suREJ3, a polycystin-1 protein, is cleaved at the GPS domain and localizes to the acrosomal region of sea urchin sperm

The sea urchin sperm acrosome reaction (AR) is a prerequisite for sperm-egg fusion. This report identifies sea urchin sperm receptor for egg jelly-3 (suREJ3) as a new member of the polycystin-1 family (the protein mutated in autosomal dominant polycystic kidney disease). suREJ3 is a multidomain, 2,681-amino acid, heavily glycosylated orphan receptor with 11 putative transmembrane segments (TMS) that localize to the plasma membrane covering the sperm acrosomal vesicle. Like the latrophilins and other members of the secretin family of G-protein-coupled receptors, suREJ3 is cleaved at the consensus GPS (G-protein-coupled receptor proteolytic site) domain. Antibodies to the extracellular 1,455-residue NH2-terminal portion identify a band at 250 kDa that shifts in electrophoretic mobility to 180 kDa upon glycosidase digestion. Antibodies to the 1,226-residue COOH-terminal portion identify a band at 150 kDa that shifts to 140 kDa after glycosidase treatment. Antibodies to both portions of suREJ3 localize exclusively to the plasma membrane over the acrosomal vesicle. Immunoprecipitation shows that both portions of suREJ3 are associated in detergent extracts. This is the first report showing that a polycystin family member is cleaved at the GPS domain. Localization of suREJ3 to the acrosomal region provides the first suggestion for the role of a polycystin-1 protein (components of nonselective cation channels) in a specific cellular process.