SH2 domain-mediated activation of phospholipase Cgamma is not required to initiate Ca2+ release at fertilization of mouse eggs.

The initiation of Ca2+ release at fertilization of mammalian eggs requires inositol trisphosphate (Miyazaki et al., 1992, Science 257, 251-255), indicating that an enzyme of the phospholipase C family is probably activated. Because Ca2+ release at fertilization in echinoderm eggs is initiated by SH2 domain-mediated activation of phospholipase Cgamma (Carroll et al., 1997, J. Cell Biol. 138, 1303-1311), we examined the possible role of PLCgamma in initiating Ca2+ release at fertilization in mouse eggs. Both PLCgamma isoforms, PLCgamma1 and PLCgamma2, are present in mouse eggs and sperm, and stimulation of these enzymes in the egg by way of an exogenously expressed PDGF receptor causes Ca2+ release. Recombinant SH2 domains of PLCgamma1 and PLCgamma2 inhibit PLCgamma1 and PLCgamma2 activation by the PDGF receptor, completely preventing Ca2+ release in response to PDGF when injected at an approximately 20- to 40-fold excess over the concentrations of endogenous proteins. However, even at an approximately 100- to 400-fold excess over endogenous protein levels, PLCgamma1 and PLCgamma2 SH2 domains do not inhibit Ca2+ release at fertilization. These findings indicate that Ca2+ release at fertilization of mouse eggs does not require SH2-domain-mediated activation of PLCgamma. However, activation of PLCgamma in the egg by an alternative pathway, or introduction of activated PLCgamma from the sperm, may be important.