INVIVO ACTIVATION OF A MICROTUBULE-ASSOCIATED PROTEIN-KINASE DURING MEIOTIC MATURATION OF THE XENOPUS OOCYTE

We have characterized a serine/threonine protein kinase from Xenopus metaphase-II-blocked oocytes, which phosphorylates in vitro the microtubule-associated protein 2 2 (MAP2). The MAP2 kinase activity, undetectable in prophase oocytes, is activated during the progesterone-induced meiotic maturation (G2-M transition of the cell cycle). p-Nitrophenyl phosphate, a phosphatase inhibitor, is required to prevent spontaneous deactivation of the MAP2 kinase in crude preparations; conversely, the partially purified enzyme can be in vitro deactivated by the low-M(r) polycation-stimulated (PCS(L) phosphatase (also termed protein phosphatase 2A2), working as a phosphoserine/phosphothreonine-specific phosphatase and not as a phosphotyrosyl phosphatase indicating that phosphorylation of serine/threonine is necessary for its activity. S6 kinase, a protein kinase activated during oocyte maturation which phosphorylates in vitro ribosomal protein S6 and lamin C, can be deactivated in vitro by PCS(L) phosphatase. S6 kinase from prophase oocytes can also be activated in vitro in fractions known to contain all the factors necessary to convert pre-M-phase-promoting factor (pre-MPF) to MPF. Active MAP2 kinase can activate in vitro the inactive S6 kinase present in prophase oocytes or reactivate S6 kinase previously inactivated in vitro by PCS(L) phosphatase. These data are consistent with the hypothesis that the MAP2 kinase is a link of the meiosis signalling pathway and is activated by a serine/threonine kinase. This will lead to the regulation of further steps in the cell cycle, such as microtubular reorganisation and S6 kinase activation.