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 fertil ization requires phospholipase C gamma-mediated production of inositol tris phosphate (IP3), indicating that a tyrosine kinase is a likely upstream reg ulator. Because previous work has shown a fertilization-dependent associati on between the Src homology 2 (SH2) domains of phospholipase C gamma and a Src family kinase, we examined whether a Src family kinase was required for Ca2+ release at fertilization. To inhibit the function of kinases in this family. we injected starfish eggs with the SH2 domains of Src and Fyn kinas es. This inhibited Ca2+ release in response to fertilization but not in res ponse to injection of IP3. We further established the specificity of the in hibition 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. Als o, a point-mutated Src SH2 domain, which has reduced affinity for phosphoty rosine, was a correspondingly less effective inhibitor of fertilization-ind uced Ca2+ release. These results indicate that a Src family kinase, by way of its SH2 domain, links sperm-egg interaction to IP3-mediated Ca2+ release at fertilization in starfish eggs.