Evidence that fertilization activates starfish eggs by sequential activation of a Src-like kinase and phospholipase C gamma

Recent evidence has indicated a requirement for a Src family kinase in init iating Ca2+ release at fertilization in starfish eggs (Giusti, A. F., Carro ll, D, J., Abassi, Y. A, Terasaki, M., Foltz, K. R, and Jaffe, L. A. (1999) J. Biol. Chem. 274, 29318-29322). We now show that injection of Src protei n into starfish eggs initiates Ca2+ release and DNA synthesis, as occur at fertilization. These responses depend on the phosphorylation state of the S rc protein; only the kinase active form is effective. Like Ca2+ release at fertilization, the Ca2+ release in response to Src protein injection is inh ibited by prior injection of the SH2 domains of phospholipase C gamma. Thes e findings support the conclusion that in starfish, sperm-egg interaction c auses egg activation by sequential activation of a Src-like kinase and phos pholipase C gamma. Injection of the SH2 domain of Src, which inhibits Ca2release at fertilization, does not inhibit Ca2+ release caused by Src prote in injection. This indicates that the requirement for a Src SH2 domain inte raction is upstream of Src activation in the pathway leading to Ca2+ releas e at fertilization.