Molecular cloning and sequence analysis of an ascidian egg β-N-acetylhexosaminidase with a potential role in fertilization

β-N-Acetylhexosaminidase, which is found almost ubiquitously in sperm of invertebrates and vertebrates, supposedly mediates a carbohydrate-based transient sperm-egg coat binding. In ascidians and mammals, β-hexosaminidase released at fertilization from eggs has been proposed to modify sperm receptor glycoproteins of the egg envelope, thus setting up a block to polyspermy. Previously, it was shown that in potential sperm receptor glycoproteins of the ascidian Phallusia mammillata, N-acetylglucosamine is the prevailing glycoside residue and that the egg harbors three active molecular forms of β-hexosaminidase. In the present study, P. mammillata β-hexosaminidase cDNA was isolated from an ovarian cDNA library and characterized. The deduced amino acid sequence showed a high similarity with other known β-hexosaminidases; however, P. mammillata β-hexosaminidase had a unique potential N-glycosylation site. A phylogenetic analysis suggested that P. mammillata β-hexosaminidase developed independently after having branched off from the common ancestor gene of the chordate enzyme before two isoforms of the mammalian enzyme appeared. In situ hybridization revealed stage-specific expression of β-hexosaminidase mRNA during oogenesis in the oocyte and in the accessory test and follicle cells. This suggests that the three egg β-hexosaminidase forms are specific for the oocyte, test cells and follicle cells.