@entity71158 Controls Mitotic Microtubule Generation by Stabilizing the XXXX Complex.

@entity238 @entity59867 @entity44073 @entity71158
UNASSIGNED: Mitotic spindles are primarily composed of microtubules (MTs), generated by polymerization of a- and b- @entity44073 hetero-dimers [1, 2]. Tubulins undergo a series of protein folding and post-translational modifications in order to fulfill their functions [3, 4]. Defects in @entity44073 polymerization dramatically affect spindle formation and disrupt chromosome segregation. We recently described a role for the product of the conserved @entity71158 (mst) gene in regulating mitotic MT generation in flies [5], but the molecular function of Mst remains unknown. Here, we use affinity purification mass spectrometry (AP-MS) to identify interacting partners of Mst in the @entity238 embryo. We demonstrate that Mst associates stoichiometrically with the hetero-octameric @entity59867 ( @entity59867 ) complex, with the hetero-hexameric @entity44073 Prefoldin complex, and with proteins having conserved roles in generating MT-competent @entity44073 . We show that RNAi-mediated in  vivo depletion of any @entity59867 subunit phenocopies the effects of mutations in mst or the Prefoldin-encoding gene merry-go-round (mgr), leading to monopolar and disorganized mitotic spindles containing few MTs. Crucially, we demonstrate that Mst, but not Mgr, is required for @entity59867 complex stability and that both the efficiency of @entity44073 polymerization and @entity44073 stability are drastically compromised in mst mutants. Moreover, our structural bioinformatic analyses indicate that Mst resembles the three-dimensional structure of @entity44073 monomers and might therefore occupy the @entity59867 complex central cavity. Collectively, our results suggest that Mst acts as a co-factor of the @entity59867 complex, playing an essential role in the @entity44073 -folding processes required for proper assembly of spindle MTs.
