title: MHC class I/beta 2-microglobulin complexes associate with TAP transporters before peptide binding.
abstract: Major histocompatibility complex class I molecules bind antigenic peptides in the endoplasmic reticulum (ER) and transport them to the cell surface for recognition by cytotoxic T lymphocytes. The peptides are predominantly generated from cytoplasmic proteins, probably by the action of the multicatalytic proteinase complex, or proteasome. They are transported into the ER by the transporters associated with antigen processing (TAP), a complex formed from two subunits, TAP.1 and TAP.2 (refs 3-5). Here we show that the TAP molecules are intimately involved in the assembly of the class I/beta 2-microglobulin (beta 2m) peptide complex. Free class I heavy chains are associated in the ER with the chaperone calnexin. In human B-cell lines, however, class I/beta 2m dimers in the ER are physically associated with TAP molecules rather than calnexin. Our results suggest that calnexin mediates class I/beta 2m dimerization, and subsequent binding of the dimers to TAP molecules facilitates their association with TAP-transported peptides.
authors.affiliation: Section of Immunobiology, Howard Hughes Medical Institute, Yale University School of Medicine, New Haven, Connecticut 06520.
authors.forename: B
authors.initials: B
authors.lastname: Ortmann
authors.forename: M J
authors.initials: MJ
authors.lastname: Androlewicz
authors.forename: P
authors.initials: P
authors.lastname: Cresswell
mesh_terms: D000071181:ATP Binding Cassette Transporter, Subfamily B, Member 2; D000071450:ATP Binding Cassette Transporter, Subfamily B, Member 3; D018528:ATP-Binding Cassette Transporters; D000595:Amino Acid Sequence; D000818:Animals; D001402:B-Lymphocytes / Q000378:metabolism; D001692:Biological Transport; D002135:Calcium-Binding Proteins / Q000378:metabolism; D037281:Calnexin; D002352:Carrier Proteins / Q000378:metabolism*; D002460:Cell Line; D004072:Digitonin / Q000494:pharmacology; D004721:Endoplasmic Reticulum / Q000378:metabolism; D015791:HLA-A3 Antigen / Q000378:metabolism; D015395:Histocompatibility Antigens Class I / Q000378:metabolism*; D006801:Humans; D051379:Mice; D008969:Molecular Sequence Data; D011485:Protein Binding; D011817:Rabbits; D012995:Solubility; D001613:beta 2-Microglobulin / Q000378:metabolism*
chemical_list: D000071181:ATP Binding Cassette Transporter, Subfamily B, Member 2; D000071450:ATP Binding Cassette Transporter, Subfamily B, Member 3; D018528:ATP-Binding Cassette Transporters; D002135:Calcium-Binding Proteins; D002352:Carrier Proteins; D015791:HLA-A3 Antigen; D015395:Histocompatibility Antigens Class I; C073119:TAP1 protein, human; C486171:Tap1 protein, mouse; C489516:Tap2 protein, mouse; D001613:beta 2-Microglobulin; D037281:Calnexin; C071692:TAP2 protein, human; D004072:Digitonin