The microtubule depolymerase MCAK influences chromosomal instability (CIN), but what controls its activity remains unclear. Bendre et al. show that GTSE1, a protein found overexpressed in tumors, regulates microtubule stability and chromosome alignment during mitosis by inhibiting MCAK. High levels of GTSE1 are linked to chromosome missegregation and CIN.

Clathrin ensures mitotic spindle stability and efficient chromosome alignment, independently of its vesicle trafficking function. Although clathrin localizes to the mitotic spindle and kinetochore fiber microtubule bundles, the mechanisms by which clathrin stabilizes microtubules are unclear. We show that clathrin adaptor interaction sites on clathrin heavy chain (CHC) are repurposed during mitosis to directly recruit the microtubule-stabilizing protein GTSE1 to the spindle. Structural analyses reveal that these sites interact directly with clathrin-box motifs on GTSE1. Disruption of this interaction releases GTSE1 from spindles, causing defects in chromosome alignment. Surprisingly, this disruption destabilizes astral microtubules, but not kinetochore-microtubule attachments, and chromosome alignment defects are due to a failure of chromosome congression independent of kinetochore–microtubule attachment stability. GTSE1 recruited to the spindle by clathrin stabilizes microtubules by inhibiting the microtubule depolymerase MCAK. This work uncovers a novel role of clathrin adaptor-type interactions to stabilize nonkinetochore fiber microtubules to support chromosome congression, defining for the first time a repurposing of this endocytic interaction mechanism during mitosis.

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Clathrin plays an important role to ensure mitotic spindle stability and efficient chromosome alignment, independently of its well-characterized functions in vesicle trafficking. While clathrin clearly localizes to the mitotic spindle and kinetochore-fiber microtubule bundles, the mechanisms by which clathrin stabilizes microtubules have remained elusive. Here we show that clathrin adaptor interaction sites on clathrin heavy chain (CHC) are repurposed during mitosis to directly recruit the microtubule-stabilizing protein GTSE1 to the mitotic spindle.Structural analyses reveal that multiple clathrin-box motifs on GTSE1 interact directly with different clathrin adaptor interaction sites on CHC, in a manner structurally analogous to that which occurs between adaptor proteins and CHC near membranes. Specific disruption of this interaction in cells releases GTSE1 from spindles and causes defects in chromosome alignment. Surprisingly, this disruption causes destabilization of astral microtubules, but not kinetochore-microtubule attachments, and the resulting chromosome alignment defect is due to a failure of chromosome congression independent of kinetochore-microtubule attachment stability. Finally, we show that GTSE1 recruited to the spindle by clathrin stabilizes 2 microtubules and promotes chromosome congression by inhibiting the activity of the microtubule depolymerase MCAK. This work thus uncovers a novel role of clathrin to stabilize non-kinetochore-fiber microtubules to support chromosome congression. This role is carried out via clathrin adaptor-type interactions of CHC with GTSE1, defining for the first time an important repurposing of this endocytic interaction mechanism during mitosis. Rizk et al., 2009; Walczak et al., 1996;Wordeman and Mitchison, 1995) (Moore et al., 2005). It was proposed that a function of TACC3 is to antagonize MCAK's potent activity at centrosomes to stabilize astral MTs, presumably by promoting activity of the microtubule depolymerase XMAP215/ch-Tog (Kinoshita et al., 2005). However, whether clathrin (or the CHC/TACC3 complex) is required for astral microtubule stabilization has not been addressed.Another potential functional member of the CHC/TACC3 complex is GTSE1 (pronounced "jitsee-one" (Monte et al., 2000)), an intrinsically disordered protein that has been shown by proteomic analysis to interact with the CHC/TACC3 complex in cells (Hubner et al., 2010). Two recent studies showed that GTSE1 localizes to the spindle during mitosis, and like TACC3, is necessary for astral microtubule stabilization and efficient chromosome alignment (Bendre et al., 2016;Tipton et al., 2017). We showed that GTSE1 stabilizes MTs in mitosis by inhibiting MCAK activity (Bendre et al., 2016). In that study, cancer cells either depleted or knocked-out of GTSE1 showed destabilization of both astral MTs and kinetochore-MT attachments.Another study reported that a specific "slow turnover" population of MTs become stabilized upon depletion of GTSE1(Tipton et al., 2017). The reason for these seemingly contr...

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