title: Processing of autophagic protein LC3 by the 20S proteasome.
abstract: Ubiquitin-proteasome system and autophagy are the two major mechanisms for protein degradation in eukaryotic cells. LC3, a ubiquitin-like protein, plays an essential role in autophagy through its ability to be conjugated to phosphatidylethanolamine. In this study, we discovered a novel LC3-processing activity, and biochemically purified the 20S proteasome as the responsible enzyme. Processing of LC3 by the 20S proteasome is ATP- and ubiquitin-independent, and requires both the N-terminal helices and the ubiquitin fold of LC3; addition of the N-terminal helices of LC3 to the N terminus of ubiquitin renders ubiquitin susceptible to 20S proteasomal activity. Further, the 20S proteasome processes LC3 in a stepwise manner, it first cleaves LC3 within its ubiquitin fold and thus disrupts the conjugation function of LC3; subsequently and especially at high concentrations of the proteasome, LC3 is completely degraded. Intriguingly, proteolysis of LC3 by the 20S proteasome can be inhibited by p62, an LC3-binding protein that mediates autophagic degradation of polyubiquitin aggregates in cells. Therefore, our study implicates a potential mechanism underlying interplay between the proteasomal and autophagic pathways. This study also provides biochemical evidence suggesting relevance of the controversial ubiquitin-independent proteolytic activity of the 20S proteasome.
authors.affiliation: Cell Biology Program, Memorial Sloan-Kettering Cancer Center, New York, NY, USA.
authors.forename: Zhonghua
authors.initials: Z
authors.lastname: Gao
authors.forename: Noor
authors.initials: N
authors.lastname: Gammoh
authors.forename: Pui-Mun
authors.initials: PM
authors.lastname: Wong
authors.forename: Hediye
authors.initials: H
authors.lastname: Erdjument-Bromage
authors.forename: Paul
authors.initials: P
authors.lastname: Tempst
authors.forename: Xuejun
authors.initials: X
authors.lastname: Jiang
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000378:metabolism / Q000502:physiology; D000818:Animals; D001343:Autophagy / Q000187:drug effects / Q000502:physiology; D002457:Cell Extracts / Q000494:pharmacology; D002478:Cells, Cultured; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D006367:HeLa Cells; D006801:Humans; D007976:Leupeptins / Q000494:pharmacology; D051379:Mice; D008869:Microtubule-Associated Proteins / Q000737:chemistry / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism* / Q000502:physiology; D061988:Proteasome Inhibitors; D011485:Protein Binding; D017510:Protein Folding / Q000187:drug effects; D011499:Protein Processing, Post-Translational / Q000187:drug effects; D017434:Protein Structure, Tertiary / Q000187:drug effects; D000071456:Sequestosome-1 Protein; D013379:Substrate Specificity; D025801:Ubiquitin / Q000737:chemistry / Q000378:metabolism; D054875:Ubiquitination / Q000187:drug effects
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D002457:Cell Extracts; D015853:Cysteine Proteinase Inhibitors; D007976:Leupeptins; C477012:MAP1LC3A protein, human; D008869:Microtubule-Associated Proteins; D061988:Proteasome Inhibitors; C099718:SQSTM1 protein, human; D000071456:Sequestosome-1 Protein; D025801:Ubiquitin; D046988:Proteasome Endopeptidase Complex; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde