Binding of human cytomegalovirus US2 to major histocompatibility complex class I and II proteins is not sufficient for their degradation.
Human cytomegalovirus (HCMV) glycoprotein US2 causes degradation of major histocompatibility complex (MHC) class I heavy-chain (HC), class II DR-alpha and DM-alpha proteins, and HFE, a nonclassical MHC protein. In US2-expressing cells, MHC proteins present in the endoplasmic reticulum (ER) are degraded by cytosolic proteasomes. It appears that US2 binding triggers a normal cellular pathway by which misfolded or aberrant proteins are translocated from the ER to cytoplasmic proteasomes. To better understand how US2 binds MHC proteins and causes their degradation, we constructed a panel of US2 mutants. Mutants truncated from the N terminus as far as residue 40 or from the C terminus to amino acid 140 could bind to class I and class II proteins. Nevertheless, mutants lacking just the cytosolic tail (residues 187 to 199) were unable to cause degradation of both class I and II proteins. Chimeric proteins were constructed in which US2 sequences were replaced with homologous sequences from US3, an HCMV glycoprotein that can also bind to class I and II proteins. One of these US2/US3 chimeras bound to class II but not to class I, and a second bound class I HC better than wild-type US2. Therefore, US2 residues involved in the binding to MHC class I differ subtly from those involved in binding to class II proteins. Moreover, our results demonstrate that the binding of US2 to class I and II proteins is not sufficient to cause degradation of MHC proteins. The cytosolic tail of US2 and certain US2 lumenal sequences, which are not involved in binding to MHC proteins, are required for degradation. Our results are consistent with the hypothesis that US2 couples MHC proteins to components of the ER degradation pathway, enormously increasing the rate of degradation of MHC proteins.
Department of Molecular Microbiology and Immunology, Oregon Health Sciences University, Portland 97201, USA.
Mathieu S
MS
Chevalier
Gwynn M
GM
Daniels
David C
DC
Johnson
D000595:Amino Acid Sequence; D001483:Base Sequence; D001693:Biological Transport, Active; D002460:Cell Line; D003546:Cysteine Endopeptidases / Q000276:immunology / Q000378:metabolism; D003587:Cytomegalovirus / Q000235:genetics / Q000378:metabolism*; D004279:DNA, Viral / Q000235:genetics; D004721:Endoplasmic Reticulum / Q000276:immunology / Q000378:metabolism / Q000821:virology; D006023:Glycoproteins; D006680:HLA Antigens / Q000378:metabolism; D006681:HLA-D Antigens / Q000378:metabolism; D006684:HLA-DR Antigens / Q000378:metabolism; D000071020:Hemochromatosis Protein; D015395:Histocompatibility Antigens Class I / Q000378:metabolism*; D000949:Histocompatibility Antigens Class II / Q000378:metabolism*; D006801:Humans; D017874:Immediate-Early Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D008565:Membrane Proteins; D008969:Molecular Sequence Data; D009097:Multienzyme Complexes / Q000276:immunology / Q000378:metabolism; D009154:Mutation; D046988:Proteasome Endopeptidase Complex; D011485:Protein Binding; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017384:Sequence Deletion; D014759:Viral Envelope Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
D004279:DNA, Viral; D006023:Glycoproteins; C103673:HFE protein, human; D006680:HLA Antigens; D006681:HLA-D Antigens; C084245:HLA-DM antigens; D006684:HLA-DR Antigens; D000071020:Hemochromatosis Protein; D015395:Histocompatibility Antigens Class I; D000949:Histocompatibility Antigens Class II; D017874:Immediate-Early Proteins; D008565:Membrane Proteins; D009097:Multienzyme Complexes; D011993:Recombinant Fusion Proteins; C118670:US2 protein, Varicellovirus; C095111:US3 protein, cytomegalovirus; D014759:Viral Envelope Proteins; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex