Ubiquitin- and ATP-Independent Proteolytic Turnover of p21 by the REGγ-Proteasome Pathway

SummaryWe previously demonstrated that the proteasome activator REGγ directs degradation of the steroid receptor coactivator SRC-3 by the 20S proteasome in an ATP- and ubiquitin-independent manner. Our efforts to identify additional endogenous direct targets of the REGγ proteasome revealed that p21Waf/Cip1, a central cyclin-dependent kinase inhibitor, is another endogenous target. Gain-of-function analysis, RNAi knockdown, REGγ-deficient MEF analysis, and pulse-chase experiments substantiate that REGγ promotes degradation of unbound p21. Cell-free proteasome proteolysis assays using purified REGγ, p21, and the 20S proteasome confirm that REGγ directly mediates degradation of free p21 in an ATP- and ubiquitin-independent manner. Depletion of REGγ in a thyroid carcinoma cell line results in cell-cycle and proliferative alterations. Our study reveals that, in addition to degrading the SRC-3 growth coactivator, REGγ also has a role in the regulation of the cell cycle through its ability to influence the level of a cell-cycle regulator(s).