A role for Drosophila Polo protein in chromosome resolution and segregation during mitosis.

Correct chromosome structure is essential to ensure faithful segregation during mitosis. Chromosome condensation occurs at the same time as cohesion is released from the arms of the sister chromatids. It is not until metaphase-anaphase transition that chromosomes lose cohesion completely, by proteolysis of the component of the cohesin complex Scc1 (Sister chromatid cohesion 1). It has been shown in vertebrates that the Polo-like kinase, Plk1, is important for this process by inducing the destabilization of Scc1 from the chromosome arms. It is still unclear if this process is conserved in other high eukaryotes, namely in Drosophila. Here we analysed the consequences over chromosome resolution of the downregulation of Drosophila Polo, both by mutant analysis and by RNAi-depletion in S2 cells. We show that the depletion of Polo results in a strong prometa/metaphase arrest with the spindle checkpoint activated in response to lack of tension. In addition, the checkpoint protein ROD fails to stream over the kinetochore microtubules in the lack of Polo activity. We also show that loss of Polo causes strong defects in chromosome resolution, a phenotype we partially rescued by depleting Scc1. Importantly, we show Scc1 fails to accumulate on the kinetochores during mitosis and remains on the chromosome arms in the absence of Polo. We therefore propose an alternative role for Drosophila Polo in Scc1 redistribution during mitosis.