title: Evidence for a conformational change in a class II major histocompatibility complex molecule occurring in the same pH range where antigen binding is enhanced.
abstract: Many class II histocompatibility complex molecules bind antigenic peptides optimally at low pH, consistent with their exposure to antigen in acidic endosomal compartments. While it has been suggested that a partially unfolded state serves as an intermediate involved in peptide binding, very little evidence for such a state has been obtained. In this report, we show that the murine class II molecule IE becomes increasingly less stable to sodium dodecyl sulfate-induced dissociation since the pH is decreased in the same range that enhances antigenic peptide binding. Furthermore, at mildly acidic pH levels, IEk binds the fluorescent dye 1-anilino-naphthalene-8-sulfonic acid (ANS), a probe for exposed nonpolar sites in proteins, suggesting that protonation produces a molten globule-like state. The association of IEk with a single high-affinity peptide had only a small effect in these two assays, indicating that the changes that occur are distal to the peptide-binding groove. Circular dichroism analysis shows that a pH shift from neutral to mildly acidic pH causes subtle changes in the environment of aromatic residues but does not grossly disrupt the secondary structure of IEk. We propose a model in which perturbations in interdomain contacts outside the peptide-binding domain of IEk occur at acidic pH, producing a partially unfolded state that facilitates optimal antigen binding.
authors.affiliation: Department of Microbiology and Immunology, Stanford University School of Medicine, California 94305, USA.
authors.forename: J J
authors.initials: JJ
authors.lastname: Boniface
authors.forename: D S
authors.initials: DS
authors.lastname: Lyons
authors.forename: D A
authors.initials: DA
authors.lastname: Wettstein
authors.forename: N L
authors.initials: NL
authors.lastname: Allbritton
authors.forename: M M
authors.initials: MM
authors.lastname: Davis
mesh_terms: D000818:Animals; D000941:Antigens / Q000737:chemistry / Q000378:metabolism; D016466:CHO Cells; D002942:Circular Dichroism; D006224:Cricetinae; D004591:Electrophoresis, Polyacrylamide Gel; D000949:Histocompatibility Antigens Class II / Q000737:chemistry* / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D051379:Mice; D010446:Peptide Fragments / Q000276:immunology / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D013050:Spectrometry, Fluorescence
chemical_list: D000941:Antigens; D000949:Histocompatibility Antigens Class II; C024138:I-E-antigen; D010446:Peptide Fragments