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

Recent evidence has indicated a requirement for a Src family kinase in initiating Ca2+ release at fertilization in starfish eggs (Giusti, A. F., Carroll, 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 protein into starfish eggs initiates Ca2+ release and DNA synthesis, as occur at fertilization. These responses depend on the phosphorylation state of the Src protein; only the kinase active form is effective. Like Ca2+ release at fertilization, the Ca2+ release in response to Src protein injection is inhibited by prior injection of the SH2 domains of phospholipase C gamma. These findings support the conclusion that in starfish, sperm-egg interaction causes egg activation by sequential activation of a Src-like kinase and phospholipase C gamma. Injection of the SH2 domain of Src, which inhibits Ca2+ release at fertilization, does not inhibit Ca2+ release caused by Src protein injection. This indicates that the requirement for a Src SH2 domain interaction is upstream of Src activation in the pathway leading to Ca2+ release at fertilization.