title: Evidence for the existence of a non-catalytic modifier site of peptide hydrolysis by the 20 S proteasome.
abstract: The 20 S proteasome is an endoprotease complex that preferentially cleaves peptides C-terminal of hydrophobic, basic, and acidic residues. Recently, we showed that these specific activities, classified as chymotrypsin-like, trypsin-like, and peptidylglutamyl peptide-hydrolyzing (PGPH) activity, are differently affected by Ritonavir, an inhibitor of human immunodeficiency virus-1 protease. Ritonavir competitively inhibited the chymotrypsin-like activity, whereas the trypsin-like activity was enhanced. Here we demonstrate that the Ritonavir-mediated up-regulation of the trypsin-like activity is not affected by specific active site inhibitors of the chymo-trypsin-like and PGPH activity. Moreover, we show that the mutual regulation of chymotrypsin-like and PGPH activities by their substrates as described previously by a "cyclical bite-chew" model is not affected by selective inhibitors of the respective active sites. These data challenge the bite-chew model and suggest that effectors of proteasome activity can act by binding to non-catalytic sites. Accordingly, we propose a kinetic "two-site modifier" model that assumes that the substrate (or effector) may bind to an active site as well as to a second non-catalytic modifier site. This model appears to be valid as it describes the complex kinetic effects of Ritonavir very well. Since Ritonavir partially inhibits major histocompatibility complex class I restricted antigen presentation, the postulated modifier site may be required to coordinate the active centers of the proteasome for the production of class I peptide ligands.
authors.affiliation: Research Department, Cantonal Hospital St. Gall, CH-9007 St. Gallen, Switzerland.
authors.forename: G
authors.initials: G
authors.lastname: Schmidtke
authors.forename: S
authors.initials: S
authors.lastname: Emch
authors.forename: M
authors.initials: M
authors.lastname: Groettrup
authors.forename: H G
authors.initials: HG
authors.lastname: Holzhutter
mesh_terms: D000818:Animals; D001665:Binding Sites; D003546:Cysteine Endopeptidases / Q000737:chemistry* / Q000378:metabolism; D004789:Enzyme Activation; D006801:Humans; D006868:Hydrolysis; D008099:Liver / Q000201:enzymology; D051379:Mice; D009097:Multienzyme Complexes / Q000737:chemistry* / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex; D013379:Substrate Specificity
chemical_list: D009097:Multienzyme Complexes; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex