TAPBPR promotes antigen loading on MHC-I molecules using a peptide trap

Abstract

Chaperones Tapasin and TAP-binding protein related (TAPBPR) perform the important functions of stabilizing nascent MHC-I molecules (chaperoning) and selecting high-affinity peptides in the MHC-I groove (editing). While X-ray and cryo-EM snapshots of MHC-I in complex with TAPBPR and Tapasin, respectively, have provided important insights into the peptide-deficient MHC-I groove structure, the molecular mechanism through which these chaperones influence the selection of specific amino acid sequences remains incompletely characterized. Based on structural and functional data, a loop sequence of variable lengths has been proposed to stabilize empty MHC-I molecules through direct interactions with the floor of the groove. Using deep mutagenesis on two complementary expression systems, we find that important residues for the Tapasin/TAPBPR chaperoning activity are located on a large scaffolding surface, excluding the loop. Conversely, loop mutations influence TAPBPR interactions with properly conformed MHC-I molecules, relevant for peptide editing. Detailed biophysical characterization by solution NMR, ITC and FP-based assays shows that the loop hovers above the MHC-I groove to promote the capture of incoming peptides. Our results suggest that the longer loop of TAPBPR lowers the affinity requirements for peptide selection to facilitate peptide loading under conditions and subcellular compartments of reduced ligand concentration, and to prevent disassembly of high-affinity peptide-MHC-I complexes that are transiently interrogated by TAPBPR during editing.

Molecular basis of MHC I quality control in the peptide loading complex

“…This arrangement resembles the position of the scoop loop in TAPBPR, which is larger (16 amino acids vs. 10 in tapasin) but is very similarly positioned and also disturbs the interaction between Tyr84 in MHC I and the C terminus of the cargo peptide by inserting residues 34–36 into the MHC I peptide binding groove 24 . This creates a larger interface with the binding groove than the tapasin loop 25 . The displacement of Tyr84 is stabilized by the interaction with Glu72 of tapasin, consistent with TAPBPR-MHC I crystal structures 24 , 26 .…”

In vivo CRISPR screens reveal Serpinb9 and Adam2 as regulators of immune therapy response in lung cancer

“…The prominent role of IFNα/β, IFNγ and TNFα pathways in the induction of antigen processing and MHC-I-mediated antigen presentation on tumor cells prompted us to test H-2K b -OVA expression in CTLR and Adam2 O/E cells in response to IFNβ, IFNγ and TNFα treatment 34,35,38,40,66–75 . Flow cytometry revealed markedly delayed and significantly reduced levels of MHCI-OVA surface expression in Adam2 O/E cells in comparison to CTLR cells in response to IFNβ, IFNγ or TNFα stimulation ( Fig.…”

l- to d-Amino Acid Substitution in the Immunodominant LCMV-Derived Epitope gp33 Highlights the Sensitivity of the TCR Recognition Mechanism for the MHC/Peptide Structure and Dynamics

“… 25 We speculate here that the p3P rigidification may be important for more efficient selection and binding to MHC class I molecules by the peptide loading complex (PLC). 43 , 44 Interestingly, a previous study in which a succession of crystal structures of the same MHC class I molecule was determined in complex with the peptide of different lengths and a fragment from the tapasin scoop loop revealed that the N-terminal part of the epitopes is loaded first before being tested for the affinity of the C-terminal to the F-pocket within the MHC peptide-binding cleft. 38 Thus, the specific rigidification of the N-terminal part of the peptide could lead to more enhanced selection and binding efficiency.…”