Sperm glycosidase as a plausible mediator of sperm binding to the vitelline envelope in Ascidians.

In the Ascidian, Ciona intestinalis, sperm alpha-L-fucosidase is concluded to be a recognition protein for the sperm receptor in the vitelline envelope. The spermatozoa bind to the vitelline envelope probably by forming rather stable substrate (alpha-L-fucoside)-enzyme complex at many spots. Experimental evidence indicates a good correspondence in the properties between receptor binding activity and alpha-L-fucosidase activity such as the effects of saccharides, of pH and of monoclonal antibodies raised against sperm alpha-L-fucosidase, and locus in the sperm. A general mechanism involving an enzyme as a recognition protein for the cell to cell binding is also discussed in relation to the molecular evolution of the recognition proteins.