REPORT◥PROTEOSTASISUBE2OisaqualitycontrolfactorfororphansofmultiproteincomplexesKotaYanagitani,*SzymonJuszkiewicz,RamanujanS.Hegde†Manynascentproteinsareassembledintomultiproteincomplexesofdefinedstoichiometry.Imbalancesinthesynthesisofindividualsubunitsresultinorphans.Howorphansareselectivelyeliminatedtomaintainproteinhomeostasisispoorlyunderstood.Here,wefoundthattheconservedubiquitin-conjugatingenzymeUBE2Odirectlyrecognizedjuxtaposedbasicandhydrophobicpatchesonunassembledproteinstomediateubiquitinationwithoutaseparateubiquitinligase.Inreticulocytes,whereUBE2Oishighlyup-regulated,unassembleda-globinmoleculesthatfailedtoassemblewithb-globinwereselectivelyubiquitinatedbyUBE2O.Innonreticulocytes,ribosomalproteinsthatdidnotengagenuclearimportfactorsweretargetsforUBE2O.Thus,UBE2Oisaself-containedqualitycontrolfactorthatcomprisessubstraterecognitionandubiquitintransferactivitieswithinasingleproteintoefficientlytargetorphansofmultiproteincomplexesfordegradation.Cellshaveevolvedawiderangeofqualitycontrolpathwaystomonitorfailuresinproteinbiogenesisandpromptlytargetdefectiveproductsfordegradation(1,2).Ineffectivequalitycontrolisimplicatedinagingandcancauseneurodegeneration(3);con-versely,robustqualitycontrolmaybeespeciallycrucialincancertosupporthighratesofpro-teinsynthesisinthefaceofanaberrantgenomeandenvironmentalstresses(4).Thesuiteofproteinqualitycontrolpathwaysneededforcellularho-meostasisinmetazoansisincompletelydefined(1).Toidentifyadditionalcytosolicqualitycontrolpathways,wesoughtanaberrantproteinwhoseubiquitinationinacell-freecytosolicextractoc-curswithoutengagingknownqualitycontrolfactors.Atransmembrane(TM)domaininter-ruptedbythreebasicresidues(hereaftertermed3R)(fig.S1A)isnotrecognizedbyqualitycontrolfactorsspecializedformislocalizedmembraneproteins,suchasBAG6ortheubiquilinfamilymembers(5–7);yet,3Rwasubiquitinatedsim-ilarlytoaBAG6substrate(fig.S1B).Although3Risanartificialmutant,wereasonedthatmech-anisticdissectionofitsubiquitinationpathwaymightleadustoaqual...
