Drosophila Mis12 complex acts as a single functional unit essential for anaphase chromosome movement and a robust spindle assembly checkpoint

The kinetochore is a dynamic multiprotein complex assembled at the centromere in mitosis. Exactly how the structure of the kinetochore changes during mitosis and how its individual components contribute to chromosome segregation is largely unknown. Here we have focused on the contribution of the Mis12 complex to kinetochore assembly and function throughout mitosis in Drosophila. We show that despite the sequential kinetochore recruitment of Mis12 complex subunits Mis12 and Nsl1, the complex acts as a single functional unit. mis12 and nsl1 mutants show strikingly similar developmental and mitotic defects in which chromosomes are able to congress at metaphase, but their anaphase movement is strongly affected. While kinetochore association of Ndc80 absolutely depends on both Mis12 and Nsl1, BubR1 localization shows only partial dependency. In the presence of residual centromeric BubR1 the checkpoint still responds to microtubule depolymerization but is significantly weaker. These observations point to a complexity of the checkpoint response that may reflect subpopulations of BubR1 associated with residual kinetochore components, the core centromere, or elsewhere in the cell. Importantly our results indicate that core structural elements of the inner plate of the kinetochore have a greater contribution to faithful chromosome segregation in anaphase than in earlier stages of mitosis.