Int6, a New Regulator of Metaphase-Anaphase Transition in Drosophila S2 Cells

We identified int6 in recent RNAi-based screen aimed at the identification of new Drosophila mitotic genes. int6 encodes a protein product that is a component of the translation initiation complex and is probably also part of both the 26S proteasome and the COP9 signalosome. RNAi-mediated depletion of Int6 in Drosophila S2 cells results in a metaphase arrest phenotype, with short spindles and elongated/distorted centromere/kinetochore regions. To elucidate the role of Int6 we performed double RNAi experiments to simultaneously deplete both Int6 and another protein required for spindle formation. We found that co-depletion of the MT-interacting kinetochore component Ndc80 or the kinesin-type MT depolymerases Klp10A and Klp67A, or the spindle assembly checkpoint (SAC) proteins Mad2 and BubR1 rescues the phenotypes elicited by Int6 single depletion. These results indicate that the int6 mutant phenotype requires a functional SAC and is partially suppressed when the kinetochore-MT interaction or the MT flux are inhibited. We thus propose that the int6 phenotype results MT-mediated tension at the kinetochore in the presence of a SAC mechanism that persistently blocks metaphase-anaphase transition.