THE SEA-URCHIN EGG RECEPTOR FOR SPERM - ISOLATION AND CHARACTERIZATION OF THE INTACT, BIOLOGICALLY-ACTIVE RECEPTOR

The species-specific binding of sea urchin sperm to the egg is mediate d by an egg cell surface receptor. Although earlier studies have resul ted in the cloning and sequencing of the receptor, structure/function studies require knowledge of the structure of the mature cell surface protein. In this study, we report the purification of this glycoprotei n to homogeneity from a cell surface complex of Strongylocentrotus pur puratus eggs using lectin and ion exchange chromatography. Based on th e yield of receptor it can be calculated that each egg contains approx imately 1.25 x 10(6) receptor molecules on its surface. The receptor, which has an apparent M(r) of 350 kD, is a highly glycosylated transme mbrane protein composed of approximately 70% carbohydrate. Because ear lier studies on the partially purified receptor and on a pure, extrace llular fragment of the receptor indicated that the carbohydrate chains were important in sperm binding, we undertook compositional analysis of the carbohydrate in the intact receptor. These analyses and lectin binding studies revealed that the oligosaccharide chains of the recept or are sulfated and that both N- and O-linked chains are present. Func tional analyses revealed that the purified receptor retained biologica l activity; it inhibited fertilization in a species-specific and dose- dependent manner, and polystyrene beads coated with it bound to acroso me-reacted sperm in a species-specific manner. The availability of bio chemical quantities of this novel cell recognition molecule opens new avenues to studying the interaction of complementary cell surface liga nds in fertilization.