An Mr 63-kD sea urchin sperm flagellar membrane protein has been previously 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 frame codes for a protein of 470 amino acids which contains a putative signal sequence of 25 residues. Western blots using antibodies to two synthetic 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 vesicles by treatment with phosphatidylinositol-specific phospholipase C, showing that it is anchored to the flagellar membrane by glycosylphosphatidyl inositol (GPI). Although we cannot demonstrate involvement of the 63-kD protein in the AR, it is of potential interest because it shares 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.

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