Heterologous expression of mammalian Plk1 in Drosophila reveals divergence from Polo during late mitosis.

Drosophila Polo kinase is the founder member of a conserved kinase family required for multiple stages of mitosis. We assessed the ability of mouse Polo-like kinase 1 (Plk1) to perform the multiple mitotic functions of Polo kinase, by expressing a Plk1-GFP fusion in Drosophila. Consistent with the previously reported localization of Polo kinase, Plk1-GFP was strongly localized to centrosomes and recruited to the centromeric regions of condensing chromosomes during early mitosis. However, in contrast to a functional Polo-GFP fusion, Plk1-GFP failed to localize to the central spindle midzone in both syncytial embryo mitosis and the conventional mitoses of cellularized embryos and S2 cells. Moreover, unlike endogenous Polo kinase and Polo-GFP, Plk1-GFP failed to associate with the contractile ring. Expression of Plk1-GFP enhanced the lethality of hypomorphic polo mutants and disrupted the organization of the actinomyosin cytoskeleton in a dominant-negative manner. Taken together, our results suggest that endogenous Polo kinase has specific roles in regulating actinomyosin rearrangements during Drosophila mitoses that its mammalian counterpart, Plk1, cannot fulfill. Consistent with this hypothesis, we observed defects in the cortical recruitment of myosin and myosin regulatory light chain in Polo deficient cells.