Enzymatic processing and MHC loading 3Molecular mechanisms of chaperone-mediated MHC-I peptide loading

Peptide loading of MHC-I molecules occurs as a result of dynamic protein folding and assembly processes involving not only the MHC-I heavy chain, the light chain, beta 2-m, and antigenic peptide, but also the components of the peptide loading complex (PLC), including tapasin, as well as the PLC-independent chaperone, TAP binding protein, related (TAPBPR). Structural studies of tapasin and TAPBPR reveal that these, like many other proteins, contain both well-ordered domains as well as regions of disorder. To examine the contributions of disordered regions of both MHC-I molecules and of TAPBPR, we have examined X-ray structures of MHC-I molecules assembled with disulfide linked truncated peptides, of TAPBPR/MHC-I complexes containing truncated peptides, and of nanobody/TAPBPR complexes. Different disordered regions of TAPBPR rigidify on interaction with MHC-I or with specific nanobodies. Conversely, the peptide binding groove of MHC-I structurally rearranges on interaction with TAPBPR. These observations are consistent with solution nuclear magnetic resonance (NMR) spectroscopy data examining the interaction of TAPBPR with isotope-labeled peptide-MHC-I complexes. The dynamic NMR data reveal a transient pMHC-I/TAPBPR intermediate that controls the selection of high-affinity peptides. We propose a model of negative allosteric coupling that involves the entire peptide binding groove. Such a model, based primarily on observations of TAPBPR should be applicable to tapasin in the PLC as well.