title: The base of the proteasome regulatory particle exhibits chaperone-like activity.
abstract: Protein substrates of the proteasome must apparently be unfolded and translocated through a narrow channel to gain access to the proteolytic active sites of the enzyme. Protein folding in vivo is mediated by molecular chaperones. Here, to test for chaperone activity of the proteasome, we assay the reactivation of denatured citrate synthase. Both human and yeast proteasomes stimulate the recovery of the native structure of citrate synthase. We map this chaperone-like activity to the base of the regulatory particle of the proteasome, that is, to the ATPase-containing assembly located at the substrate-entry ports of the channel. Denatured but not native citrate synthase is bound by the base complex. Ubiquitination of citrate synthase is not required for its binding or refolding by the base complex of the proteasome. These data suggest a model in which ubiquitin-protein conjugates are initially tethered to the proteasome by specific recognition of their ubiquitin chains; this step is followed by a nonspecific interaction between the base and the target protein, which promotes substrate unfolding and translocation.
authors.affiliation: Institut für Biochemie, Medizinische Fakultät, Humboldt Universität zu Berlin, Charité, Germany.
authors.forename: B C
authors.initials: BC
authors.lastname: Braun
authors.forename: M
authors.initials: M
authors.lastname: Glickman
authors.forename: R
authors.initials: R
authors.lastname: Kraft
authors.forename: B
authors.initials: B
authors.lastname: Dahlmann
authors.forename: P M
authors.initials: PM
authors.lastname: Kloetzel
authors.forename: D
authors.initials: D
authors.lastname: Finley
authors.forename: M
authors.initials: M
authors.lastname: Schmidt
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000595:Amino Acid Sequence; D020134:Catalytic Domain; D002950:Citrate (si)-Synthase / Q000737:chemistry / Q000378:metabolism; D003546:Cysteine Endopeptidases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D004793:Enzyme Reactivators / Q000378:metabolism; D006801:Humans; D066298:In Vitro Techniques; D018832:Molecular Chaperones / Q000737:chemistry / Q000378:metabolism*; D008969:Molecular Sequence Data; D009097:Multienzyme Complexes / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex; D017510:Protein Folding; D020673:Protein Renaturation; D012441:Saccharomyces cerevisiae / Q000201:enzymology; D013379:Substrate Specificity; D014452:Ubiquitins / Q000378:metabolism
chemical_list: D004793:Enzyme Reactivators; D018832:Molecular Chaperones; D009097:Multienzyme Complexes; D014452:Ubiquitins; D000255:Adenosine Triphosphate; D002950:Citrate (si)-Synthase; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex