GPI-anchored aminopeptidase is involved in the acrosome reaction in sperm of the mussel mytilusedulis.

The sperm of the mussel Mytilus had hydrolytic activities against substrates for aminopeptidase. Acrosome reaction (AR) was suppressed in the presence of aminopeptidase substrate, Phe-4-methylcoumaryl-7-amide (MCA), and an aminopeptidase inhibitor, bestatin. Treatment of sperm with phosphatidylinositol-specific phospholipase C (PI-PLC) released aminopeptidase activity from sperm and suppressed AR. These results suggest that the enzyme is located on the sperm surface via glycosylphosphatidylinositol (GPI)-anchor and is involved in the AR. Immunoblot analysis showed that tyrosine residues of 40, 59, 68, and 72 kDa proteins were phosphorylated during induction of the AR. The 40 kDa protein was also recognized by anti-c-Src antibody by immunoblotting. The tyrosine phosphorylation of these proteins was inhibited when sperm were inseminated in the presence of Phe-MCA, and by PI-PLC treatment. Treatment of sperm with tyrosine kinase activator, 9,10-dimethyl-1,2-benzanthracene, induced AR, and its inhibitor, genistein, suppressed AR. These results suggest that tyrosine phosphorylation of 40, 59, 68, and 72 kDa proteins, induced by the interaction of GPI-anchored aminopeptidase with oocyte surface, triggers AR in Mytilus sperm.