A 225 K dalton glycoprotein is the active core structure of the sperm-binding factor of the sea urchin, Anthocidaris crassispina.

Fab fragments against 225 000 D glycoprotein (225 K), 87 000 D protein (87K) 80 000 D glycoprotein (80 K) of partially purified sperm-binding factor of Anthocidaris crassispina were prepared, and their effects upon fertilizability of dejellied homologous and heterologous eggs examined. Only the 225 K Fab impaired the fertilizability of homologous, not heterologous eggs by decreasing their sperm-binding capacity. It was concluded that 225 K glycoprotein is the active core structure of the sperm-binding factor of this species. The possible participation of the other two proteins as residual ingredients of the sperm-binding factor was also discussed. Immunofluorescence studies showed that 225 K core protein is localized on the whole surface of unfertilized eggs and of fully-grown oocytes. The fluorescence disappeared following fertilization.