Sea urchin egg receptor for sperm: the oligosaccharide chains stabilize sperm binding

Sulfated O-linked oligosaccharides from the sea urchin egg receptor have been shown to bind to acrosome-reacted sperm and to inhibit fertilization in a competitive bioassay. However, the inhibitory activity of these isolated chains was much lower than that of a recombinant protein representing a portion of the extracellular domain of the receptor. Because the isolated oligosaccharides lacked the potential polyvalency that they might have when linked to the polypeptide backbone, in the current study we asked if their inhibitory activity could be increased by chemically coupling them to a protein to form a neoglycoprotein. Using a recombinant fragment of the receptor we could not detect an oligosaccharide dependent increase in inhibitory activity with this neoglycoprotein, probably because of the much higher inhibitory activity of the polypeptide backbone. Therefore, we examined the activity of the oligosaccharides coupled to a protein lacking the ability to inhibit fertilization, namely, bovine serum albumin. A marked increase in the inhibitory activity of the oligosaccharides was observed with this neoglycoprotein. Finally, because inhibition by the oligosaccharides and the polypeptide was measured in an end point assay, namely, inhibition of fertilization, we sought a more direct, kinetically sensitive way to measure their properties. Accordingly, an assay was devised (R.L. Stears and W.J. Lennarz, unpublished observations) involving measurement of sperm binding to beads that was dependent on the presence of the receptor or its components. This assay revealed that sperm binding to beads via the recombinant protein peaked at 10 sec and then declined. In contrast, binding mediated by neoglycosylated recombinant protein reached a plateau. Thus, binding of sperm to the oligosaccharides resulted in a more stable interaction than that observed in binding to the polypeptide backbone.