Bound peptide-dependent thermal stability of major histocompatibility complex class II molecule I-Ek.
We used differential scanning calorimetry to study the thermal denaturation of murine major histocompatibility complex class II, I-E(k), accommodating hemoglobin (Hb) peptide mutants possessing a single amino acid substitution of the chemically conserved amino acids buried in the I-Ek pocket (positions 71 and 73) and exposed to the solvent (position 72). All of the I-Ek-Hb(mut) molecules exhibited greater thermal stability at pH 5.5 than at pH 7.4, as for the I-Ek-Hb(wt) molecule, which can explain the peptide exchange function of MHC II. The thermal stability was strongly dependent on the bound peptide sequences; the I-Ek-Hb(mut) molecules were less stable than the I-Ek-Hb(wt) molecules, in good correlation with the relative affinity of each peptide for I-Ek. This supports the notion that the bound peptide is part of the completely folded MHC II molecule. The thermodynamic parameters for I-Ek-Hb(mut) folding can explain the thermodynamic origin of the stability difference, in correlation with the crystal structural analysis, and the limited contributions of the residues to the overall conformation of the I-Ek-peptide complex. We found a linear relationship between the denaturation temperature and the calorimetric enthalpy change. Thus, although the MHC II-peptide complex could have a diverse thermal stability spectrum, depending on the amino acid sequences of the bound peptides, the conformational perturbations are limited. The variations in the MHC II-peptide complex stability would function in antigen recognition by the T cell receptor by affecting the stability of the MHC II-peptide-T cell receptor ternary complex.
Saito
Keigo
K
Research Institute for Biological Sciences, Tokyo University of Science, 2669, Yamazaki, Noda, Chiba 278-0022, Japan.
Oda
Masayuki
M
Sarai
Akinori
A
Azuma
Takachika
T
Kozono
Haruo
H
D000818:Animals; D017951:Antigen Presentation; D001224:Aspartic Acid / Q000235:genetics; D015496:CD4-Positive T-Lymphocytes / Q000378:metabolism; D002152:Calorimetry, Differential Scanning; D002850:Chromatography, Gel; D018698:Glutamic Acid / Q000235:genetics; D006454:Hemoglobins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D000949:Histocompatibility Antigens Class II / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D051379:Mice; D016297:Mutagenesis, Site-Directed; D010446:Peptide Fragments / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D011485:Protein Binding; D011487:Protein Conformation; D011489:Protein Denaturation; D013816:Thermodynamics
D006454:Hemoglobins; D000949:Histocompatibility Antigens Class II; C024138:I-E-antigen; D010446:Peptide Fragments; D001224:Aspartic Acid; D018698:Glutamic Acid