cells using Agrobacterium tumefaciens is typically best carried out on dicotyledonous plants. Monocots are usually most readily transformed by so-called direct gene transfer of protoplasts. Direct gene transfer is usually carried out by electroportation, by polyethyleneglycol-mediated transfer or bombardment of cells by microprojectiles carrying the needed DNA. These methods of transfection are well-known in the art and need not be further discussed herein. It is also noted that at lest rice and maize can be transformed by Agrobacterium. Methods of regenerating whole plants from transfected cells and protoplasts are also well-known, as are techniques for obtaining a desired protein from plant tissues. See, also, U.S. Pat. No. 5,618,988 and No. 5,679,880 and the citations therein.
A contemplated method can also include the recovery and solubilization of a recombinant fusion protein. Thus, for example, RPBLAs collected from a step-density gradient or one-step density cushion are suspended in a buffered solution containing a reducing agent and centrifuged. The pellet is discarded and the recombinant protein recovered from the supernatant to be further purified as desired such as by classical chromatographic methods.
Without further elaboration, it is believed that one skilled in the art can, using the preceding description and the detailed examples below, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limiting of the remainder of the disclosure in any way whatsoever.
Experimental Procedure
The coding sequences of T20 and human epidermal growth factor (hEGF) were obtained synthetically and were modified in order to optimize its codon usage for expression in plants.
The first strand of the cDNA sequence encoding the 36 amino acids of T20 was obtained by chemical oligonucleotide synthesis, and the sequence corresponding to the Factor Xa specific cleavage site and enzyme restriction site were added at 5′ end of the sequence. This synthetic construction was purified by polyacrylamide denaturing gel.
The double-stranded cDNA was obtained by PCR using specific T20 primers containing restriction sites for further cloning.
Primers:
The synthetic gene encoding the 53 amino acids of active hEGF was obtained by primer overlap extension PCR method, using 4 oligonucleotides of around 60 bases, with 20 overlapping bases. The synthetic hEGF cDNA included a 5′ linker sequence corresponding to the Factor Xa specific cleavage site. The oligonucleotides were purified by polyacrylamide denaturing gel.
EGF1:
EGF2:
EGF3:
EGF4:
The synthetic gene encoding the 191 amino acids of active hGH was obtained by primer overlap extension PCR met Example 2: Plasmid construction hod, using 15 oligonucleotides of about 60 bases, with 20 overlapping bases. The synthetic hGH cDNA included a 5′ linker sequence corresponding to the enterokinase specific cleavage site. The oligonucleotides were purified by polyacrylamide denaturing gel.
hGH1:
hGH2:
hGH3:
hGH4:
hGH5:
hGH6:
hGH7:
hGH8:
hGH9:
hGH10:
hGH11:
hGH12:
hGH13:
hGH14:
hGH15:
Synthetic T20 and hEGF cDNA were purified from agarose gel (Amersham) and cloned into pGEM vector (Promega). The RX3 cDNA fragment (coding for an N-terminal domain of gamma-zein) containing cohesive ends of BspHI and NcoI, was inserted into the vector pCKGFPS65C (Reichel et al., 1996 Proc. Natl. Acad. Sci. USA 93:5888-5893) previously digested with NcoI (as described in patent application WO2004003207). The sequences coding for T20 and EGF were fused in frame to the RX3 sequence. The constructs RX3-T20 and RX3-EGF were prepared by substitution of the GFP coding sequence for the T20 and EGF synthetic gene.
The resulting constructs named pCRX3T20 and pCRX3EGF contained a nucleic acid sequence that directs transcription of a protein as the enhanced 35S promoter, a translation enhancer as the tobacco etch virus (TEV), the T20 and EGF coding sequences and the 3′ polyadenylation sequences from the cauliflower mosaic virus (CaMV). Effective plant transformation vectors p19RX3T20 and p19RX3EGF were ultimately obtained by inserting the HindIII/HindIII expression cassettes into the binary vector pBin19 (Bevan, 1984 Nucleic Acids Research 12:8711-8721).
The cDNA encoding the hGH was fused to the RX3 N-terminal gamma-zein coding sequence (patent WO2004003207) and inserted into a pUC18 derived plasmid containing the enhanced CaMV 35S promoter and 3′ ocs terminator. The expression cassette from the pUC18 derived plasmid named pUC18RX3hGH, that contained the corresponding fusion protein RX3-hGH sequence, was introduced in the pBin19 binary vector (Bevan, 1984 Nucleic Acids Research 12:8711-8721).
