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* Cell Biology Group, Worcester Foundation for Biomedical Research, Shrewsbury, Massachusetts 01545; and Through association with CDK1, cyclin B accumulation and destruction govern the G2/M/G1 transitions in eukaryotic cells. To identify CDK1 inactivation-dependent events during late mitosis, we
expressed a nondestructible form of cyclin B (cyclin
B Mitotic Spindle
Group, Cell Biology Program, European Molecular Biology Laboratory, 69117 Heidelberg, Germany
90) by microinjecting its mRNA into prometaphase
normal rat kidney cells. The injection inhibited chromosome decondensation and nuclear envelope formation. Chromosome disjunction occurred normally, but
anaphase-like movement persisted until the chromosomes reached the cell periphery, whereupon they often somersaulted and returned to the cell center. Injection of rhodamine-tubulin showed that this movement
occurred in the absence of a central anaphase spindle. In 82% of cells cytokinesis was inhibited; the remainder
split themselves into two parts in a process reminiscent
of Dictyostelium cytofission. In all cells injected, F-actin
and myosin II were diffusely localized with no detectable organization at the equator. Our results suggest
that a primary effect of CDK1 inactivation is on spindle
dynamics that regulate chromosome movement and cytokinesis. Prolonged CDK1 activity may prevent cytokinesis through inhibiting midzone microtubule formation, the behavior of proteins such as TD60, or through
the phosphorylation of myosin II regulatory light chain.
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