Ubiquitin- and ATP-independent proteolytic turnover of p21 by the REGgamma-proteasome pathway.

We previously demonstrated that the proteasome activator REGgamma 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 REGgamma proteasome revealed that p21(Waf/Cip1), a central cyclin-dependent kinase inhibitor, is another endogenous target. Gain-of-function analysis, RNAi knockdown, REGgamma-deficient MEF analysis, and pulse-chase experiments substantiate that REGgamma promotes degradation of unbound p21. Cell-free proteasome proteolysis assays using purified REGgamma, p21, and the 20S proteasome confirm that REGgamma directly mediates degradation of free p21 in an ATP- and ubiquitin-independent manner. Depletion of REGgamma 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, REGgamma also has a role in the regulation of the cell cycle through its ability to influence the level of a cell-cycle regulator(s).