Specificity of binding of zona pellucida glycoproteins to sperm proacrosin and related proteins.

A major regulatory site for species specificity of fertilization in mammals lies at the level of sperm binding to the zona pellucida. This implies a high degree of complementarity between gamete receptor molecules. One putative receptor molecule on spermatozoa is proacrosin/acrosin which binds to zona pellucida glycoproteins (ZPGPs) non-enzymically and with high affinity. The mechanism of recognition involves polysulphate groups in a particular stereochemical orientation and it has been suggested that this may contribute to species specificity of sperm-egg binding. In the present work this hypothesis has been tested by challenging 125I-labelled ZPGPs from pig, cow, and hamster eggs to recognize heterologous sperm proacrosins as well as a variety of sequence-related and unrelated proteins. Results show that pig and cow 125I-ZPGPs bind readily to boar, ram, and bull proacrosin but do not recognize guinea-pig proacrosin or any of the polysulphate binding proteins from rat, hamster, or mouse spermatozoa. Hamster 125I-ZPGPs also recognise boar, ram, and bull proacrosin as well as a range of unidentified proteins in pH3 extracts of hamster, rat, and mouse spermatozoa. The binding of ZPGPs to a variety of proteins not related to proacrosin is strongest to those with a high content of basic residues (i.e., pI > 8.5), although the secondary folding of the target protein is of major importance as boar proacrosin (pI 6.75) has the highest binding capacity of all proteins tested. Cross-reaction of ZPGP probes was not observed to guinea-pig proacrosin, suggesting that in this species the conformation of the protein is different to other sperm proacrosins.(ABSTRACT TRUNCATED AT 250 WORDS)