COMPONENTS OF THE SIGNALING PATHWAY LINKING THE 1-METHYLADENINE RECEPTOR TO MPF ACTIVATION AND MATURATION IN STARFISH OOCYTES

Starfish oocytes are arrested at the G2/M-phase border of meiosis I. E xposure to their natural mitogen, 1-methyladenine (1-MA), leads to the activation of MPF and MAP kinase, resumption of the meiotic cell cycl e, and fertilization competency. The 1-MA receptor has not yet been id entified, but it is known to be linked functionally to a pertussis tox in-sensitive G-protein. G beta gamma appears to be the major effector of the 1-MA receptor, since injection of G beta gamma, but not activat ed G alpha(i), leads to the activation of MPF, entry into meiosis, and oocyte maturation. The components that connect G beta gamma to MPF an d MAP kinase activation in oocytes are unknown. In mammalian cells, a novel phosphatidylinositol 3-kinase, PI-3 kinase-gamma, links G beta g amma to the MAP kinase activation pathway. Here we show that PI-3 kina se is required for starfish oocyte maturation. LY294002 and wortmannin , inhibitors of PI-3 kinase, block MPF and MAP kinase activation and e ntry into meiosis. Inhibition by LY294002 is reversible and limited to the hormone dependent period. Neither inhibitor, however, blocks the earliest hormone-induced event, formation of actin spikes at the cell membrane. By contrast, pertussis toxin blocks both actin spiking and l ater events, arguing that PI-3 kinase functions downstream of G beta g amma. Finally, we show that unlike the well-studied case in Xenopus oo cytes, where MAP kinase is an essential component of the MPF activatio n pathway, MAP kinase is not required for either MPF activation or sub sequent oocyte maturation in starfish. Instead, its major role appears to be suppression of DNA synthesis in unfertilized, haploid eggs. (C) 1998 Academic Press.