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* Department of Physiology, University of Connecticut Health Center, Farmington, Connecticut 06032; and Although inositol trisphosphate (IP3) functions in releasing Ca2+ in eggs at fertilization, it is not
known how fertilization activates the phospholipase C
that produces IP3. To distinguish between a role for
PLC Institut National de
la Santé et de la Recherche Medicale CJF9207, Faculté de Pharmacie 15, F-34060 Montpellier, France
, which is activated when its two src homology-2 (SH2) domains bind to an activated tyrosine kinase,
and PLC
, which is activated by a G protein, we injected starfish eggs with a PLC SH2 domain fusion
protein that inhibits activation of PLC. In these eggs,
Ca2+ release at fertilization was delayed, or with a high
concentration of protein and a low concentration of
sperm, completely inhibited. The PLCSH2 protein is a
specific inhibitor of PLC in the egg, since it did not inhibit PLC activation of Ca2+ release initiated by the
serotonin 2c receptor, or activation of Ca2+ release by
IP3 injection. Furthermore, injection of a PLC SH2 domain protein mutated at its phosphotyrosine binding
site, or the SH2 domains of another protein (the phosphatase SHP2), did not inhibit Ca2+ release at fertilization. These results indicate that during fertilization of
starfish eggs, activation of phospholipase C by an SH2
domain-mediated process stimulates the production of IP3 that causes intracellular Ca2+ release.
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