Peptide-independent stabilization of MHC class I molecules breaches cellular quality control.

The intracellular trafficking of major histocompatibility complex class I (MHC-I) proteins is directed by three quality control mechanisms that test for their structural integrity, which is correlated to the binding of high-affinity antigenic peptide ligands. To investigate which molecular features of MHC-I these quality control mechanisms detect, we have followed the hypothesis that suboptimally loaded MHC-I molecules are characterized by their conformational mobility in the F-pocket region of the peptide-binding site. We have created a novel variant of an MHC-I protein, K(b)-Y84C, in which two α-helices in this region are linked by a disulfide bond that mimics the conformational and dynamic effects of bound high-affinity peptide. K(b)-Y84C shows a remarkable increase in the binding affinity to its light chain, beta-2 microglobulin (ß2m), and bypasses all three cellular quality control steps. Our data demonstrate (1) that coupling between peptide and ß2m binding to the MHC-I heavy chain is mediated by conformational dynamics; (2) that the folded conformation of MHC-I, supported by ß2m, plays a decisive role in passing the ER-to-cell-surface transport quality controls; and (3) that ß2m association is also tested by the cell surface quality control that leads to MHC-I endocytosis.

Distinct mechanisms survey the structural integrity of HLA-B*27:05 intracellularly and at the surface.

HLA-B*27:05 is associated with the development of autoimmune spondyloarthropathies, but the precise causal relationship between the MHC haplotype and disease pathogenesis is yet to be elucidated. Studies focusing on the structure and cellular trafficking of HLA-B*27:05 implicate several links between the onset of inflammation and the unusual conformations of the molecule inside and at the surface of antigen presenting cells. Several lines of evidence emphasize the emergence of those unnatural protein conformations under conditions where peptide loading onto B*27:05 is impaired. To understand how cellular factors distinguish between poorly loaded molecules from the optimally loaded ones, we have investigated the intracellular transport, folding, and cell surface expression of this particular B27 subtype. Our findings show that B*27:05 is structurally unstable in the absence of peptide, and that an artificially introduced disulfide bond between residues 84 and 139 conferred enhanced conformational stability to the suboptimally loaded molecules. Empty or suboptimally loaded B*27:05 can escape intracellular retention and arrive at the cell surface leading to the appearance of increased number of ß2m-free heavy chains. Our study reveals a general mechanism found in the early secretory pathways of murine and human cells that apply to the quality control of MHC class I molecules, and it highlights the allotype-specific structural features of HLA-B*27:05 that can be associated with aberrant antigen presentation and that might contribute to the etiology of disease.

Correction: Distinct mechanisms survey the structural integrity of HLA-B*27:05 intracellularly and at the surface.

[This corrects the article DOI: 10.1371/journal.pone.0200811.].

The Carboxy Terminus of the Ligand Peptide Determines the Stability of the MHC Class I Molecule H-2Kb: A Combined Molecular Dynamics and Experimental Study.

Major histocompatibility complex (MHC) class I molecules (proteins) bind peptides of eight to ten amino acids to present them at the cell surface to cytotoxic T cells. The class I binding groove binds the peptide via hydrogen bonds with the peptide termini and via diverse interactions with the anchor residue side chains of the peptide. To elucidate which of these interactions is most important for the thermodynamic and kinetic stability of the peptide-bound state, we have combined molecular dynamics simulations and experimental approaches in an investigation of the conformational dynamics and binding parameters of a murine class I molecule (H-2Kb) with optimal and truncated natural peptide epitopes. We show that the F pocket region dominates the conformational and thermodynamic properties of the binding groove, and that therefore the binding of the C terminus of the peptide to the F pocket region plays a crucial role in bringing about the peptide-bound state of MHC class I.

Not all empty MHC class I molecules are molten globules: tryptophan fluorescence reveals a two-step mechanism of thermal denaturation.

When major histocompatibility complex (MHC) class I molecules bind peptide, they change their conformation and their dynamics. The structure and properties of the peptide-empty class I are still largely unknown. We have investigated the thermal denaturation of the murine class I allotypes H-2D(b) and H-2K(b) through the fluorescence of their intrinsic tryptophans, and we find that it occurs via an empty form that can also be produced by folding denatured recombinant class I molecules. It rapidly binds exogenous peptides. Our data demonstrate that the empty form of class I is a distinct conformational state with at least transient stability.