The outer kinetochore serves as a platform for the initiation of the spindle assembly checkpoint (SAC) and for mediating kinetochore–microtubule attachments. How the inner kinetochore subcomplex CENP-S–CENP-X is involved in regulating the SAC and kinetochore–microtubule attachments has not been well characterized. Using live-cell microscopy and yeast genetics, we found that Mhf1–Mhf2, the CENP-S–CENP-X counterpart in the fission yeast Schizosaccharomyces pombe, plays crucial roles in promoting the SAC and regulating chromosome segregation. The absence of Mhf2 attenuates the SAC, impairs the kinetochore localization of most of the components in the constitutive centromere-associated network (CCAN), and alters the localization of the kinase Ark1 (yeast homolog of Aurora B) to the kinetochore. Hence, our findings constitute a model in which Mhf1–Mhf2 ensures faithful chromosome segregation by regulating the accurate organization of the CCAN complex, which is required for promoting SAC signaling and for regulating kinetochore–microtubule attachments.
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