A ROLE FOR THE DISINTEGRIN DOMAIN OF CYRITESTIN, A SPERM SURFACE PROTEIN BELONGING TO THE ADAM FAMILY, IN MOUSE SPERM-EGG PLASMA-MEMBRANE ADHESION AND FUSION

Sperm-egg plasma membrane fusion is preceded by sperm adhesion to the egg plasma membrane, Cell-cell adhesion frequently involves multiple a dhesion molecules on the adhering cells, One sperm surface protein wit h a role in sperm-egg plasma membrane adhesion is fertilin, a transmem brane heterodimer (alpha and beta subunits), Fertilin alpha and beta a re the first identified members of a new family of membrane proteins t hat each has the following domains: pro-, metalloprotease, disintegrin , cysteine-rich, EGF-like, transmembrane, and cytoplasmic domain. This protein family has been named ADAM because all members contain a disi ntegrin and metalloprotease domain, Previous studies indicate that the disintegrin domain of fertilin beta functions in sperm-egg adhesion l eading to fusion, Full length cDNA clones have been isolated for five ADAMs expressed in mouse testis: fertilin alpha, fertilin beta, cyrite stin, ADAM 4, and ADAM 5, The presence of the disintegrin domain, a kn own integrin ligand, suggests that like fertilin beta, other testis AD AMs could be involved in sperm adhesion to the egg membrane. We tested peptide mimetics from the predicted binding sites in the disintegrin domains of the five testis-expressed ADAMs in a sperm-egg plasma membr ane adhesion and fusion assay. The active site peptide from cyritestin strongly inhibited (80-90%) sperm adhesion and fusion and was a more potent inhibitor than the fertilin beta active site peptide, Antibodie s generated against the active site region of either cyritestin or fer tilin beta also strongly inhibited (80-90%) both sperm-egg adhesion an d fusion. Characterization of these two ADAM family members showed tha t they are both processed during sperm maturation and present on matur e sperm. Indirect immunofluorescence on live, acrosome-reacted sperm u sing antibodies against either cyritestin or fertilin beta showed stai ning of the equatorial region, a region of the sperm membrane that par ticipates in the early steps of membrane fusion. Collectively, these d ata indicate that a second ADAM family member, cyritestin, functions w ith fertilin beta in sperm-egg plasma membrane adhesion leading to fus ion.