title: Proteasomes can either generate or destroy MHC class I epitopes: evidence for nonproteasomal epitope generation in the cytosol.
abstract: Proteasomes have been implicated in the production of the majority of peptides that associate with MHC class I molecules. We used two different proteasome inhibitors, the peptide aldehyde N-acetyl-L-leucyl-L-leucyl-L-norleucinal (LLnL) and the highly specific inhibitor lactacystin, to examine the role of proteasomes in generating peptide epitopes associated with HLA-A*0201. Neither LLnL nor lactacystin was able to completely block the expression of the HLA-A*0201. Furthermore, the effects of LLnL and lactacystin on the expression of different categories of specific epitopes, TAP independent vs TAP dependent and derived from either cytosolic or membrane proteins, were assessed. As predicted, presentation of two TAP-dependent epitopes was blocked by LLnL and lactacystin, while a TAP-independent epitope that is processed in the endoplasmic reticulum was unaffected by either inhibitor. Surprisingly, both LLnL and lactacystin increased rather than inhibited the expression of a cytosolically transcribed and TAP-dependent peptide from the influenza A virus M1 protein. Mass spectrometric analyses of in vitro proteasome digests of a synthetic 24 mer containing this epitope revealed no digestion products of any length that included the intact epitope. Instead, the major species resulted from cleavage sites within the epitope. Although cleavage at these sites was inhibitable by LLnL and lactacystin, epitope-containing species were still not produced. We conclude that proteasomes may in some cases actually destroy epitopes that would otherwise be destined for presentation by class I molecules. These results suggest that some epitopes are generated by nonproteasomal proteases in the cytosol.
authors.affiliation: Department of Microbiology, Beirne Carter Center for Immunology Research, University of Virginia, Charlottesville 22908, USA.
authors.forename: C J
authors.initials: CJ
authors.lastname: Luckey
authors.forename: G M
authors.initials: GM
authors.lastname: King
authors.forename: J A
authors.initials: JA
authors.lastname: Marto
authors.forename: S
authors.initials: S
authors.lastname: Venketeswaran
authors.forename: B F
authors.initials: BF
authors.lastname: Maier
authors.forename: V L
authors.initials: VL
authors.lastname: Crotzer
authors.forename: T A
authors.initials: TA
authors.lastname: Colella
authors.forename: J
authors.initials: J
authors.lastname: Shabanowitz
authors.forename: D F
authors.initials: DF
authors.lastname: Hunt
authors.forename: V H
authors.initials: VH
authors.lastname: Engelhard
mesh_terms: D000071181:ATP Binding Cassette Transporter, Subfamily B, Member 2; D018528:ATP-Binding Cassette Transporters / Q000502:physiology; D000111:Acetylcysteine / Q000031:analogs & derivatives / Q000494:pharmacology; D000595:Amino Acid Sequence; D000818:Animals; D002460:Cell Line; D002474:Cell-Free System / Q000276:immunology; D003546:Cysteine Endopeptidases / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology*; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D003600:Cytosol / Q000201:enzymology* / Q000276:immunology*; D003602:Cytotoxicity, Immunologic / Q000187:drug effects; D018984:Epitopes, T-Lymphocyte / Q000096:biosynthesis / Q000187:drug effects / Q000378:metabolism*; D005998:Glycine / Q000494:pharmacology; D015234:HLA-A Antigens / Q000096:biosynthesis / Q000187:drug effects; D015395:Histocompatibility Antigens Class I / Q000378:metabolism*; D006801:Humans; D007976:Leupeptins / Q000494:pharmacology; D008565:Membrane Proteins / Q000378:metabolism; D051379:Mice; D008969:Molecular Sequence Data; D009097:Multienzyme Complexes / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology*; D046988:Proteasome Endopeptidase Complex; D013602:T-Lymphocytes, Cytotoxic / Q000201:enzymology / Q000276:immunology; D013997:Time Factors; D014763:Viral Matrix Proteins / Q000096:biosynthesis / Q000276:immunology
chemical_list: D000071181:ATP Binding Cassette Transporter, Subfamily B, Member 2; D018528:ATP-Binding Cassette Transporters; D015853:Cysteine Proteinase Inhibitors; D018984:Epitopes, T-Lymphocyte; D015234:HLA-A Antigens; D015395:Histocompatibility Antigens Class I; D007976:Leupeptins; C031540:M-protein, influenza virus; C488936:M1 protein, Influenza A virus; D008565:Membrane Proteins; D009097:Multienzyme Complexes; C073119:TAP1 protein, human; C486171:Tap1 protein, mouse; D014763:Viral Matrix Proteins; C063261:acetylleucyl-leucyl-norleucinal; C067713:lactacystin; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; D005998:Glycine; D000111:Acetylcysteine