Another possibility is that the sperm/SF may carry an activator of the egg PLCs. Mammalian eggs express PLC|31, PLC(33, PLC71, and PLC72, and because of their much greater volume, they are likely to contain in excess of 1000-fold greater amounts of the isoforms present in sperm. In addition, the egg PLCs are easily activated to produce [Ca2+]i oscillations. For example, [Ca2+]i rises are initiated in mammalian eggs after injection of GTP7S or when eggs expressing foreign cell-surface receptors coupled to G-proteins or tyrosine kinase pathways are stimulated with appropriate agonists. Thus, it appears unlikely that SF would induce [Ca2+]i oscillations without involving the egg PLCs.
In echinoderms and ascidians, fertilization and SF (as-cidians) appear to activate the egg PLCs; expression of Src homology 2 (SH2) domains of PLC71 inhibited the sperm-induced Ca2+ responses. In these eggs, a Src-like tyrosine kinase appears to be required to induce activation of PLC7 and initiation of Ca2+ release. In vertebrates (Xenopus and mouse), the role of eggs PLCs is not as obvious because fertilization-induced Ca2+ responses were not inhibited by competing PLC71 SH2 domains or injection of antibodies against Gaq proteins, which are supposed to prevent the activation of PLCp [8, 34, 74]. PLO7 can be activated by mechanisms other than tyrosine phosphorylation at SH2 domains, and in most studies, the anti-Gaq antibody was raised against the C-terminal portion of this protein, a domain that does not appear to be important for interaction with PLCs. In Xenopus and mouse eggs, fertilization-induced Ca2+ responses appear sensitive to the addition of tyrosine kinase inhibitors, and in Xenopus eggs, Src family kinases seem to activate PLC7 at fertilization. Whether and how SF may activate a Src-like kinase in mammalian eggs is not known, although in somatic cells an Src kinase was shown to be activated by Ga-type proteins, and it has been reported that mammalian sperm express a Gaq protein. Thus, the active component(s) of SF may stimulate IP3 production by acting upstream of the egg PLCs, although the precise mechanism remains to be elucidated.