The cDNA encoding the alpha zein of 22 kD (22aZ) and the rice prolamin of 13 kD (rP13) were amplified by RT-PCR from a cDNA library from maize W64A and Senia rice cultivar, respectively. The oligonucleotides used in the PCR reaction were:
The corresponding PCR fragments were cloned in the pCRII vector (Invitrogen), sequenced and cloned in pUC18 vectors containing the enhanced CaMV 35S promoter, the TEV sequence and 3′ ocs terminator. The pCRII-rP13 was digested by SalI and NcoI, and cloned in the pUC18RX3Ct, pUC18RX3hGH and pUC18RX3EGF plasmids digested by the same enzymes to obtain respectively: pUC18rP13Ct, pUC18rP13hGH and pUC18rP13EGF. The pCRII-22aZ was digested by SalI/NcoI and cloned in the pUC18RX3Ct and pUC18RX3EGF plasmid digested by the same enzymes to obtain pUC1822aZtCt and pUC1822aZtEGF respectively. The pCRII-22aZ was also digested by SalI/RcaI and cloned in the pUC18RX3hGH plasmid digested by SalI/NcoI to obtain the clone pUC1822aZhGH. Finally, all these pUC18-derived vectors were cloned in pCambia 5300 by HindIII/EcoRI.
Animal Cells
The synthetic gene corresponding to the mature calcitonin sequence (Ct, WO2004003207) and EGF sequences as well the cDNA encoding the hGH were fused to the RX3 N-terminal gamma-zein coding sequence (patent WO2004003207) and were introduced into the vector pUC18. SalI-BamHI restriction fragments from the pUC18 derived plasmids pUC18RX3Ct, pUC18RX3EGF and pUC18RX3hGH, containing the corresponding fusion protein RX3-Ct, RX3-EGF and RX3-hGH sequences, were introduced in the vector pcDNA3.1—(Invitrogen) restricted with Xho I-Bam HI. In the resulting constructs named p3.1RX3CT, p3.1RX3EGF and p3.1RX3hGH, the fusion protein sequences were under the CMV promoter and the terminator pA BGH.
Yeast Cells
SalI (blunt ended)—BamHI restriction fragments from the pUC18 derived plasmids described above, containing the corresponding fusion protein RX3-EGF and RX3-hGH sequences were introduced in the vector pYX243 (R&D Systems) restricted with EcoRI (blunt ended)—Bam HI. In the resulting constructs named, respectively, c117 and c118, the fusion protein sequences were under the inducible GAL promoter.
Yeast
The Saccharomyces cerevisiae strain leu2) was transformed with the plasmid constructs c117 and c118 by the LiAc method (Ito et al. 1983, J. Bacteriol. 153:163-168) and transformants were selected on Leu− plates. Expression analyses were made by growing the transformants in a galactose-containing medium.
Plant Material
Tobacco (Nicotiana tabacum var. Wisconsin) plants were grown in an in vitro growth chamber at 24-26° C. with a 16 hour photoperiod. Adult plants were grown in greenhouse between at 18-28° C., humidity maintained between 55 and 65% with average photoperiod of 16 hours.
Plantlets for Agroinfiltration (Vaquero et al., 1999 Proc. Natl. Acad. Sci., USA 96(20):11128-11133; Kapila et al., 1997 Plant Sci. 122:101-108) method were grown from seeds for 4-6 weeks in the in vitro conditions described above.
Tobacco Stable Transformation
The binary vectors were transferred into LBA4404 strain of A. tumefaciens. Tobacco (Nicotiana tobaccum, W38) leaf discs were transformed as described by Draper and Hamil 1988. In: Plant Genetic Transformation and Gene Expression. A Laboratory Manual (Eds. Draper, J., Scott, R., Armitage, P. and Walden, R.), Blackwell Scientific Publications. Regenerated plants were selected on medium containing 200 mg/L kanamycin and transferred to a greenhouse. Transgenic tobacco plants having the highest transgene product levels were cultivated in order to obtain T1 and T2 generations.
Recombinant protein level was detected by immunoblot. Total protein extracts from tobacco leaves were quantified by Bradford assay, separated onto 15% SDS-PAGE and transferred to nitrocellulose membranes using a Mini Trans-Blot Electrophoretic Transfer Cell (Bio Rad). Membranes were incubated with gamma-zein antiserum (dilution 1/7000) (Ludevid et al. 1985, Plant Science 41:41-48) and were then incubated with horseradish peroxidase-conjugated antibodies (dilution 1/10000, Amersham Pharmacia). Immunoreactive bands were detected by enhanced chemiluminescence (ECL western blotting system, Amersham Pharmacia).
