Getting in the groove: Editing of MHC-I antigen repertoires by molecular chaperones is governed by a network of protein dynamics

Abstract Molecular chaperones TAPBPR (TAP-binding protein related) and tapasin associate with class-I major histocompatibility complex (MHC-I) molecules to promote optimization (editing) of peptide cargo. Using solution NMR, we investigate the molecular mechanism of peptide exchange performed by the 90 kDa chaperone protein complex. We identify TAPBPR-induced conformational changes on conserved MHC-I surfaces, consistent with our independently determined X-ray structure of the empty complex. Conformational dynamics present in the empty MHC-I are stabilized by TAPBPR in a peptide-deficient complex, and become progressively dampened with increasing peptide occupancy. Incoming peptides are recognized by the chaperoned groove according to the global stability of the final pMHC-I product, and anneal in a native-like conformation. Our results demonstrate an inverse relationship between MHC-I occupancy by peptide and the affinity of TAPBPR for such pMHC-I molecules, where the lifetime of transiently bound peptides controls the dynamic regulation of a conformational switch, located near the TAPBPR binding site, which triggers TAPBPR release. We further discuss the role of protein dynamics in shaping chaperone specificity towards different human and murine class-I MHC allotypes, and present the high-resolution cryoEM structure of a human A*02/TAPBPR complex with novel insights into the antigen editing mechanism. These results suggest a similar mechanism for the editing function of tapasin in the peptide-loading complex, and provide a molecular blueprint for the design of novel chaperones with tailored antigen editing functions.