Project Summary: Müllerian Inhibiting Substance (MIS) or Anti- Müllerian Hormone (AMH), originally identified for its role in male sex differentiation during development, has emerged as a significant molecule in female reproduction. For example, MIS plays an important role in regulating follicle development and serum levels and is now used as a measure of ovarian reserve. Mutations in MIS are associated with both male and female reproductive disorders, including Persistent Müllerian Duct Syndrome (PMDS) in males and Polycystic Ovary Syndrome (PCOS) in females. As a member of the TGF? family, MIS signals through a type I and type II receptor. Uniquely, MIS signals through its own type II receptor MISRII and the type I receptor Alk2, which is utilized by multiple ligands. While previous studies have detailed TGF? family ligand interactions, how MIS interacts, at the molecular level, with MISRII and Alk2 is unknown. The objective of this proposal is to understand how MIS interacts with its cognate receptors and how specific MIS signaling is generated through these receptors. Here, we will combine both structural and functional approaches, including X-ray crystallography, cell-based signaling assays, structure-based engineering, and biological models of fertility and ovarian protection. We will pursue the following three specific aims (1) determine the crystal structure of MIS with MISRII and Alk2, (2) characterize the MIS receptors and determine how the generate MIS-specific signaling, using both the extracellular and intercellular domains and (3) generate MIS analogs that will be tested in both in vitro binding and cell-based assays, along with in vivo assay including an AAV9 model of follicle suppression. Collectively, this proposal will uncover how MIS interacts with its receptors providing a platform for developing reagents that modify MIS activity with the potential for future application in reproductive therapies.