Calcium oscillations in mammalian eggs triggered by a soluble sperm protein

AT fertilization in mammals, the sperm induces a characteristic series of Ca2+ oscillations in the egg which serve as the essential trigger for egg activation and early development of the embryo(1-3). It is not known how the sperm initiates this fundamental process, however(4-6), nor has any pathway linking sperm-egg membrane-receptor binding with intracellular Ca2+ release been demonstrated(7-11). Microinjection of sperm extracts into mammalian eggs elicits Ca2+ oscillations identical to those occurring at fertilization(12-14), which suggests that sperm may introduce a Ca2+ oscillation-inducing factor into the egg on gamete membrane fusion(12,15-18). Here we identify a soluble sperm protein that exhibits Ca2+ oscillation-inducing ('oscillogen') activity in eggs, Sperm oscillogen exists as an oligomer with a subunit of M(r) 33K and a specific intracellular localization at the equatorial segment of the sperm head. Cloning of the 33K oscillogen complementary DNA indicates similarity with a hexose phosphate isomerase found in prokaryotes. This sperm-derived oscillogen, termed oscillin, may represent the physiological trigger for development in mammals.