Phosphatase-Regulated Recruitment of the Spindle and Kinetochore Associated (SKA) Complex to Kinetochores

ABSTRACT Kinetochores move chromosomes on dynamic spindle microtubules and regulate cell cycle progression by signaling the spindle checkpoint. The Spindle and Kinetochore-Associated (Ska) Complex, a hexamer composed of two copies of Ska1, Ska2 and Ska3, participates in both roles. The mitotic kinases, Cdk1, Aurora B, Plk1, Mps1 and Bub1 play key, overlapping tasks in regulating chromosome movement and checkpoint signaling. However, roles for the phosphatases that oppose these kinases are more poorly defined. Recently, we showed that Ska1 is important for recruiting protein phosphatase 1 (PP1) to kinetochores. Here we show that PP1 and protein phosphatase 2A (PP2A) both promote accumulation of Ska at kinetochores. Depletion of PP1 or PP2A by siRNA reduces Ska binding at kinetochores, impairs alignment of chromosomes to the spindle midplane, and causes metaphase delay or arrest, phenotypes also seen after depletion of Ska. Tethering of PP1 to the kinetochore protein Nuf2 promotes Ska recruitment to kinetochores, and reduces mitotic defects seen after Ska depletion. We propose that kinetochore-associated phosphatases generate a positive feedback cycle to reinforce Ska complex accumulation and function at kinetochores. SUMMARY STATEMENT Phosphatases reinforce recruitment of the Ska complex at kinetochores to stabilize microtubule attachment and oppose spindle checkpoint signaling.