1. Program in Molecular, Cell and Developmental Biology, Oklahoma Medical Research Foundation, 825 NE 13th Street, Oklahoma City, Oklahoma 73104, USA
2. Department of Biochemistry and Molecular Genetics, University of Virginia Medical School, 1300 Jefferson Park Avenue, Charlottesville, Virginia 22908, USA

Correspondence to: Gary J. Gorbsky¹ Correspondence and requests for materials should be addressed to G.J.G. (Email: GJG@omrf.ouhsc.edu).

Abstract

A guiding hypothesis for cell-cycle regulation asserts that regulated proteolysis constrains the directionality of certain cell-cycle transitions^{1, 2}. Here we test this hypothesis for mitotic exit, which is regulated by degradation of the cyclin-dependent kinase 1 (Cdk1) activator, cyclin B^{3, 4, 5}. Application of chemical Cdk1 inhibitors to cells in mitosis induces cytokinesis and other normal aspects of mitotic exit, including cyclin B degradation. However, chromatid segregation fails, resulting in entrapment of chromatin in the midbody. If cyclin B degradation is blocked with a proteasome inhibitor or by expression of non-degradable cyclin B, Cdk inhibitors will nonetheless induce mitotic exit and cytokinesis. However, if after mitotic exit, the Cdk1 inhibitor is washed free from cells in which cyclin B degradation is blocked, the cells can revert back to M phase. This reversal is characterized by chromosome recondensation, nuclear envelope breakdown, assembly of microtubules into a mitotic spindle, and in most cases, dissolution of the midbody, reopening of the cleavage furrow, and realignment of chromosomes at the metaphase plate. These findings demonstrate that proteasome-dependent degradation of cyclin B provides directionality for the M phase to G1 transition.

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