IN-VIVO REGULATION OF CYTOSTATIC ACTIVITY IN XENOPUS METAPHASE II-ARRESTED OOCYTES

Metaphase II arrest of Xenopus oocyte is characterized by the presence of M-phase-promoting factor (MPF) and of a microtubular spindle, both of which are stable in the presence of protein synthesis inhibitors. We studied in vivo this equilibrium state that is settled during meiot ic maturation. At time of germinal vesicle breakdown (GVBD), cdc2 kina se and MAP kinase activities are stimulated. A component of the cyclin ubiquitin ligase, CDC27, is phosphorylated at the same time and remai ns phosphorylated until fertilization, indicating that an important co mponent of the ligase complex is modified as early as GVBD. During a f irst period extending from GVBD until the cortical anchorage of the me taphase II spindle, homogeneous pools of cdc2 kinase and mitogen-activ ated protein (MAP) kinase activities are present in oocyte and are str ictly dependent on protein turnover, since protein synthesis inhibitio n induces their total inactivation and drives oocytes into interphase. The metaphase II spindle, once anchored into the cortex, is no more s ensitive to protein synthesis inhibition, likewise MAP kinase activity . During this cellular arrest, cdc2 kinase is divided into two distinc tly regulated pools. The first one contains cyclin B that actively tur ns over and is subjected to a microtubular checkpoint. The second one is stable. Alteration of intracellular compartmentation of metaphase I I oocytes either by gentle centrifugation or by cold shock inactivates MAP kinase and targets all cyclin B molecules for full destruction. W e therefore suggest that MAP kinase participates to the cytostatic act ivity by preventing part of cyclin B molecules from entering the ibiqu itination/degradation machinery which is still turned on in metaphase II oocytes. (C) 1997 Academic Press.