PP6 phosphatase and Elongator contribute to kinesin 5-dependent spindle assembly by controlling microtubule regulators levels.

Eukaryotic chromosome segregation relies on the assembly of a bipolar machinery based on microtubules (MTs), named the mitotic spindle. Formation of the mitotic spindle follows a force balance mechanism that ensures the proper capture and separation of sister chromatids. Many proteins have been involved in the establishment of this force balance, although kinesin 5 is well recognized as the major outward pushing force generator, since its inactivation results in monopolar, non-functional spindles. In order to find additional players in the force balance mechanism, we have performed a suppressor screen using a conditional allele of the fission yeast kinesin 5 ortholog Cut7. This screen identified that the lack of the PP6 phosphatase partially suppresses cut7 phenotypes, at least by defective translation of MT regulators, such as the minus end-directed kinesin Klp2, the MT stabilizer Alp7 and the MT bundler Ase1, impacting on the force balance mechanism. Additionally, our data show that the Elongator complex, a target activated by PP6 for efficient tRNA modification, also contributes to the force balance, albeit to a lesser extent. Importantly, this complex has recently been implicated in direct MT polymerization in metazoans, a role not shared by its fission yeast counterpart.