Coordination of NDC80 and Ska complexes at the kinetochore-microtubule interface in human cells

The conserved kinetochore-associated NDC80 complex (comprised of Hec1/Ndc80, Nuf2, Spc24, and Spc25) has well-documented roles in mitosis including (1) connecting mitotic chromosomes to spindle microtubules to establish force-transducing kinetochore-microtubule attachments, and (2) regulating the binding strength between kinetochores and microtubules such that correct attachments are stabilized and erroneous attachments are released. Although the NDC80 complex plays a central role in forming and regulating attachments to microtubules, additional factors support these processes as well, including the spindle and kinetochore-associated (Ska) complex. Multiple lines of evidence suggest that Ska complexes strengthen attachments by increasing the ability of NDC80 complexes to bind microtubules, especially to depolymerizing microtubule plus-ends, but how this is accomplished remains unclear. Using cell-based and in vitro assays, we demonstrate that the Hec1 tail domain is dispensable for Ska complex recruitment to kinetochores and for generation of kinetochore-microtubule attachments in human cells. We further demonstrate that Hec1 tail phosphorylation regulates kinetochore-microtubule attachment stability independently of the Ska complex. Finally, we map the location of the Ska complex in cells to a region near the coiled-coil domain of the NDC80 complex, and demonstrate that this region is required for Ska complex recruitment to the NDC80 complex-microtubule interface.