Characterization of fertilin beta-disintegrin binding specificity in sperm-egg adhesion.

An essential step leading to fertilization is the binding of a sperm to the egg plasma membrane. Fertilin beta, a membrane bound protein on the extracellular surface of sperm, partially mediates this binding via the alpha6beta1 integrin. Fertilin beta is a member of the still expanding family of ADAM proteins (a disintegrin and metalloprotease) that are implicated in many cellular functions ranging from neurogenesis to myoblast fusion and cytokine processing. Fertilin beta contains a highly conserved motif (D/E)ECD in the disintegrin domain. This suggests that (D/E)ECD could be the consensus sequence for recognition of disintegrins by alpha6beta1 integrins. Previously, it has been demonstrated that small peptides containing different moieties of this consensus sequence are inhibitors of in vitro fertilization. In the present study, we sought to determine whether a four amino acid peptide sequence with two adjacent acidic residues improved inhibition, and investigated the importance for inhibition of a cysteine versus a cystine. A series of linear and cyclic peptides were synthesized, in which either one or both adjacent acidic residues in the sequence DECD were mutated to their corresponding amides (N or Q). To explore the required oxidation state of the cysteine in the (D/E)ECD sequence, it was protected as a mixed disulfide. Our results indicate that only one acidic residue is required for inhibition of fertilization and a reduced C is required.