Polo regulates Spindly to prevent premature stabilization of kinetochore–microtubule attachments

Accurate chromosome segregation in mitosis requires sister kinetochores to bind to microtubules from opposite spindle poles. The stability of kinetochore–microtubule attachments is fine‐tuned to prevent or correct erroneous attachments while preserving amphitelic interactions. Polo kinase has been implicated in both stabilizing and destabilizing kinetochore–microtubule attachments. However, the mechanism underlying Polo–destabilizing activity remains elusive. Here, resorting to an RNA i screen in Drosophila for suppressors of a constitutively active Polo mutant, we identified a strong genetic interaction between Polo and the Rod– ZW 10–Zwilch ( RZZ ) complex, whose kinetochore accumulation has been shown to antagonize microtubule stability. We find that Polo phosphorylates Spindly and impairs its ability to bind to Zwilch. This precludes dynein‐mediated removal of the RZZ from kinetochores and consequently delays the formation of stable end‐on attachments. We propose that high Polo‐kinase activity following mitotic entry directs the RZZ complex to minimize premature stabilization of erroneous attachments, whereas a decrease in active Polo in later mitotic stages allows the formation of stable amphitelic spindle attachments. Our findings demonstrate that Polo tightly regulates the RZZ –Spindly–dynein module during mitosis to ensure the fidelity of chromosome segregation.