CONTROL OF METAPHASE-I FORMATION IN XENOPUS-OOCYTE - EFFECTS OF AN INDESTRUCTIBLE CYCLIN-B AND OF PROTEIN-SYNTHESIS

A cytological analysis was performed in order to determine how the formation of the metaphase I- and metaphase II-spindles is dependent upon p34cdc2 kinase activity and protein synthesis during the meiotic maturation of Xenopus oocytes. The p34cdc2 kinase activity increases abruptly during the prophase-prometaphase I transition, then drops to a minimum level at the metaphase I/anaphase I transition and further increases again until reaching a maximum stable level at metaphase II. The injection of an indestructible cyclin B into oocytes arrests the maturation process at the onset of anaphase I and prevents the re-increase of p34cdc2 activity which accompanies normal entry into metaphase II. Inhibition of protein synthesis between the germinal vesicle breakdown and the onset of metaphase I spindle induces exit from M-phase and leads to an 'interphase-like' state characterized by the organization of nuclear-like structures. In contrast, inhibition of protein synthesis at metaphase II stage does not affect the metaphase 11 spindle nor p34cdc2 activity, indicating that metaphase I- and metaphase II-spindles are not regulated by the same effectors. When protein synthesis is inhibited before induction of M-phase by MPF transfer, it prevents the formation of the metaphase I spindle, despite a transient elevated level of p34cdc2 activity. To dissociate the role of protein synthesis and of p34cdc2 kinase activity, the indestructible cyclin B was microinjected in the absence of protein synthesis. This allows the in vivo maintenance of a stable p34cdc2 activity. Under these conditions, microtubules do not polymerize and no metaphase I spindle is formed, demonstrating that the synthesis of proteins, different from cyclin B, is required for the organization of the metaphase I spindle in addition to the presence of p34cdc2 activity.