Isolation, characterization, and primary structure of three major proteins obtained from Mytilus edulis sperm

Acrosomal proteins from Mytilus edulis sperms were separated into 11 fractions by reverse phase HPLC. The three major proteins, named M3, M6, and M7, showed strong egg vitelline coat lysin and first polar body releasing activities. The amino acid sequences of these proteins were determined. M6 and M7 were composed of 180 amino acid residues and showed high sequence homology (76%), while M3 was composed of 149 residues and showed 26% homology with M6 and M7. The disulfide linkage motif of the three proteins was similar and resembled the carbohydrate recognition domain (CRD) of C-type lectin. The C-terminal half of these proteins showed sequence homology with CRD of C-type lectins, but no homology with vitelline coat lysins of other mollusks. The proteins bound to asialofetuin-Sepharose in the presence of Ca2+ and were eluted with EDTA, indicating that they are Ca(2+)-dependent carbohydrate-binding proteins.