Characterization of the sperm receptor on the surface of eggs of Strongylocentrotus purpuratus.

Eggs of the sea urchins Strongylocentrotus purpuratus and Arbacia punctulata bind sperm with a high degree of species specificity. By use of an in vitro assay that utilizes bindin (the protein from sperm that mediates sperm-egg binding) egg surface-derived glycoconjugates that function as receptors in this adhesion process have been identified and purified. These glycoconjugates are of extraordinarily high molecular weight and exhibit some properties expected for a proteoglycan. The isolated receptors from both species bind to sperm and inhibit fertilization species specifically. Both receptors contain active carbohydrate-rich fragments that can be liberated by proteolytic digestion. The carbohydrate-rich receptor fragment from S. purpuratus is a very high-molecular-weight (greater than 10(6)), negatively charged glycosaminoglycan-like polymer containing fucose, galactosamine, iduronic acid, and sulfate esters. By contrast, the carbohydrate-rich fragment derived from the A. punctulata receptor is of defined molecular weight (6000) and has no net charge. Incubation of acrosome-reacted sperm with nanomolar amounts of the carbohydrate-rich fragments from either species results in inhibition of fertilization, indicating that these receptor fragments retain sperm binding activity. However, studies utilizing heterologous gametes show that the carbohydrate-rich receptor fragments are not species specific in binding. Thus, it appears that although the carbohydrate chains of the receptor are an adhesive element of the receptor, the intact glycoconjugate is required for species-specific binding.