ABSTRACT
Upon encountering the jelly coat of a homologous egg, starfish spermatozoa immediately undergo the acrosome reaction that ensures a spatio-temporally matched exposure of their devices for penetration through the egg coats and for fusion with the egg plasma membrane. To induce the acrosome reaction, three jelly components act in concert on the spermatozoa: a sulfated glycoprotein named acrosome reaction-inducing substance (ARIS), a group of sulfated steroid saponins named Co-ARIS, and an oligopeptide(s) presumably having an ability to increase the intracellular pH of the spermatozoa and therefore named sperm activating peptide (SAP). All three are required to mimic the full ability of jelly coat to induce the acrosome reaction. A combination of ARIS and Co-ARIS is enough for induction in normal sea water, although its action is significantly slower than the jelly. ARIS can species-specifically induce the acrosome reaction by itself in high Ca** or high pH sea water. When spermatozoa meet the homologous jelly coat, the acrosome reaction is eventually induced in a few seconds because ARIS and Co-ARIS cooperatively increase the intracellular Ca** by stimulating verapamil- and maitotoxin-sensitive Ca** channels and SAP increases the intracellular pH by stimulating Na*/H* exchange systems.
