Independent and hetero-oligomeric-dependent sperm binding to egg envelope glycoprotein ZPC in Xenopus laevis

Vitelline envelopes are composed of glycoproteins that participate in sperm -egg interactions during the initial stages of fertilization, in Xenopus la evis, the vitelline envelope is composed of at least 4 glycoproteins (ZPA, ZPB, ZPC, and ZPX). A sperm binding assay involving the covalent coupling o f envelope glycoproteins to silanized glass slides was developed. In our as say, sperm bound to the egg envelopes derived from oviposited eggs but not activated eggs. The majority of the egg envelope ligand activity for sperm binding was derived from the complex N-linked oligosaccharides of ZPC. This sperm binding involved N-acetylglucosamine and fucose residues, as binding was abolished after treatment with cortical granule beta-N-acetylglucosami nidase and commercial beta-N-acetylglucosaminidases and was reduced by 44% after treatment with alpha-fucosidase. Although both the envelope glycoprot eins ZPA and ZPC possessed independent ligand activity, ZPC was the major l igand for sperm binding (75%). Mixing of isolated ZPA, ZPB, and ZPC in a ra tio of 1:4:4 (equal to that in the egg envelope) resulted in sperm binding that was greater than that of the sum of the separate components. The egg g lycoproteins acted in synergy to increase sperm binding. Thus, ZPC possesse d both independent and hetero-oligomeric-dependent ligand activities for sp erm binding.