Both these properties are characteristic of hydrophobic interactions. If the antigen had bound covalently to an integral membrane receptor, then it would have migrated with a higher molecular mass after SDS-PAGE. That a significant proportion of 24-kDa antigen remains in the aqueous layer after detergent-phase separation suggests significant structural heterogeneity, conceivably mediated by differences in the level of glycosylation, that could influence the conformational changes necessary to interact with the plasma membrane.
The failure of PI-PLC to release the 24-kDa antigen from cauda spermatozoa suggests absence of a GPI anchor. Although several sperm proteins with known GPI anchors are PI-PLC resistant (e.g., rat 2B1), the characteristic motif for a lipid attachment site is not present in the primary sequence of 2D6 glycoprotein. The mechanism of binding of the 24-kDa antigen to the sperm plasma membrane, therefore, must involve either a covalent attachment to a preexisting membrane receptor or a protein-lipid interaction that is specific for the sperm tail. The credibility of the latter possibility is enhanced by recent photobleaching experiments that have shown that lipid diffusion in the tail plasma membrane is —5 times slower than on the sperm head and has a higher proportion of immobile-phase lipid. Taken together, it would seem that some special properties of the sperm plasma membrane determine binding of epididymal glycoproteins rather than the glycoproteins in question. buy tavist online