PP6 phosphatase and Elongator contribute to kinesin 5-dependent spindle assembly by controlling microtubule regulator 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, impacting on the force balance mechanism. Additionally, our data show that the Elongator complex, a target regulated by PP6 involved in tRNA modification, also ensures the force balance, albeit to a lesser extent. Importantly, this complex has been recently involved in direct MT polymerization in metazoans, a role probably not shared by its fission yeast counterpart.