title: NOXA, a sensor of proteasome integrity, is degraded by 26S proteasomes by an ubiquitin-independent pathway that is blocked by MCL-1.
abstract: Ubiquitin (Ub)-mediated proteasome-dependent proteolysis is critical in regulating multiple biological processes including apoptosis. We show that the unstructured BH3-only protein, NOXA, is degraded by an Ub-independent mechanism requiring 19S regulatory particle (RP) subunits of the 26S proteasome, highlighting the possibility that other unstructured proteins reported to be degraded by 20S proteasomes in vitro may be bona fide 26S proteasome substrates in vivo. A lysine-less NOXA (NOXA-LL) mutant, which is not ubiquitinated, is degraded at a similar rate to wild-type NOXA. Myeloid cell leukemia 1, but not other anti-apoptotic BCL-2 family proteins, stabilizes NOXA by interaction with the NOXA BH3 domain. Depletion of 19S RP subunits, but not alternate proteasome activator REG subunits, increases NOXA half-life in vivo. A NOXA-LL mutant, which is not ubiquitinated, also requires an intact 26S proteasome for degradation. Depletion of the 19S non-ATPase subunit, PSMD1 induces NOXA-dependent apoptosis. Thus, disruption of 26S proteasome function by various mechanisms triggers the rapid accumulation of NOXA and subsequent cell death strongly implicating NOXA as a sensor of 26S proteasome integrity.
authors.affiliation: MRC Toxicology Unit, Hodgkin Building, University of Leicester, Lancaster Road, Leicester LE1 9HN, UK.
authors.forename: A
authors.initials: A
authors.lastname: Craxton
authors.forename: M
authors.initials: M
authors.lastname: Butterworth
authors.forename: N
authors.initials: N
authors.lastname: Harper
authors.forename: L
authors.initials: L
authors.lastname: Fairall
authors.forename: J
authors.initials: J
authors.lastname: Schwabe
authors.forename: A
authors.initials: A
authors.lastname: Ciechanover
authors.forename: G M
authors.initials: GM
authors.lastname: Cohen
mesh_terms: D017209:Apoptosis / Q000502:physiology; D006367:HeLa Cells; D006801:Humans; D020125:Mutation, Missense; D064549:Myeloid Cell Leukemia Sequence 1 Protein; D046988:Proteasome Endopeptidase Complex / Q000235:genetics / Q000378:metabolism*; D059748:Proteolysis; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000235:genetics / Q000378:metabolism*; D025801:Ubiquitin / Q000235:genetics / Q000378:metabolism*; D054875:Ubiquitination / Q000502:physiology
chemical_list: D064549:Myeloid Cell Leukemia Sequence 1 Protein; C498702:PMAIP1 protein, human; C484162:PSMD1 protein, human; D019253:Proto-Oncogene Proteins c-bcl-2; D025801:Ubiquitin; D046988:Proteasome Endopeptidase Complex; C061553:ATP dependent 26S protease