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p62- and ubiquitin-dependent stress-induced autophagy of the mammalian 26S proteasome
Cohen-Kaplan, Victoria,Livneh, Ido,Avni, Noa,Fabre, Bertrand,Ziv, Tamar,Kwon, Yong Tae,Ciechanover, Aaron National Academy of Sciences 2016 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF Vol.113 No.47
<P>The ubiquitin-proteasome system and autophagy are the two main proteolytic systems involved in, among other functions, the maintenance of cell integrity by eliminating misfolded and damaged proteins and organelles. Both systems remove their targets after their conjugation with ubiquitin. An interesting, yet incompletely understood problem relates to the fate of the components of the two systems. Here we provide evidence that amino acid starvation enhances polyubiquitination on specific sites of the proteasome, a modification essential for its targeting to the autophagic machinery. The uptake of the ubiquitinated proteasome is mediated by its interaction with the ubiquitin-associated domain of p62/SQSTM1, a process that also requires interaction with LC3. Importantly, deletion of the PB1 domain of p62, which is important for the targeting of ubiquitinated substrates to the proteasome, has no effect on stress-induced autophagy of this proteolytic machinery, suggesting that the domain of p62 that binds to the proteasome determines the function of p62 in either targeting substrates to the proteasome or targeting the proteasome to autophagy.</P>
KPC1-Mediated Ubiquitination and Proteasomal Processing of NF-κB1 p105 to p50 Restricts Tumor Growth
Kravtsova-Ivantsiv, Y.,Shomer, I.,Cohen-Kaplan, V.,Snijder, B.,Superti-Furga, G.,Gonen, H.,Sommer, T.,Ziv, T.,Admon, A.,Naroditsky, I.,Jbara, M.,Brik, A.,Pikarsky, E.,Kwon, Y.,Doweck, I.,Ciechanover, Cell Press ; MIT Press 2015 Cell Vol.161 No.2
NF-κB is a key transcriptional regulator involved in inflammation and cell proliferation, survival, and transformation. Several key steps in its activation are mediated by the ubiquitin (Ub) system. One uncharacterized step is limited proteasomal processing of the NF-κB1 precursor p105 to the p50 active subunit. Here, we identify KPC1 as the Ub ligase (E3) that binds to the ankyrin repeats domain of p105, ubiquitinates it, and mediates its processing both under basal conditions and following signaling. Overexpression of KPC1 inhibits tumor growth likely mediated via excessive generation of p50. Also, overabundance of p50 downregulates p65, suggesting that a p50-p50 homodimer may modulate transcription in place of the tumorigenic p50-p65. Transcript analysis reveals increased expression of genes associated with tumor-suppressive signals. Overall, KPC1 regulation of NF-κB1 processing appears to constitute an important balancing step among the stimulatory and inhibitory activities of the transcription factor in cell growth control.