A novel protein for Ca2+ signaling at fertilization

At fertilization in all species studied the sperm activates the egg by causing an increase in the level of cytoplasmic free Ca2+ concentration. It is still not established how the sperm causes the changes in Ca2+ in the egg, which in the majority of eggs is due to release from internal stores. Current hypotheses about the signaling molecules involved in fertilization are confounded by the fact that for many eggs the fertilization-associated Ca2+ increase is readily mimicked by parthenogenetic activating agents. One exception to this is found for mammalian eggs where there are a series of Ca2+ oscillations observed at fertilization that have distinct characteristics. In this context we discuss three different theories of how sperm trigger Ca2+ release in eggs. We present the case that the sperm mediates its Ca2+ mobilization effects after gamete membrane fusion by introducing a specific protein into the egg cytoplasm. Our argument is based upon the fact that only the mammalian sperm protein factor can trigger a pattern of Ca2+ oscillations that is similar to that induced by the sperm in mammalian eggs. The sperm factor activity is correlated with a novel signaling protein that we have called oscillin and which may mediate Ca2+ release via a novel mechanism.