title: A three-step kinetic mechanism for peptide binding to MHC class II proteins.
abstract: Peptide binding reactions of class II MHC proteins exhibit unusual kinetics, with extremely slow apparent rate constants for the overall association (<100 M(-)(1) s(-)(1)) and dissociation (<10(-)(5) s(-)(1)) processes. Various linear and branched pathways have been proposed to account for these data. Using fluorescence resonance energy transfer between tryptophan residues in the MHC peptide binding site and aminocoumarin-labeled peptides, we measured real-time kinetics of peptide binding to empty class II MHC proteins. Our experiments identified an obligate intermediate in the binding reaction. The observed kinetics were consistent with a binding mechanism that involves an initial bimolecular binding step followed by a slow unimolecular conformational change. The same mechanism is observed for different peptide antigens. In addition, we noted a reversible inactivation of the empty MHC protein that competes with productive binding. The implications of this kinetic mechanism for intracellular antigen presentation pathways are discussed.
authors.affiliation: Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: R V
authors.initials: RV
authors.lastname: Joshi
authors.forename: J A
authors.initials: JA
authors.lastname: Zarutskie
authors.forename: L J
authors.initials: LJ
authors.lastname: Stern
mesh_terms: D000944:Antigens, Differentiation, B-Lymphocyte / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D004735:Energy Transfer; D004926:Escherichia coli / Q000235:genetics; D005822:Genetic Vectors / Q000737:chemistry / Q000378:metabolism; D015798:HLA-DR1 Antigen / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D019267:Hemagglutinin Glycoproteins, Influenza Virus / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D000949:Histocompatibility Antigens Class II / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D008956:Models, Chemical; D009842:Oligopeptides / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D011485:Protein Binding / Q000235:genetics; D013050:Spectrometry, Fluorescence
chemical_list: D000944:Antigens, Differentiation, B-Lymphocyte; D015798:HLA-DR1 Antigen; D019267:Hemagglutinin Glycoproteins, Influenza Virus; D000949:Histocompatibility Antigens Class II; D009842:Oligopeptides; C045084:invariant chain