We report an entirely new role for the HSP70 chaperone in dissociating 265 proteasome complexes (into free 20S proteasomes and bound 19S regulators), preserving 19S regulators, and reconstituting 26S proteasomes in the first 1-3 h after mild oxidative stress. These responses, coupled with direct 20S proteasome activation by poly(ADP ribose) polymerase in the nucleus and by PA28 alpha beta in the cytoplasm, instantly provide cells with increased capacity to degrade oxidatively damaged proteins and to survive the initial effects of stress exposure. Subsequent adaptive (hormetic) processes (3-24 h after stress exposure), mediated by several signal transduction pathways and involving increased transcription/translation of 20S proteasomes, immunoproteasomes, and PA28 alpha beta, abrogate the need for 26S proteasome dissociation. During this adaptive period, HSP70 releases its bound 19S regulators, 26S proteasomes are reconstituted, and ATP-stimulated proteolysis is restored. The 265 proteasome-dependent, and ATP-stimulated, turnover of ubiquitinylated proteins is essential for normal cell metabolism, and its restoration is required for successful stress adaptation. (C) 2011 Elsevier Inc. All rights reserved.