N-Acetylglucosaminidase Inhibitor Isolated from the Vitelline Coat of Ascidian Eggs Is a Candidate Sperm Receptor

Abstract

An inhibitor against the egg β-D- N-acetylglucosaminidase (GlcNAcase) was purified from the vitelline coat of eggs of the ascidian, Halocynthia roretzi. The purified inhibitor gave a single band with an apparent molecular mass of 70 kDa on SDS-PAGE under non-reducing conditions. The GlcNAcase inhibitor potently inhibited the ascidian egg GlcNAcase activity, and also substantially inhibited the GlcNAcase activity in the ascidian sperm extract, although to lesser extent. In contrast, it showed little inhibitory effect on the activity of GlcNAcase from Jack beans. This inhibitor was found to block the binding of sperm to the vitelline coat of the eggs in a concentration-dependent manner. These results strongly suggest that the GlcNAcase inhibitor isolated from the vitelline coat functions in the binding of sperm to the egg coat, as a possible sperm receptor.