Anaphase onset and dephosphorylation of mitotic phosphoproteins occur concomitantly.

The cyclical phosphorylation and dephosphorylation of the centrosome during mitosis was analyzed by immunofluorescence methods using the MPM-2 antibody, which reacts with a subset of mitotic phosphoproteins. Quantification of MPM-reactivity indicated that centrosomal phosphorylation attained a maximal level just prior to anaphase onset. This level was maintained in metaphase cells blocked from further mitotic progression with the microtubule depolymerizing agent nocodazole. However, when nocodazole was added to cells that had just initiated anaphase, the level of centrosomal phosphorylation decreased rapidly as in untreated anaphase cells. We conclude that the onset of dephosphorylation of the centrosome coincided with the onset of anaphase and continued in the absence of chromosome movement. Dephosphorylation of MPM-2 reactive phosphoproteins may be taken as a biochemical indicator of anaphase onset.