The proteasome activator REGγ, a new Chk2-interacting protein, regulates chromosome stability and centrosome amplification

Chk2 is a DNA damage-responsive checkpoint kinase with an oncosuppressor role, implicated in repair events, cell cycle arrest and apoptosis. We have identified the proteasome activator REGgamma (PSME3, PA28gamma) as a new Chk2-interacting protein in yeast two-hybrid screenings, and confirmed this association in vivo in human cells. Overexpression or depletion of REGgamma, however, had no effect on Chk2 protein levels, phosphorylation or catalytic activity in response to DNA damage induced by ionizing radiation (IR). REGgamma was expressed in all phases of the cell cycle with a diffuse nuclear localization, but in mitotic telophases showed a distinctive localization on chromosomes, hence suggesting a specific role for REGgamma in mitosis progression. In accordance with this, whereas REGgamma overexpression markedly suppressed the mitotic index, REGgamma depletion by RNA interference significantly increased it. Furthermore, human fibroblasts depleted of REGgamma protein by siRNA or shRNA, and primary cells from REGgamma-/- mice demonstrated a marked aneuploidy (chromosomal gains and losses), supernumerary centrosomes and multipolar spindles. These results underscore a previously uncharacterized function of the proteasome activator REGgamma in maintenance of chromosomal stability.