The 220-kDa vitelline coat glycoprotein mediates sperm binding in the polarized egg of Unio elongatulus through O-linked oligosaccharides.

In previous studies we found that two glycoproteins of 220 and 180 kDa account for 80-90% of the material dissolved from the vitelline coat (VC) of Unio elongatulus egg (Focarelli and Rosati, 1993). The two glycoproteins were purified by electroelution and used to raise the corresponding polyclonal antibodies. Immunofluorescence experiments showed that the 180-kDa protein species is ubiquitous in the VC, whereas the 220-kDa protein is concentrated in a restricted region of the vegetal pole, the crater region, where the sperm-egg interaction occurs. Binding assays indicated that only the 220-kDa protein interacted with the sperm and that the protein bound in the apical region and triggered acrosomal changes in sperm. Competition binding assays showed that O- and not N-linked oligosaccharides derived from the 220-kDa protein competed with the binding of the protein to sperm and that fucose is involved in the ligand role of the 220-kDa protein.