A GPI-ANCHORED SEA-URCHIN SPERM MEMBRANE-PROTEIN CONTAINING EGF DOMAINS IS RELATED TO HUMAN UROMODULIN

An M(r) 63-kD sea urchin sperm flagellar membrane protein has been pre viously implicated as a possible receptor for egg jelly ligand(s) that trigger the sperm acrosome reaction (AR). The cDNA and deduced amino acid sequences of the 63-kD protein are presented. The open reading fr ame codes for a protein of 470 amino acids which contains a putative s ignal sequence of 25 residues. Western blots using antibodies to two s ynthetic peptides confirm the sequence to be that of the 63-kD protein . The mRNA is approximately 2,300 bases in length and the gene appears to be single copy. The protein is released from sperm membrane vesicl es by treatment with phosphatidylinositol-specific phospholipase C, sh owing that it is anchored to the flagellar membrane by glycosylphospha tidyl inositol (GPI). Although we cannot demonstrate involvement of th e 63-kD protein in the AR, it is of potential interest because it shar es significant similarity with the developmentally expressed proteins crumbs, notch and xotch as well as human uromodulin over a region that includes two separate EGF repeats.