Requirement of a Src Family Kinase for Initiating Calcium Release at Fertilization in Starfish Eggs

Signal transduction leading to calcium release in echinoderm eggs at fertilization requires phospholipase Cγ-mediated production of inositol trisphosphate (IP 3 ), indicating that a tyrosine kinase is a likely upstream regulator. Because previous work has shown a fertilization-dependent association between the Src homology 2 (SH2) domains of phospholipase Cγ and a Src family kinase, we examined whether a Src family kinase was required for Ca 2+ release at fertilization. To inhibit the function of kinases in this family, we injected starfish eggs with the SH2 domains of Src and Fyn kinases. This inhibited Ca 2+ release in response to fertilization but not in response to injection of IP 3 . We further established the specificity of the inhibition by showing that the SH2 domains of several other tyrosine kinases (Abl, Syk, and ZAP-70), and the SH3 domain of Src, were not inhibitory. Also, a point-mutated Src SH2 domain, which has reduced affinity for phosphotyrosine, was a correspondingly less effective inhibitor of fertilization-induced Ca 2+ release. These results indicate that a Src family kinase, by way of its SH2 domain, links sperm-egg interaction to IP 3 -mediated Ca 2+ release at fertilization in starfish eggs.