Tobacco Agroinfiltration
Plantlets for Agroinfiltration method were grown from seeds for 4-6 weeks in an in vitro growth chamber at 24-26° C. with a 16 hour photoperiod.
A. tumefaciens strain LB4404 containing a desired construct was grown on LB medium (Triptone 10 g/l, yeast extract 5 g/l, NaCl 10 g/l) supplemented with kanamycin (50 mg/l) and rifampicine (100 mg/l) at 28° C. with shaker (250 rpm) overnight (about 18 hours). Agrobacteria were then inoculated in 30 ml of LB also supplemented with kanamycin (50 mg/l) and rifampicine (100 mg/l). After overnight culture at 28° C. (about 18 hours), agrobacterial cells were collected by centrifugation for 10 minutes at 3000×g and resuspended in 10 ml of liquid MS medium with MES (Sigma Chemical) 4.9 g/l and sucrose 30 g/l at pH 5.8. Bacterial culture was adjusted to a final OD600 of 0.1 for agroinfiltration. Then, cell culture was supplemented with acetosyringone to a final concentration of 0.2 mM and incubated for 90 minutes at 28° C.
For agroinfiltration, the plantlets were totally covered with the suspension and vacuum was applied (100 KPa) for 5-6 seconds. The suspension was removed and plantlets maintained in a growth chamber at 24-26° C. under a photoperiod of 16 hours for four days. The plantlet material was recovered and total protein extraction analyzed by immunoblot using anti-gamma-zein antibody.
Animal Cell Transformation
The constructs p3.1RX3.Ct, p3.1RX3.EGF and p3.1RX3.hGH were introduced in 293T, Cos1 or CHO cultured mammalian cells by the lipofectamine-based transfection method (Invitrogen). Cells transfected with plasmid pECFP-N1 (Clontech) containing the gene sequence of an enhanced cyan fluorescent modified GFP, were used as a control.
Fresh Plant Material
Plant material (wet or dry tobacco leaves or plantlets) was ground in liquid nitrogen and homogenized with extraction buffer T containing Tris-HCl 50 mM pH 8, 200 mM dithiothreitol (DTT) and protease inhibitors [10 μM Aprotinin, 1 μM pepstatin, 100 μM leupeptine, 100 μM phenylmethylsulphonyl floride (PMSF) and 100 μM E64 (Sigma Chemical)]. The homogenates were centrifuged at 10000×g for 30 minutes at 4° C. to remove insoluble material. Total soluble proteins (TSP) were quantified using Bradford protein assay (BioRad).
Dry Tobacco Leaves and Protein Extraction
Adult transgenic and wild type tobacco leaves were dried in a 37° C. room for two weeks in a filter paper. After two weeks, leaves were cut up and stored at room temperature for 5 months. Total soluble protein of dry material was extracted as described for fresh material and was analyzed by western blot.
Homogenization
Fresh and dried transgenic tobacco leaves and agroinfiltrated tobacco plantlets (transient transformation) were ground in a mortar and pestle at 0° C. in a PBP extraction buffer containing Tris 100 mM pH 8, KCl 50 mM, MgCl2 6 mM, EDTA 10 mM supplemented with 10% sucrose and protease inhibitors (PMSF, Leupeptine, Aprotinine, E-64). The homogenate was additionally ground using a polytron (IKA T25 Basic, 24.000 rpm) with a small rotor (7.5 mm diameter), about ten times for 3-4 seconds in ice. The solid material was removed by filtering through four layers of Miracloth (22-24 micrometer) (Calbiochem) to remove unruptured tissue and cells.
Transfected cells were recovered from culture plates by scraping and they were suspended in the homogenization B medium (10 mM Tris-HCl pH 8.0, 0.9% NaCl, 5 mM EDTA with protease inhibitors). The suspension was taken into a 5 ml syringe fitted with a 23 gauge needle and it was expelled approximately 30 times. Cell rupture was monitored by a phase contrast microscope. The homogenate was loaded on a step sucrose gradient and centrifuged as described for tobacco leaf homogenates.
