CELL-CYCLE CONTROL - POLO-LIKE KINASES JOIN THE OUTER CIRCLE

Named after the polo gene of Drosophila, POLO-like kinases (PLKs) cons titute a novel, evolutionarily conserved family of essential cell-cycl e regulators. As emphasized in this review, recent studies identify, i mportant roles for vertebrate PLKs at the onset of mitosis: Plx1, a Xe nopus PLK, has been implicated in the activation of Cdc25 phosphatase (and hence the activation of Cdc2) while human Plk1 is required for th e proper maturation of the poles of mitotic spindles. These studies su ggest a major role for Plk1/Plx1 in coordinating spindle assembly with the activation of Cdc2-cyclin complexes, and they establish a direct link between PLKs and the core cell-cycle-regulatory machinery. Geneti c and biochemical studies in yeasts and Drosophila point to additional roles for PLKs at later stages of mitosis. Finally, mammals express m ultiple PLKs, suggesting that different family members might function at distinct cell-cycle transitions, reminiscent of cyclin-dependent ki nases.