Formation of two peptide/MHC II isomers is catalyzed differentially by HLA-DM.
Major histocompatability class II proteins are transmembrane alphabeta-heterodimers that present peptides to T-cells. MHC II may bind exogenous peptides directly at the cell surface. Alternatively, peptides derived from processing of endosomal protein may bind to MHC II in endosomal compartments. There, HLA-DM catalyzes the formation of peptide/MHC complexes, which are then transported to the cell surface. Here we report evidence that the peptide Ii CLIP 81-104 binds to DR*0404 in two alternate registries, whose dissociation rates, while kinetically indistinguishable at pH 5.3 and 37 degrees C, are kinetically resolved in the presence of HLA-DM. In one registry isomer, CLIP Met 91 is placed in the N-terminal P1 pocket of DR*0404, and peptide dissociation is readily catalyzed by HLA-DM. In a second proposed registry, likely with CLIP Leu 97 in the P1 pocket, the complex is substantially less sensitive to HLA-DM catalysis. Without HLA-DM, or at pH 7, the fraction of each isomer formed in solution is relatively insensitive to the duration of incubation with peptide. However, with HLA-DM, the fraction of the DM-insensitive isomer is dramatically influenced by peptide incubation time. The mechanism of isomer formation appears to be determined by the HLA-DM-modified relative association to the two registries, followed by HLA-DM-catalyzed dissociation of each isomer and rebinding, leading to a final isomer composition determined by these kinetic constants. Intramolecular isomer interconversion does not appear to be involved. The behavior of these complexes may provide a model for peptide editing by DM in endosomes.
Belmares
Michael P
MP
Departments of Chemistry and Pediatrics, Stanford University, Stanford, California 94305, USA.
Busch
Robert
R
Mellins
Elizabeth D
ED
McConnell
Harden M
HM
D000595:Amino Acid Sequence; D000818:Animals; D000944:Antigens, Differentiation, B-Lymphocyte / Q000378:metabolism; D002384:Catalysis; D002460:Cell Line; D004331:Drosophila melanogaster; D006681:HLA-D Antigens / Q000737:chemistry / Q000378:metabolism*; D015804:HLA-DR4 Antigen / Q000378:metabolism; D000949:Histocompatibility Antigens Class II / Q000378:metabolism; D006801:Humans; D007536:Isomerism; D007700:Kinetics; D008969:Molecular Sequence Data; D004676:Myelin Basic Protein / Q000378:metabolism; D010446:Peptide Fragments / Q000737:chemistry / Q000378:metabolism*; D011485:Protein Binding; D011487:Protein Conformation
D000944:Antigens, Differentiation, B-Lymphocyte; D006681:HLA-D Antigens; C084245:HLA-DM antigens; D015804:HLA-DR4 Antigen; D000949:Histocompatibility Antigens Class II; D004676:Myelin Basic Protein; D010446:Peptide Fragments; C045084:invariant chain; C407155:myelin basic protein 84-102