Calcium oscillations in mammalian eggs triggered by a soluble sperm protein

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Abstract
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. It is not known how the sperm initiates this fundamental process, however, nor has any pathway linking sperm-egg membrane-receptor binding with intracellular Ca2+ release been demonstrated. Microinjection of sperm extracts into mammalian eggs elicits Ca2+ oscillations identical to those occurring at fertilization, which suggests that sperm may introduce a Ca2+ oscillation-inducing factor into the egg on gamete membrane fusion. 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.