Peptides corresponding to the epidermal growth factor-like domain of mousefertilin: Synthesis and biological activity

A key step leading to fertilization is the binding of sperm to the egg plas ma membrane. When a mammalian sperm reaches the egg plasma membrane, fertil in, and extracellular sperm membrane protein, is believed to bind to an egg plasma membrane receptor mediating fusion. Fertilin is composed of two sub units, and each subunit contains several domains, i.e., metalloprotease, di sintegrin, epidermal growth factor (EGF)-like and fusion domains. This inve stigation examined the role of the EGF-like domains of mouse fertilin alpha and fertilin beta. Peptides corresponding to the N-terminal subdomain, con taining four cysteines, and the C-terminal subdomain, containing two cystei nes, were synthesized by solid-phase synthesis methods. Disulfide bonds wer e formed regioselectively according to the canonical EGF-like disulfide pat tern. The activity of these peptides and their linear counterparts were tes ted for activity in a mouse in vitro fertilization assay. One peptide, 4a, corresponding to the cystine-constrained N-terminal subdomain of fertilin b eta, had an activating effect on fertilization. The fertilization rate (num ber of eggs fertilized), fertilization index (number of sperm fused per egg ), and level of polyspermy (three or more sperm fused per egg) increased in the presence of 500 mu M 4a (12, 56, and 190%, respectively). Its linear c ounterpart, 4b, had no effect on in vitro fertilization. These data suggest that the EGF-like domain of fertilin beta has a function in sperm-egg bind ing and fusion. Previously, it has been shown that the fertilin beta disint egrin domain has a role in sperm-egg binding. Considered together, these st udies suggest that fertilin is a modular, multidomain protein with more tha n one mechanism of action. This modularity may be used to design inhibitors of fertilin-receptor interactions that have high specificities for the fer tilization process. (C) 1998 John Wiley & Sons. Inc.