The Kinetochore-Bound Ska1 Complex Tracks Depolymerizing Microtubules and Binds to Curved Protofilaments

Abstract

Summary To ensure equal chromosome segregation during mitosis, the macromolecular kinetochore must remain attached to depolymerizing microtubules, which drive chromosome movements. How kinetochores associate with depolymerizing microtubules, which undergo dramatic structural changes forming curved protofilaments, has yet to be defined in vertebrates. Here, we demonstrate that the conserved kinetochore-localized Ska1 complex tracks with depolymerizing microtubule ends and associates with both the microtubule lattice and curved protofilaments. In contrast, the Ndc80 complex, a central player in the kinetochore-microtubule interface, binds only to the straight microtubule lattice and lacks tracking activity. We demonstrate that the Ska1 complex imparts its tracking capability to the Ndc80 complex. Finally, we present a structure of the Ska1 microtubule binding domain that reveals its interaction with microtubules and its regulation by Aurora B. This work defines an integrated kinetochore-microtubule interface formed by the Ska1 and Ndc80 complexes that associates with depolymerizing microtubules, potentially by interacting with curved microtubule protofilaments.

CENP-A Ubiquitylation Is Inherited through Dimerization between Cell Divisions

“…In our study, overexpression of the monoubiquitin fusion protein Flag-CENP-A K124R-Ub (K48R) led to sufficient recruitment of HJURP and central-outer kinetochore components (i.e., CENP-I, HEC1, and SKA1) to noncentromeric chromatin regions. In particular, SKA1 recruitment on the ectopic centromere verified that the neocentromeres are functional, because SKA1 centromere localization requires the formation of kinetochore–microtubule interactions (Hanisch et al, 2006; Schmidt et al, 2012). In our experiment, SKA1-positive, putative neocentromeres replicated and were inherited epigenetically between cell divisions.…”

Regulation of Chromosome Speeds in Mitosis

“…There might, nonetheless, be an alternative device that functions in a manner analogous to the Dam1 complex. Currently, the best candidate is the Ska complex, which binds more tightly to the bent protofilaments of tubulin characteristic of a microtubule end than to the microtubule wall [49]. When Ska is microtubule-associated, it also binds near the N-terminus of Ndc80/Nuf2, the microtubule-binding end of the ubiquitous Ndc80 complex.…”

Centromeric Heterochromatin: The Primordial Segregation Machine

“…The Dam1 complex is essential in budding yeast and is one of the few components that is not evolutionarily conserved. The functional equivalent in mammalian kinetochores may be the Ska1 complex that binds depolymerizing microtubules and enhances the ability of Ndc80 to track microtubule ends (139, 173). The linkage to the chromatin is established via the CCAN complex (45, 150) and the centromere-specific histone H3 variant CENP-A (45, 145).…”