Fission yeast CENP-S/X regulates the spindle assembly checkpoint and faithful chromosome segregation.

The outer kinetochore serves as a platform for initiating the spindle assembly checkpoint and for mediating kinetochore-microtubule attachments. How the inner kinetochore subcomplex CENP-S/CENP-X is involved in regulating the spindle assembly checkpoint and kinetochore-microtubule attachments has not been well characterized. Employing live-cell microscopy and yeast genetics, we found that the fission yeast CENP-S/CENP-X counterpart Mhf1/Mhf2 plays crucial roles in promoting the spindle assembly checkpoint and regulating chromosome segregation. The absence of Mhf2 attenuates the spindle assembly checkpoint, impairs the kinetochore localization of most of the components in the constitutive centromere-associated network (CCAN), and alters the localization of the kinase Aurora-B/Ark1 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 that is required for promoting SAC signaling and for regulating kinetochore-microtubule attachments.