My laboratory is interested in the molecular basis of cellular interactions during mammalian fertilization and development. Our previous studies have identified a novel sperm surface receptor. galactosyltransferase(GalTase), that facilitates gamete recognition by binding to its specific oligosaccharide ligand on the extracellular coat of the egg. Although GalTase is traditionally viewed as an intracellular biosynthetic enzyme, we now know that an isozymic variant of GalTase is expressed on the cell surface due, in part, to a unique cytoplasmic sequence that overrides the Golgi retention- signal, transporting GalTase to the cell surface. The purpose of this renewal application is to continue our analysis of GalTase function during fertilization in the mouse. Thus far, HD 23479 has supported the work reported in 14 peer-reviewed publications, the results of which can be summarized as follows. GalTase is first expressed on the germ cell surface during early spermatogenesis, where it may function during germ cell adhesion to Sertoli cells. In the epididymis, soluble glycosides bind to the sperm surface, masking GalTase. These competitive glycosides are shed from the sperm surface during capacitation. thus exposing the GalTase binding site for interaction with its ligand on the egg coat glycoprotein, ZP3. Aggregation of GalTase by multivalent ZP3 oligosaccharides activates a heterotrimeric 0-protein cascade, leading to the acrosome reaction. The acrosome releases hydrolytic enzymes. including N-acetylglucosaminidase. that facilitates sperm penetration through the zona. After egg activation, the global release of N-acetylglucosaminidase from the cortical granules destroys the GalTase recognition site on ZP3, thus leading to the block in polyspermic binding. In this renewal application, we wish to address GalTase function in light of other sperm proteins with which it may associate. We intend to: I) examine the effect of ZP3 binding on the phosphorylation of the GalTase cytoplasmic domain, 2) identify the kinase responsible for phosphorylating sperm GalTase, 3) examine the effects of phosphorylation on 0-protein activation and identify amino acid residues within the cytoplasmic domain responsible for G-protein activation, 4) identify the repertoire of sperm proteins that constitute the signal transduction complex in association with the GalTase cytoplasmic domain, 5) determine whether the cytoplasmic domain of sperm GalTase defines a novel class of testis-specific receptors, and 6) examine whether the extracellular domain of sperm GalTase complexes with other membrane proteins that may also contribute to ZP3 binding.
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