Abstract
The basic outline of the events controlling entry into mitosis has been clear for more than 20 y: the essential mitotic driver M-phase promoting factor [MPF; a cyclin-dependent kinase (CDK) composed of CDK1 and cyclin B] becomes rapidly activated and phosphorylates a wide variety of targets, creating mitotic phosphoproteins that contribute to nuclear membrane breakdown, chromosome condensation, and spindle assembly. But is this the entire story? The article in PNAS by Burgess et al. (1) adds to evidence accumulating over the last 2 y arguing that half the picture has been missing. The levels of mitotic phosphoproteins are governed not only by MPF but also by the specific phosphatases that remove MPF-catalyzed phosphorylations. Importantly, both entry into M phase and the maintenance of M phase require the operation of a pathway, mediated by a kinase called Greatwall (Gwl), that can inactivate these phosphatases.
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