Roles of the Disintegrin Domains of Mouse Fertilins α and β in Fertilization1

Abstract

Fertilin is a heterodimer of α and β 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 α and fertilin β bind to mouse eggs and inhibit sperm-egg membrane binding. In this study, we examined the roles of the disintegrin domains of fertilins α and β by producing recombinant forms of fertilins α and β that included the disintegrin domains (αDCE and βDCE) or that were truncated so that they lack the disintegrin domains (αCE and βCE) and tested the abilities of these proteins to bind to eggs and to inhibit sperm-egg binding. Fertilin βDCE was able to inhibit sperm-egg binding, but fertilin βCE was relatively ineffective, indicating that the disintegrin domain of fertilin β is required for interactions with egg binding sites and/or for proper protein folding. Fertilins αDCE and αCE both inhibited sperm-egg interactions, but fertilin αDCE tended to be more effective. Thus, the presence of the disintegrin domain in fertilin αDCE 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 α 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 α may participate in sperm-egg adhesion.