Degradation rates of most
proteins in eukaryotic cells are determined by their rates of ubiquitination. However, possible regulation of the
proteasome's capacity to degrade
ubiquitinated proteins has received little attention, although
proteasome inhibitors are widely used in research and
cancer treatment. We show here that mammalian 26S proteasomes have five associated
ubiquitin ligases and that multiple
proteasome subunits are ubiquitinated in cells, especially the
ubiquitin receptor subunit, Rpn13. When proteolysis is even partially inhibited in cells or purified 26S proteasomes with various inhibitors, Rpn13 becomes extensively and selectively poly-ubiquitinated by the
proteasome-associated
ubiquitin ligase, Ube3c/Hul5. This modification also occurs in cells during heat-shock or
arsenite treatment, when poly-
ubiquitinated proteins accumulate. Rpn13 ubiquitination strongly decreases the
proteasome's ability to bind and degrade
ubiquitin-conjugated
proteins, but not its activity against
peptide substrates. This autoinhibitory mechanism presumably evolved to prevent binding of
ubiquitin conjugates to defective or stalled proteasomes, but this modification may also be useful as a
biomarker indicating the presence of proteotoxic stress and reduced proteasomal capacity in cells or patients.