The accumulation of fusion proteins in the transiently transfected cells was analyzed by Western blot, using the gamma-zein antibodies raised against the gamma-zein. After 48 hours of transfection, total soluble cell proteins were extracted with buffer A (100 mM Tris-HCl pH 8.0, 150 mM NaCl, 5 mM EDTA, 0.5% SDS, 0.5% Triton X-100, 2% 2-mercaptoethanol and protease inhibitors). Aliquots of the cell incubation media were precipitated and stored at −20° C. Proteins extracted from equivalent amounts of transfected cells and media were separated by SDS polyacrylamide gel electrophoresis and transferred to nitrocellulose sheets for immunodetection.
A homogenate was centrifuged at 50×g for 5 minutes at 4° C. to obtain a clarified extract. For discontinuous sucrose gradient, this clarified homogenate (supernatant) was layered onto a step gradient composed of 2.5 ml of 20%-30%-42%-56% (w/w) or 42%-49%-56%-65% (w/w) sucrose in buffer PBP and it was centrifuged (Beckman Coulter Optima™ XL-100K ultracentrifuge) at 80,000×g in a swinging-bucket rotor (SW41Ti) at 4° C. for 120 minutes without brake.
The supernatant, interphases and pellet fractions were collected. Equivalent aliquots of supernatant, interphase fractions and pellet were precipitated with 15% TCA and analyzed by SDS-PAGE and immunoblot by using specific antibodies against the fusion expressed proteins. Proteins RX3-EGF, RX3-T20, RX3-Ct, and RX3-INF were detected by Western blot using the gamma-zein antibody. The electrophoretic gels were analyzed by silver staining according to Morrissey et al., 1981 Anal. Biochem. 117:307-310 to evaluate the enrichment of recombinant protein vs. contaminant proteins inside PBs.
Homogenate prepared as described above, was centrifuged onto a 42% (w/w) sucrose cushion of 8 ml (1.18 g/cm3) for 120 minutes at 24,000 g at 4° C. Supernatant, interface and pellet fractions were recovered. RPBLAs sediment at the bottom of the cushion. For protein analysis, equivalent aliquots of these fractions were precipitated in 15% TCA and samples were separated on 15% SDS-PAGE and analyzed by silver staining. Recombinant proteins present in PB fraction were detected by immunoblot using gamma-zein antibody.
RPBLAs isolated from the 42%-56% (w/w) interphase of step sucrose gradients or isolated by density cushion of 42% (w/w) of sucrose were washed in PBP Buffer and recovered by a brief centrifugation for 5 minutes at 16000×g. Recombinant proteins accumulated inside RPBLAs were solubilized in one volume of SB buffer containing sodium borate 12.5 mM pH 8, 0.1% SDS and 2% 2-mercaptoethanol. The solution was incubated overnight (about 18 hours) at 37° C. One aliquot was centrifuged 10 minutes at 16000×g at room temperature and the supernatant and pellet were analyzed by SDS-PAGE and Western blot to evaluate complete protein solubilization.
S. cerevisiae expressing recombinant fusion proteins were pelleted. Aliquots of the respective incubation media were precipitated and stored at −20° C. to be analyzed. The cell pellets were also frozen and after thawing, the cells were broken by standard methods using glass beads and medium Y (50 mM HCl-Tris pH 8.0, 150 mM NaCl, 5 mM EDTA, 200 mM DTT and protease inhibitors). Equivalent amounts of both, cells and media, were analyzed by SDS-PAGE and immunoblot by using specific antibodies against the recombinant expressed proteins.
The disruption method applied to isolate organelles from transformed yeast cells was based on the gentle lysis of spheroplasts described in Zinser et al., 1995 Yeast, 11:493-536. Thirty mL of cultured transformed yeast cells (DO600 around 0.5) were pelleted, washed with 1 M sorbitol and suspended in 1 mL of spheroplasting buffer (1 M sorbitol, 50 mM potassium phosphate pH 7.5, 14 mM 2-mercaptoethanol) containing 100 units/ml of zymolase. Spheroplast formation was allowed to proceed for 20-30 minutes at 30° C. with occasional gentle agitation. After sedimentation at 1000 g for 6 minutes, the spheroplasts were washed with spheroplasting buffer without 2-mercaptoethanol, and resuspended in 0.5 mL of ice-cold lysis buffer (0.3 M sorbitol, 10 mM triethanolamine, 1 mM EDTA and protease inhibitors). After 20 minutes on ice with occasional gentle agitation, lysates were adjusted to a final concentration of 1.0 M sorbitol. The lysates were loaded on a step sucrose gradient and centrifuged as described for tobacco leaf homogenates. Fractions were analyzed by SDS-PAGE and immunoblot.
