A PROPAGATED WAVE OF MPF ACTIVATION ACCOMPANIES SURFACE CONTRACTION WAVES AT FIRST MITOSIS IN XENOPUS

During the period of mitosis, two surface contraction waves (SCWs) pro gress from the animal to vegetal poles of the Xenopus egg, It has been shown that these SCWs occur in parallel with the activation of MPF an d with its subsequent inactivation in the animal and vegetal hemispher es, suggesting that they are responses to propagated waves of MPF acti vity across the egg, We have analysed the mechanism of MPF regulation in different regions of the egg in detail in relation to SCW progressi on, The distributions of histone HI kinase activity and of Cdc2 and cy clin B (the catalytic and regulatory subunits of MPF) were followed by dissection of intact eggs following freezing and in cultured fragment s separated by ligation, Cdc2 was found to be distributed evenly throu ghout the egg cytoplasm, Loss of phosphorylated (inactive) forms of Cd c2 coincided spatially with the wave of MPF activation, while cyclin B 2 accumulation occurred in parallel in animal and vegetal regions, In ligated vegetal pole fragments no MPF activation or Cdc2 dephosphoryla tion were detectable, A wave of cyclin B destruction that occurred in concert with the second SCW was also blocked, Taken together these res ults indicate that the triggering mechanism for MPF activation require s components specific to the animal cytoplasm, acting via Cdc2 dephosp horylation, and that MPF activation subsequently propagates autocataly tically across the egg. SCW progression in the vegetal hemisphere was followed directly by time-lapse videomicroscopy of subcortical mitocho ndrial islands, The first SCW traversed the vegetal pole at the time o f MPF activation in this region, Like MPF activation and inactivation, SCWs were blocked in the vegetal region by ligation, These observatio ns reinforce the hypothesis that the first SCW is a direct consequence of the MPF activation wave, It may reflect depolymerisation of the su bcortical microtubule network since it coincided exactly with the arre st of the microtubule-dependent movement of 'cortical rotation' and wa s related in direction in most eggs, The cyclin B destruction wave and associated cortical contraction of the second SCW may be localised do wnstream consequences of the MPF activation wave, or they may propagat e independently from the animal cytoplasm.