Roles of the Disintegrin Domains of Mouse Fertilins (alpha) and (beta) in Fertilization

Fertilin is a heterodimer of (alpha) and (beta) subunits, both of which are members of the ADAM (A Disintegrin and A Metalloprotease domain)/MDC (Metalloprotease-Disintegrin-Cysteine-rich) family of proteins. We have previously demonstrated that recombinant forms of the putative extracellular domains of mouse fertilin (alpha) and fertilin (beta) bind to mouse eggs and inhibit sperm-egg membrane binding. In this study, we examined the roles of the disintegrin domains of fertilins (alpha) and (beta) by producing recombinant forms of fertilins (alpha) and (beta) that included the disintegrin domains ((alpha)DCE and (beta)DCE) or that were truncated so that they lack the disintegrin domains ((alpha)CE and (beta)CE) and tested the abilities of these proteins to bind to eggs and to inhibit sperm-egg binding. Fertilin (beta)DCE was able to inhibit sperm-egg binding, but fertilin (beta)CE was relatively ineffective, indicating that the disintegrin domain of fertilin (beta) is required for interactions with egg binding sites and/or for proper protein folding. Fertilins (alpha) DCE and (alpha)CE both inhibited sperm-egg interactions, but fertilin (alpha)DCE tended to be more effective. Thus, the presence of the disintegrin domain in fertilin (alpha)DCE apparently enhanced the ability of this recombinant protein to inhibit sperm-egg binding, either by interacting with egg binding sites or by improving the efficiency of protein folding. These data also indicate that the other domains of the fertilin (alpha) extracellular region (cysteine-rich and/or epidermal growth factor-like repeat) have the ability to block sperm binding and suggest that these domains of fertilin (alpha) may participate in sperm-egg adhesion.