Hydrogen Bond Integrity Between MHC Class II Molecules and Bound Peptide Determines the Intracellular Fate of MHC Class II Molecules

Abstract

MHC class II molecules associate with peptides through pocket interactions and the formation of hydrogen bonds. The current paradigm suggests that the interaction of side chains of the peptide with pockets in the class II molecule is responsible for the formation of stable class II-peptide complexes. However, recent evidence has shown that the formation of hydrogen bonds between genetically conserved residues of the class II molecule and the main chain of the peptide contributes profoundly to peptide stability. In this study, we have used I-Ak, a class II molecule known to form strong pocket interactions with bound peptides, to probe the general importance of hydrogen bond integrity in peptide acquisition. Our studies have revealed that abolishing hydrogen bonds contributed by positions 81 or 82 in the β-chain of I-Ak results in class II molecules that are internally degraded when trafficked through proteolytic endosomal compartments. The presence of high-affinity peptides derived from either endogenous or exogenous sources protects the hydrogen bond-deficient variant from intracellular degradation. Together, these data indicate that disruption of the potential to form a complete hydrogen bond network between MHC class II molecules and bound peptides greatly diminishes the ability of class II molecules to bind peptides. The subsequent failure to stably acquire peptides leads to protease sensitivity of empty class II molecules, and thus to proteolytic degradation before export to the surface of APCs.

Conformational isomers of a peptide–class II major histocompatibility complex

“…The authors concluded that the changes to the hydrogen‐bonding network enhanced the transition to the ‘open’ form of the molecule normally catalyzed by DM (110). Sant and colleagues (111) subsequently obtained similar results with I‐A k . Most recently, Stern's group (112) analyzed the ability of DM to catalyze the release of peptides containing mutations that abolished their ability to form hydrogen bonds with the MHC molecule.…”

DM Loss in k Haplotype Mice Reveals Isotype-Specific Chaperone Requirements

“…It has long been known that peptide ligands extend intracellular class II half-lives (70). Recent evidence suggests that empty or loosely occupied class II is susceptible to proteolytic degradation inside endocytic compartments and at the cell surface (71,72). Allele-and isotype-specific sensitivities to acid-induced denaturation of empty dimers may also selectively influence the elimination of unstable complexes (8,52).…”

The optimization of peptide cargo bound to MHC class I molecules by the peptide‐loading complex

“…Interestingly, like class I, the dependence among class II alleles on chaperones is variable, and a naturally occurring mouse MHC class II allele, I‐E k , which has a shallow P1 pocket, was shown to have little dependence upon DM for peptide loading (50). Jensen and colleagues (47) first suggested that DM might act via its ability to accelerate the off‐rate of weak peptides by interrupting the formation of H bonds between key positions in class II and peptide ligand backbone, and this idea has since been strengthened by the observation that I‐A k point mutants, which lack key H‐bond donor side chains (βH81 and βD82), have dissociation rates accelerated by roughly the same degree as wildtype IA k in the presence of DM (51). In addition, Rotzschke and colleagues (52) have demonstrated that small H‐bond donors (such as ethanol) can catalyze ligand exchange on DR in a way that parallels the action of DM (except that it occurs at neutral pH).…”