Results
The genes coding for RX3-EGF and RX3-T20 gamma-zein derived fusion proteins were introduced in tobacco plants via Agrobacterium tumefaciens. Transformed plants were analyzed by immunoblot to determine those plants with higher recombinant protein expression.
Tobacco leaf extracts were loaded on density step gradients and the accumulation of recombinant proteins in the different fractions was analyzed by immunoblot (
These novel RPBLAs formed in tobacco leaves exhibit densities in the range of the natural maize protein bodies (Ludevid et al., 1984 Plant Mol. Biol. 3:227-234; Lending et al., 1989 Plant Cell 1:1011-1023), or are even more dense. It should be noted that RX3-T20 PBs are rather (slightly) less dense than RX3-EGF PBs, this is attributed to some specific characteristics of the protein of interest. Therefore, although RPBLAs accumulating recombinant fusion proteins have high densities relative to usually present soluble cell portions, the features of the protein fused to RX3 domain can determine variations in such density.
It was estimated that more than 90 percent of both recombinant proteins were recovered in the dense RPBLAs fractions and pellet (see
To evaluate the purification of the recombinant protein RX3-EGF by RPBLAs isolation, the different density fractions were analyzed by silver stain (
In respect to the degree of fusion proteins purification in the RPBLAs fractions (F56 and F65), it was estimated that RX3-EGF protein represents approximately 80 percent of the proteins detected in the PBLS-containing fractions. This result indicates that, using a RPBLAs isolation procedure, one can achieve an important enrichment of fusion proteins in only one step of purification.
An important point in molecular farming is the presence of an easy means to store plant biomass. In this context, drying can provide a convenient method to lessen storage volume and preserve the product. Nevertheless, drying frequently promotes the degradation of the proteins of interest. The use of desiccated plants to isolate RPBLAs containing recombinant proteins would be of great interest for industrial purposes.
Transformed tobacco leaves accumulating RX3-EGF and RX3-T20 fusion proteins as described above were dried as also discussed above. After 5 months of dry storage, the stability of recombinant proteins was analyzed. Protein extracts from equivalent amounts of wet (fresh) (W, lane 1) and dry (D, lane 2) leaf tissue were analyzed by immunoblot (
The distribution in step density gradients of fusion proteins (RX3EGF and RX3T20) from homogenates of dried leaves was analyzed by immunoblot (
Thus, recombinant proteins can be purified from dried tissues via isolation of RPBLAs thereby illustrating that transgenic plant collection and recombinant protein extraction and purification can be independent in time. In keeping with these results, gamma-zein fusion proteins were also accumulated in RPBLAs in rice seeds.
The transient expression systems can be a convenient tool to test the accumulation behavior of recombinant proteins in a short period of time. Thus, the recombinant proteins RX3-EGF and RX3-T20 were also expressed and accumulated in transiently transformed tobacco plantlets via agroinfiltration. The protein extracts from transformed plantlets analyzed by immunoblot (
The expression of a higher molecular weight fusion protein, RX3-hGH was also analyzed in transiently transformed tobacco (
To simplify the procedure used to purify recombinant proteins via dense recombinant protein body-like assemblies, two additional alternative methods were performed: i) clarified homogenates were centrifuged through only one dense sucrose cushion (
In agreement with the previously described results, both RX3-EGF and RX3-T20 proteins were recovered in high yields (more than 90%) in the pellets obtained after centrifugation through 1.1868 g/cm3 sucrose cushions (
The principal advantage of this method as compared to step density gradients lies in its easy scalability for industrial production of recombinant proteins. It should be noted that the cushion density as well other properties such as its viscosity and osmolarity can be adjusted in each case in order to optimize recovery and purification of the recombinant proteins.
In addition, low speed centrifugation (LSC) was also assayed to concentrate and purify fusion protein-containing protein body-like structures (
Thereafter, the first pellet obtained by low speed centrifugation was washed by using a buffer containing 5% Triton X-100. After washing, the sample was centrifuged at 12,000×g for 5 minutes and, interestingly, the bulk of contaminating proteins present in the P1 pellet were eliminated after washing and centrifugation and the new pellet (P2,
Studies were undertaken to determine whether storage protein-derived fusion proteins also induced the formation of dense recombinant PB-like assemblies in transfected animal cells. The sub-cellular distribution of organelles from homogenized transfected mammal cells was analyzed by using step density gradients. Three different cell culture types, 293T (from human), Cos1 (from monkey) and CHO (from hamster), were transfected by using the cDNA coding for three different fusion proteins, RX3-Ct, RX3-EGF and RX3-hGH. Cos1 cells transfected with pECFP-N1 (Clontech) were used as control. The gradient fractions were collected as described previously and analyzed by immunoblot (
The recombinant RX3-derived proteins expressed in transfected cells, were detected by using the gamma-zein antiserum. Detection of the control ECGP in the different collected fractions was made by using an anti-GFP antiserum raised in rabbits.
