Ca 2؉ oscillations can be induced in mammalian eggs and somatic cells by microinjection of a cytosolic sperm protein factor. The nature of the sperm factor-induced Ca 2؉ signaling was investigated by adding sperm protein extracts to homogenates of sea urchin eggs, which contain multiple classes of Ca 2؉ release mechanisms. We show that the sperm factor mobilizes Ca 2؉ from nonmitochondrial Ca 2؉ stores in egg homogenates after a distinct latency. This latency is abolished by preincubation of

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... rm extracts with egg cytosol. The preincubation step is highly temperature-dependent and generates a high molecular weight, protein-based Ca 2؉releasing agent that can also mobilize Ca 2؉ from purified egg microsomes. This Ca 2؉ release appears to be mediated via both inositol 1,4,5-trisphosphate and ryanodine receptors, since homologous desensitization of these two release mechanisms by their respective agonists inhibits further release by the sperm factor. However, sperm factor-induced Ca 2؉ release by these channels is independent of inositol 1,4,5-trisphosphate or cADPR since antagonists of either of these two messengers did not block the Ca 2؉ release effected by the sperm factor. The sperm protein factor may cause Ca 2؉ release via an enzymatic step that generates a proteinbased Ca 2؉ -releasing agent.