Fertilin ␣ (also known as ADAM1) is a member of the ADAM (A disintegrin and A metalloprotease domain) family of proteins. In this study, we examine the mechanism of mouse fertilin ␣'s in adhesion of sperm to the egg plasma membrane during fertilization. We find that recombinant forms of fertilin ␣ corresponding to either the disintegrin-like domain or the cysteine-rich domain and the EGF-like repeat can perturb sperm-egg binding, suggesting that both of these domains can participate in fertilin ␣-mediated adhesion events. In further examination of the fertilin ␣ disintegrin-like domain, we find that a subdomain of disintegrin-like domain with the sequence DLEECDCG outside the putative disintegrin loop but with homology to the fertilin ␤ disintegrin loop can inhibit the binding of both sperm and recombinant fertilin ␣ to eggs, suggesting that this is an adhesionmediating motif of the fertilin ␣ disintegrin-like domain. This sequence also inhibits the binding of recombinant fertilin ␤ to eggs and thus is the first peptide sequence found to block two different sperm ligands. Finally, a monoclonal antibody to the tetraspanin protein CD9, KMC.8, inhibited the binding of recombinant fertilin ␣ to eggs in one type of binding assay, suggesting that, under certain conditions, fertilin ␣ may interact with a KMC.8-sensitive binding site on the egg plasma membrane. ADAM1 (for A disintegrin and A metalloprotease) proteins comprise a recently identified and rapidly growing molecular family of membrane proteins. To date, ϳ30 ADAMs have been identified in a variety of animal species, including several mammals, Xenopus laevis, Drosophila melanogaster, and Caenorhabditis elegans (1, 2). The conserved domain structure of ADAMs (see Fig. 1A) and functional analyses of these proteins indicate that members of this family have functions as proteases and/or as cell adhesion molecules. In the ADAMs that have been described to function as cell adhesion molecules, the adhesive activity generally appears to be attributable to the disintegrin-like domain, a domain with homology to integrin ligand-like snake venom polypeptides. These venom polypeptides, known as disintegrins, contain an RGD tripeptide, presented at the end of an extended loop structure called the "disintegrin loop" (3). Although ADAMs share significant sequence homology in their disintegrin-like domains to snake venom disintegrins, they do not have RGD sequences in their putative disintegrin loops (with the exception of human ADAM15). Various studies have shown that the adhesion-mediating sequence of several ADAMs appears to be a short sequence within the putative disintegrin loop, such as the ECDcontaining sequences in fertilin ␤ (4, 5), ADAM9 (6), and ADAM23 (7), or the QCD-containing sequence in cyritestin (8).Among the cell adhesion events that are mediated by ADAMs are the interactions between mammalian gamete plasma membranes during fertilization. On mouse sperm, there are at least three ADAMs that appear to participate in sperm-egg binding: fertilin ␣ (ADAM1), fertili...