Calcium release at fertilization in starfish eggs is mediated by phospholipase C gamma

Although inositol trisphosphate (IP3) functions in releasing Ca2+ in eggs at fertilization, it is not known how fertilization activates the phospholipase C that produces IP3. To distinguish between a role for PLC gamma, which is activated when its two sic homology-2 (SH2) domains bind to an activated tyrosine kinase, and PLC beta, which is activated by a G protein, we injected starfish eggs with a PLC gamma SH2 domain fusion protein that inhibits activation of PLC gamma. In these eggs, Ca2+ release at fertilization was delayed, or with a high concentration of protein and a low concentration of sperm, completely inhibited. The PLC gamma SH2 protein is a specific inhibitor of PLC gamma in the egg, since it did not inhibit PLC beta activation of Ca2+ release initiated by the serotonin 2c receptor, or activation of Ca2+ release by IP3 injection. Furthermore, injection of a PLC gamma SH2 domain protein mutated at its phosphotyrosine binding site, or the SH2 domains of another protein (the phosphatase SHP2), did not inhibit Ca2+ release at fertilization. These results indicate that during fertilization of starfish eggs, activation of phospholipase C gamma by an SH2 domain-mediated process stimulates the production of IP3 that causes intracellular Ca2+ release.