Sperm-Egg binding in ascidian

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Specific binding of spermatozoa to the vitelline coat is an necessary step for fertilization in various animals. In Halocynthia roretzi and Ciona intestinalis, L-fucosyl residues of acidic saccharide chains in glycoproteins appears to be essential for sperm-receptor activity of the vitelline coat. Sperm binding was blocked by synthetic substrates (p-nitrophenyl or 4-methylumbelliferyl α-fucoside) toward the enzyme and their β-anomers that act as competitive inhibitors. The spermatozoa bound the vitelline coat in normal sea water (pH 8.2) detached at pH 4. Since the fucosylamine affinity column adsorbed the sperm extract appreciably in normal sea water at pH 8.2, though less extent than at pH 4, it was suggested that the enzyme was able to form a complex with substrates in normal sea water. Thus L-fucose residues of the sacchride chains appeared to be essential for the activity of sperm-receptor.In the present paper, we isolate a cDNA (HrFuc) encoding Halocynthia roretzi sperm α-L-fucosidase by RT-PCR method using the conservative region of α-L-fucosidase. The polyclonal antibody was prepared against a lacZ-HrFuc fusion protein made in E. coli. This antibody crossed to sperm 47 kDa protein, and inhibited ascidian fertilization in sea water. This cDNA product might be mediate the sperm binding.