M phase-specific kinetochore proteins in fission yeast: microtubule-associating Dis1 and Mtc1 display rapid separation and segregation during anaphase.

Kinetochore microtubules are made early in mitosis and link chromosomal kinetochores to the spindle poles. They are required later to move the separated sister chromatids toward the opposite poles upon the onset of anaphase. Very little is known about proteins that are responsible for the connection between kinetochores and mitotic microtubules. We here show that fission yeast Dis1 and the related protein Mtc1/Alp14 are both able to bind microtubules in vitro and share an essential function for viability in vivo. The deletion of mtc1+ results in an instability of cytoplasmic microtubules that can be suppressed by the ectopic expression of dis1+. Dis1 and Mtc1 are localized along interphase cytoplasmic microtubules and are mobilized onto the spindle upon mitotic commitment. In chromatin immunoprecipitation (CHIP) experiments Dis1 coprecipitated with the central centromeric DNA in an M phase-specific manner. Consistently, observations of both living cells in which the native, genomic copy of dis1+ tagged with GFP and cells fixed by immunostaining established that Dis1 behaves as a kinetochore protein during the progression from metaphase to anaphase. The central and C-terminal regions of Dis1 are sufficient for interactions with microtubules and the kinetochore, respectively. In anaphase, the GFP signals of both Dis1 and Mtc1 suddenly separate and move quickly toward opposite spindle poles. Fission yeast Dis1 and Mtc1 are members of an evolutionarily conserved microtubule binding protein family that includes frog XMAP215. Dis1 and Mtc1 are implicated in stabilizing kinetochore microtubules in metaphase and so counteract the action of microtubule destabilizing factors that dominate in anaphase. Dis1 may play a dual role by becoming a part of the kinetochores in an M phase-specific manner, and it may possibly generate connections between kinetochores and microtubules.