A CYTOSOLIC SPERM PROTEIN FACTOR MOBILIZES CA2-MOLECULAR-WEIGHT MESSENGERS( FROM INTRACELLULAR STORES BY ACTIVATING MULTIPLE CA2+ RELEASE MECHANISMS INDEPENDENTLY OF LOW)

Ca2+ oscillations can be induced in mammalian eggs and somatic cells b y microinjection of a cytosolic sperm protein factor. The nature of th e sperm factor-induced Ca2+ signaling was investigated by adding sperm protein extracts to homogenates of sea urchin eggs, which contain mul tiple classes of Ca2+ release mechanisms. We show that the sperm facto r mobilizes Ca2+ from nonmitochondrial Ca2+ stores in egg homogenates after a distinct latency. This latency is abolished by preincubation o f sperm extracts with egg cytosol. The preincubation step is highly te mperature-dependent and generates a high molecular weight, protein-bas ed Ca2+-releasing agent that can also mobilize Ca2+ from purified egg microsomes. This Ca2+ release appears to be mediated via both inositol 1,4,5-trisphosphate and ryanodine receptors, since homologous desensi tization of these two release mechanisms by their respective agonists inhibits further release by the sperm factor. However, sperm factor-in duced Ca2+ release by these channels is independent of inositol 1,4,5- trisphosphate or cADPR since antagonists of either of these two messen gers did not block the Ca2+ release effected by the sperm factor. The sperm protein factor may cause Ca2+ release via an enzymatic step that generates a protein-based Ca2+-releasing agent.