This study demonstrates that the Ska complex and the Hec1 tail domain contribute to kinetochore–microtubule attachment regulation independently. The Hec1 tail is shown to be dispensable for Ska complex recruitment to kinetochores and for formation of kinetochore–microtubule attachments, but required for wild-type force generation at kinetochores.

Successful mitotic cell division is critically dependent on the formation of correct attachments between chromosomes and spindle microtubules. Microtubule attachments are mediated by kinetochores, which are large proteinaceous structures assembled on centromeric chromatin of mitotic chromosomes. These attachments must be sufficiently stable to transduce force; however, the strength of these attachments are also tightly regulated to ensure timely, error-free progression through mitosis. The highly conserved, kinetochore-associated NDC80 complex is a core component of the kinetochore-microtubule attachment machinery in eukaryotic cells. A small, disordered region within the Hec1 subunit of the NDC80 complex-the N-terminal "tail" domain-has been actively investigated during the last decade due to its roles in generating and regulating kinetochore-microtubule attachments. In this review, we discuss the role of the NDC80 complex, and specifically the Hec1 tail domain, at the kinetochore-microtubule interface, and how recent studies provide a more unified view of Hec1 tail domain function.

Tubulin dimers assemble into a dynamic microtubule network throughout the cell. Microtubule dynamics and network organization must be precisely tuned for the microtubule cytoskeleton to regulate a dazzling array of dynamic cell behaviors. Since tubulin concentration determines microtubule growth, we studied here a novel regulatory mechanism of microtubule dynamics: local tubulin condensation. We discovered that two microtubule tip-binding proteins, CLIP-170 and EB3, undergo phase separation and form an EB3/CLIP-170 droplet at the growing microtubule tip. We prove that this +TIP-droplet has the capacity to locally condense tubulin. This process of tubulin co-condensation is spatially initiated at the microtubule tip and temporally regulated to occur only when there is tip growth. Tubulin condensation at the growing microtubule tip increases growth speeds three-fold and strongly reduces depolymerization events. With this work we establish a new mechanism to regulate microtubule dynamics by enrichment of tubulin at strategically important locations: the growing microtubule tips.

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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 kinetochoremicrotubule 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 complexmicrotubule interface. however, is required for high affinity binding of NDC80 complexes to microtubules in vitro (Wei et al., 2007; Ciferri et al., 2008;Miller et al., 2008; Lampert et al., 2013;Umbreit et al., 2012; Alushin et al., 2012; Cheerambathur et al., 2013;Zaytsev et al., 2015), suggesting that cellular factors likely compensate for Hec1 tail domain functions to varying degrees in different organisms.In addition to generating attachments to spindle microtubules, kinetochores also regulate their stability. In early mitosis attachments are labile and undergo rapid turnover, whereas in late mitosis,