SIGNAL-TRANSDUCTION DURING SHRIMP OOCYTE ACTIVATION BY EXTRACELLULAR MG2- ROLES OF INOSITOL 1,4,5-TRISPHOSPHATE, TYROSINE KINASES AND G-PROTEINS( )

Oocytes of the shrimp Sicyonia ingentis are naturally activated upon c ontact with seawater Mg2+. We investigated the mechanism of Mg2+-induc ed intracellular Ca2+ release and cortical contraction through treatme nt of oocytes with various activators and inhibitors of signal transdu ction pathways, Injection of oocytes with the second messenger inosito l 1,4,5-trisphosphate resulted in an immediate rise in intracellular C a2+ and normal cortical contraction, By contrast,:injection of the GTP analog guanosine 5'-0-(3'-thiotriphosphate) to activate G-proteins di d not affect intracellular Ca2+ levels but did induce cortical contrac tion, Tyrosine kinase inhibitors (tyrphostin and staurosporine) suppre ssed the Mg2+-induced Ca2+ rise and contraction, and the inhibition co uld be overcome by injection with inositol 1,4,5-trisphosphate, Immuno precipitation of phosphotyrosine-containing proteins from oocyte lysat es showed that a 110x10(3) M(r) protein was phosphorylated within seco nds of oocyte exposure to Mg2+, and tyrphostin inhibited the phosphory lation of this protein, Pretreatment of oocytes with the protease tryp sin abolished their ability to release Ca2+ in response to extracellul ar Mg2+, indicating a role for a cell surface protein during normal oo cyte activation; treated oocytes could be rescued by inositol 1,4,5-tr isphosphate injection, These results suggest that S. ingentis oocytes are activated through a Mg2+ 'receptor' which activates a tyrosine kin ase and results in the production of inositol 1,4,5-trisphosphate to r elease intracellular Ca2+ stores and induce cortical contraction, A G- protein/GTPase may also be involved in the pathway leading to cortical contraction.