The union of sperm and egg is a special membrane fusion event that gives a signal to begin development. We have hypothesized that proteins mediating cell-cell fusion events resemble viral fusion proteins and have shown that PH-30, a sperm surface protein involved in sperm-egg fusion, shares biochemical characteristics with viral fusion proteins. We report here the complementary DNA and deduced amino-acid sequences of the mature alpha and beta subunits of PH-30. Both are type-I integral membrane glycoproteins. The alpha subunit contains a putative fusion peptide typical of viral fusion proteins and the beta subunit contains a domain related to a family of soluble integrin ligands found in snake venoms. Thus, the PH-30 alpha/beta complex resembles many viral fusion proteins in both its membrane topology and its predicted binding and fusion functions.

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Binding between sperm and egg plasma membranes is an essential step in fertilization. Whereas fertilin, a mammalian sperm surface protein, is involved in this crucial interaction, sperm receptors on the egg plasma membrane have not been identified. Because fertilin contains a predicted integrin ligand domain, we investigated the expression and function of integrin subunits in unfertilized mouse eggs. Polymerase chain reactions detected mRNAs for alpha 5, alpha 6, alpha v, beta 1, beta 3, and beta 5. Immunofluorescence revealed alpha 6 beta 1 and alpha v beta 3 on the plasma membrane. GoH3, a function-blocking anti-alpha 6 monoclonal antibody, abolished sperm binding, but a nonfunction-blocking anti-alpha 6 monoclonal antibody, a function-blocking anti-alpha v beta 3 polyclonal antibody, and an RGD peptide had no effect. Somatic cells bound sperm avidly, but only if they expressed alpha 6 beta 1. A peptide analog of the fertilin integrin ligand domain inhibited sperm binding to eggs and alpha 6 beta 1+ cells and diminished GoH3 staining of eggs. Our results indicate a novel role for the integrin alpha 6 beta 1 as a cell-cell adhesion receptor that mediates sperm-egg binding.

ELL--cell and cell-matrix adhesion, as well as proteolysis of the extracellular matrix, are vital for normal processes such as tissue morphogenesis and wound healing, as well as for pathologies such as tumor cell invasion and metastasis. A variety of cell surface adhesion proteins and proteases are important players in these events. Families of membrane-anchored cell surface adhesion molecules include cadherins, immunoglobulin superfamily members, selectins, integrins, and syndecans (7). Membrane-anchored cell surface proteases include membrane-type matrix metalloprotease (4) and meprin (9). In this review we describe the ADAMs, a recently discovered gene family encoding membrane proteins with A Disintegrin And Metalloprotease domain. ADAMs 1 are unique among cell surface proteins in possessing both a potential adhesion domain as well as a potential protease domain. ADAM cDNAs have been found in a wide array of mammalian tissues as well as in lower eukaryotes (Tables I and II). The eleven full-length members described to date encode proteins of 750-800 amino acids which contain pro-, metalloprotease-like, disintegrin-like, cysteinerich, EGF-like, transmembrane, and cytoplasmic domains (Table I , legend). Although these domains are not similar in sequence to those of other membrane-anchored adhesion molecules or proteases, they are related to domains found in a family of soluble snake venom proteins, the snake venom metalloproteases (SVMPs) (5). SVMPs are encoded by three cDNA classes. N-I encodes pro and metalloprotease domains, N-II encodes pro, metalloprotease, and disintegrin domains, and N-III encodes pro, metalloprotease, disintegrin-like, and cysteine-rich domains. We will refer to the primary translation products as P-I, P-II, and P-III, respectively. In a snake bite victim, snake venom metalloproteases and disintegrins promote hemorrhage.

Fertilin alpha and beta, previously known as PH-30 alpha and beta, are two subunits of a guinea pig sperm integral membrane protein implicated in sperm-egg binding and fusion. They are derived from sequence-similar precursors which contain a metalloprotease-like and a disintegrin-like domain and which are related to a family of metalloprotease and disintegrin domain-containing snake venom proteins. We report here the cloning, sequencing, and characterization of mouse fertilin alpha and beta as well as five additional sequence-similar cDNAs from guinea pig and mouse testis. We name this gene family ADAM, for proteins containing A Disintegrin And Metalloprotease domain, and in honor of its dual origins in the fields of snakes and fertility. In situ hybridization demonstrated that, in testis, RNA encoding these ADAMs is expressed only in spermatogenic cells and that this expression is developmentally regulated. PCR analysis of mouse tissue cDNA showed that these ADAMs display different patterns of tissue distribution. Some ADAMs (e.g., fertilin alpha) have the consensus active-site sequence for a zinc-dependent metalloprotease in their metalloprotease-like domain. All have a disintegrin-like domain, which could bind integrins or other receptors. Some have sequences which may be active in membrane fusion. All encode potential membrane-spanning domains. Searches of sequence databases revealed that additional mammalian members of the ADAM gene family have been cloned from a variety of tissues. Thus, the ADAMs are a large, widely expressed, and developmentally regulated family of proteins with multiple potential functions in cell-cell and cell-matrix interactions.