Phosphorylation changes associated with the early cell cycle in Xenopus eggs

Enucleated and nondividing amphibian eggs undergo cyclic changes in cell morphology and in the level of maturation promoting factor (MPF) with a period similar to the early cleavage cycle. We show here that there is a corresponding phosphorylation and dephosphorylation of specific proteins associated with this fundamental cell cycle. M-phase is associated with a general increase in phosphatase activity and specific phosphorylation of a small set of M-phase proteins, reflected in an increased stochiometry of phosphate and increased turnover. At the end of metaphase and correlated with a drop in MPF the phosphoproteins are rapidly lost. By microinjecting M-phase phosphoproteins into arrested interphase and metaphase eggs we could show that the specific M-phase phosphorylation was not due to specificity in phosphatase action. The ability to segregate synthesis from phosphorylation demonstrates that regulation is not on the level of synthesis of the M-phase proteins. Taken together these data suggest that regulation of kinase activity in M-phase in the face of general rapid phosphate turnover in the egg plays an important role in the regulation of the fundamental mitotic cycle.