As expected, the soluble ECGP protein was recovered in the supernatant fraction (S,
A significant amount of recombinant protein was recovered in the soluble fraction of the gradients from RX3-Ct- and RX3-hGH-transfected cells. This was probably due to an excess of cell rupture during homogenization that permitted the solubilization of the yet unassembled fusion proteins contained in the ER.
Low speed centrifugation (LSC,
The formation of dense structures containing fusion proteins in transformed yeast was also analyzed by step density gradients. The cDNAs coding for RX3-EGF and RX3-hGH fusion proteins were introduced via yeast transformation vectors in Sacharomyces cerevisiae using standard procedures. The disruption method applied to isolate organelles was based in the gentle lysis of spheroplasts as described in methods. The lysates were loaded on a step sucrose gradient and centrifuged as described for mammal cells or tobacco leaf homogenates. Fractions were analyzed by SDS-PAGE and immunoblot.
The fractionation results are shown in
The purification of gamma-zein derived fusion proteins through their density properties is extensible to fusion proteins derived from other storage proteins. Here we show how fusion proteins derived from the rice prolamin of 13 kD (rP13) and from 22 kD alfa-zein (22aZt) accumulated in dense fractions corresponding to RPBLAs on step sucrose gradients (
The calcitonin sequence was fused in frame with rP13 and 22aZt sequences under the CaMV35S promoter and introduced in Agrobacterium tumefaciens. Tobacco plantlets were transiently transformed and leaf homogenates were submitted to step density gradients. The collected fractions were analyzed by SDS-PAGE and immunoblot using an anti-calcitonin antiserum raised in rabbit.
As can be seen in
Agroinfiltration studies were performed with the hGH fused to the rice prolamin (rP13-hGH) and the full length alpha zein (22aZ-hGH). Once again, the majority of the RPBLAs were observed in the F42 and F56 fractions 7A (lane 4-5 and lanes 9-10), but interestingly this time some non-assembled fusion protein was also detected in the supernatant and in the low density interfaces (lanes 1-2 and 6-7). This effect could be explained by a partial solubilizing effect of the hGH to the rP13 and the 22aZ.
Transgenic tobacco plants expressing the fusion proteins rP13-EGF and the 22aZ-EGF were produced by Agrobacterium tumefasciens transformation. The best expressers where determined by immunoblot using an antibody against the EGF, and those cell lines were used in a comparative analysis with tobacco plantlets agroinfiltrated with the same constructs. As can be seen in
Taking all these results together, it is clear that prolamins are able to induce high density RPBLAs, even when they are fused to other proteins. That is an unexpected result, mainly when almost no homology is observed between them. Moreover, there are some data suggesting that the prolamins interact to stabilize the protein bodies, and that some of them are not stable when expressed in vegetative tissue alone, as for instance alpha-zein (Coleman et al., 1996 Plant Cell 8:2335-2345)
It has been demonstrated that the isolation of dense recombinant PB-like assemblies is an advantageous method to recover recombinant proteins with high yield and high purification level from transgenic organisms. Here it is shown that these recombinant proteins can be extracted from the storage organelles. Although the recombinant PBLAs can be directly used for some applications (i.e. oral vaccine preparation), in some other cases, the disposal of purified recombinant proteins could be necessary.
After an overnight (about 18 hours) incubation of PB fractions at 37° C. in a buffer containing reducing agents (SB buffer that contained sodium borate 12.5 mM pH 8, 0.1% SDS and 2% 2-mercaptoethanol; treatment), RX3-EGF and RX3-T20 proteins were solubilized. As can be seen in
Each of the patents and articles cited herein is incorporated by reference. The use of the article “a” or “an” is intended to include one or more.
The foregoing description and the examples are intended as illustrative and are not to be taken as limiting. Still other variations within the spirit and scope of this invention are possible and will readily present themselves to those skilled in the art.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0426161.6 | Nov 2004 | GB | national |
