Calcium Release at Fertilization in Starfish Eggs Is Mediated by Phospholipase Cγ

Although inositol trisphosphate (IP(3)) functions in releasing Ca(2+) in eggs at fertilization, it is not known how fertilization activates the phospholipase C that produces IP(3). To distinguish between a role for PLCγ, which is activated when its two src homology-2 (SH2) domains bind to an activated tyrosine kinase, and PLCβ, which is activated by a G protein, we injected starfish eggs with a PLCγ SH2 domain fusion protein that inhibits activation of PLCγ. In these eggs, Ca(2+) release at fertilization was delayed, or with a high concentration of protein and a low concentration of sperm, completely inhibited. The PLCγSH2 protein is a specific inhibitor of PLCγ in the egg, since it did not inhibit PLCβ activation of Ca(2+) release initiated by the serotonin 2c receptor, or activation of Ca(2+) release by IP(3) injection. Furthermore, injection of a PLCγ SH2 domain protein mutated at its phosphotyrosine binding site, or the SH2 domains of another protein (the phosphatase SHP2), did not inhibit Ca(2+) release at fertilization. These results indicate that during fertilization of starfish eggs, activation of phospholipase Cγ by an SH2 domain-mediated process stimulates the production of IP(3) that causes intracellular Ca(2+) release.