SummaryTransforming acidic coiled-coil-containing protein 3 (TACC3) has been implicated in mitotic spindle assembly, although the mechanisms involved are largely unknown. Here we identify that clathrin heavy chain (CHC) binds specifically to phosphorylated TACC3 and recruits it to spindle poles for proper spindle assembly and chromosome alignment. Phosphorylation of Xenopus TACC3 at serine 620 (S620) and S626, but not S33, is required for its binding with CHC. Knockdown of CHC by RNA interference (RNAi) abolishes the targeting of TACC3 to spindle poles and results in abnormal spindle assembly and chromosome misalignment, similar to the defects caused by TACC3 knockdown. Furthermore, the binding of CHC with phosphorylated TACC3 is inhibited by importin  and this inhibition is reversed by the presence of the GTP-binding nuclear protein Ran in the GTP-bound state. Together, these results indicate that the recruitment of phosphorylated TACC3 to spindle poles by CHC ensures proper spindle assembly and chromosome alignment, and is regulated by Ran.

Mammalian target of rapamycin (mTOR), a serine/threonine protein kinase and a master regulator of cell growth and metabolism, forms two structurally and functionally distinct complexes, mTOR complex 1 (mTORC1) and mTORC2. While mTORC1 signaling is well characterized, mTORC2 is relatively poorly understood. mTORC2 appears to exist in functionally distinct pools, but few mTORC2 effectors/substrates have been identified. Here, we review recent advances in our understanding of mTORC2 signaling, with particular emphasis on factors that control mTORC2 activity.

Centrosome duplication is critical for cell division, and genome instability can result if duplication is not restricted to a single round per cell cycle. Centrosome duplication is controlled in part by CP110, a centriolar protein that positively regulates centriole duplication while restricting centriole elongation and ciliogenesis. Maintenance of normal CP110 levels is essential, since excessive CP110 drives centrosome over-duplication and suppresses ciliogenesis, whereas its depletion inhibits centriole amplification and leads to highly elongated centrioles and aberrant assembly of cilia in growing cells1,2. CP110 levels are tightly controlled in part through SCFcyclin F-mediated ubiquitylation during G2 and M phase of the cell cycle3. Here we report a new mechanism for regulation of centrosome duplication that requires USP33, a de-ubiquitylating enzyme (DUB) able to regulate CP110 levels. USP33 interacts with CP110 and localizes to centrioles primarily during S and G2/M phase, the period during which centrioles duplicate and elongate. USP33 potently and specifically de-ubiquitylates CP110, but not other cyclin F substrates. USP33 activity antagonizes SCFcyclin F-mediated ubiquitylation and promotes generation of supernumerary centriolar foci, whereas ablation of USP33 destabilizes CP110 and thereby inhibits centrosome amplification and mitotic defects. To our knowledge, these studies have identified the first centriolar de-ubiquitinating enzyme whose expression regulates centrosome homeostasis by countering cyclin F-mediated destruction of a key substrate and suggest potential therapeutic strategies for inhibiting tumorigenesis associated with centrosome amplification.