Xenopus laevis sperm receptor gp69/64 glycoprotein is a homolog of the mammalian sperm receptor ZP2

Little is known about sperm-binding proteins in the egg envelope of nonmamm alian vertebrate species. We report here the molecular cloning and characte rization of a recently identified sperm receptor (gp69/64) in the Xenopus l aevis egg vitelline envelope. Our data indicate that the gp69 and gp64 glyc oproteins are two glycoforms of the receptor and have the same number of N- linked oligosaccharide chains but differ in the extent of O-glycosylation, The amino acid sequence of the receptor is closely related to that of the m ouse zona pellucida protein ZP2, Most of the sequence conservation, includi ng a ZP domain, a potential furin cleavage site, and a putative transmembra ne domain are located in the C-terminal half of the receptor. Proteolytic c leavage of the gp69/64 protein by a cortical granule protease during fertil ization removes 27 amino acid residues from the N terminus of gp69/64 and r esults in loss of sperm binding to the activated eggs, Similarly, we find t hat treatment of eggs with type I collagenase removes 31 residues from the N terminus of gp69/64 and has the same effect on sperm binding, The isolate d and purified N terminus-truncated receptor protein is inactive as an inhi bitor of sperm-egg binding. Earlier studies on the effect of Pronase digest ion on receptor activity suggest that this N-terminal peptide may contain a n O-linked glycan that is involved in the binding process. Based on these r esults and the findings on the primary structure of the receptor, a pathway for the maturation and secretion of gp69/64, as well as its inactivation f ollowing fertilization, is proposed.