Misfolded major histocompatibility complex class I heavy chains are translocated into the cytoplasm and degraded by the proteasome.
N-acetyl-L-leucyl-L-leucyl-L-norleucinal (LLnL), which reversibly inhibits the proteasome in addition to other proteases, and a more specific irreversible inhibitor of the proteasome, lactacystin, were found to cause the accumulation of major histocompatibility complex (MHC) class I heavy chains in the cytosol of the beta2-microglobulin-deficient cell line Daudi and the TAP-deficient cell line .174. These cell lines, which are severely impaired in their ability to fold MHC class I heavy chain, showed an accumulation of soluble class I heavy chains at different rates over a period of hours in the presence of LLnL. The accumulation of soluble class I heavy chains in the presence of either LLnL or lactacystin was easily revealed in Daudi and .174 but almost undetectable in a Daudi transfectant expressing beta2-microglobulin and in 45.1, the wild-type parent of .174. The soluble class I heavy chain was also found to be devoid of its N-linked glycan and to be located in the cytosol. When the gene for ICP47, a herpes simplex virus protein that blocks the translocation of peptides into the endoplasmic reticulum, was transfected into 45.1, a similar accumulation of soluble MHC class I heavy chain was detectable. These data suggest that in cells where the MHC class I molecule is unable to assemble properly, the misfolded heavy chain is removed from the endoplasmic reticulum to the cytosol, deglycosylated, and degraded by the proteasome.
Hughes
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EA
Section of Immunobiology, Yale University School of Medicine, New Haven, CT 06510, USA.
Hammond
C
C
Cresswell
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P
D000111:Acetylcysteine / Q000031:analogs & derivatives / Q000494:pharmacology; D015153:Blotting, Western; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D003593:Cytoplasm / Q000201:enzymology / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006031:Glycosylation; D015395:Histocompatibility Antigens Class I / Q000737:chemistry / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D007976:Leupeptins / Q000494:pharmacology; D009097:Multienzyme Complexes / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex; D017510:Protein Folding; D012995:Solubility; D014162:Transfection; D014407:Tumor Cells, Cultured; D001613:beta 2-Microglobulin / Q000378:metabolism
D015853:Cysteine Proteinase Inhibitors; D004791:Enzyme Inhibitors; D015395:Histocompatibility Antigens Class I; D007976:Leupeptins; D009097:Multienzyme Complexes; D001613:beta 2-Microglobulin; C063261:acetylleucyl-leucyl-norleucinal; C067713:lactacystin; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; D000111:Acetylcysteine