Abstract
At fertilization, eggs undergo a cytoplasmic free Ca 2+ rise, which is necessary for stimulating embryogenesis. In starfish eggs, studies using inhibitors designed against vertebrate proteins have shown that this Ca 2+ rise requires an egg Src family kinase (SFK) that directly or indirectly activates phospholipase C-γ (PLC-γ) to produce IP 3, which triggers Ca 2+ release from the egg's endoplasmic reticulum (ER) [reviewed in Semin. Cell Dev. Biol. 12 (2001) 45]. To examine in more detail the endogenous factors in starfish eggs that are required for Ca 2+ release at fertilization, an oocyte cDNA encoding PLC-γ was isolated from the starfish Asterina miniata. This cDNA, designated AmPLC-γ, encodes a protein with 49% identity to mammalian PLC-γ1. A 58-kDa Src family kinase interacted with recombinant AmPLC-γ Src homology 2 (SH2) domains in a specific, fertilization-responsive manner. Immunoprecipitations of sea urchin egg PLC-γ using an affinity-purified antibody directed against AmPLC-γ revealed fertilization-dependent phosphorylation of PLC-γ. Injecting starfish eggs with the tandem SH2 domains of AmPLC-γ (which inhibits PLC-γ activation) specifically inhibited Ca 2+ release at fertilization. These results indicate that an endogenous starfish egg PLC-γ interacts with an egg SFK and mediates Ca 2+ release at fertilization via a PLC-γ SH2 domain-mediated mechanism.
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