Species specific agglutination of eggs by bindin isolated from sea urchin sperm

Abstract

SPERMATOZOA adhere to eggs during sea urchin fertilisation^1,2. The adhesion is between the acrosome process of the sperm and the vitelline layer covering of the egg³. During the sperm acrosome reaction, the membrane of the extending acrosome process becomes coated with protein derived from the sperm acrosome vesicle^3,6. We believe^4,5, like others^3,6, that the acrosome vesicle protein binds the sperm to the egg by interacting with glycoprotein receptors on the egg vitelline layer^4,5,7,8. We have isolated the insoluble contents of the acrosome vesicle and have shown it to be composed of a single protein of molecular weight 30,500 which we have named bindin⁴. In most interspecies inseminations sea urchin sperm fail to adhere to the vitelline layers of foreign eggs and fertilisation does not occur, even though sperm undergo the acrosome reaction in the presence of the foreign eggs⁹. Presumably this failure of gamete adhesion occurs because bindin, on the sperm acrosome process, does not bind to the glycoprotein receptors on the vitelline layers of eggs of another species. We report here the species specific agglutination of unfertilised eggs by isolated bindin. This demonstrates the preservation of the species specific recognition event between the isolated protein and its receptor which should facilitate the in vitro reconstruction and analysis of the molecular mechanism of gamete adhesion.