Molecular delineation of BubR1 kinase function in kinetochore microtubule attachments

Accurate segregation of chromosomes into two daughter cells is essential for cell health, which is governed by spindle checkpoint surveillance. The spindle checkpoint is a signaling cascade ensures the correct kinetochore‐microtubule attachment and prevents aberrant chromosome segregation. It has been shown that checkpoint kinase BubR1 stabilizes kinetochore‐microtubule attachment and facilitates correcting aberrant microtubule attachments. To dissect spatiotemporal property of BubR1 during cell division, a combination of a phenotype‐based screens judging kinetochore microtubule attachment with enzymatic assay was employed to identify compounds that affect mitotic checkpoint kinase BubR1. One compound, here named bubrestin, prevents chromosome alignment and stable kinetochore attachment without alteration of cell cycle progression through S and G2. The bubrestin‐treated cells exhibit aberrant kinetochore attachments such as syntelic and merotelic. A combination of bubrestin with proteomic analyses has uncovered a list of BubR1 substrates implicated in faithful kinetochore microtubule attachment. One candidate is TIP150, a newly characterized microtubule plus‐end tracking protein. Importantly, inhibition of BubR1 kinase activity perturbs the localization of TIP150 and plus‐end microtubule dynamics. Consistent with this finding, non‐phosphorylatable TIP150 failed to track plus‐end and exhibited a dominant negative phenotype. We propose that BubR1 links regulation of chromosome‐spindle attachment to microtubule plus‐end dynamics.