Insights into MHC-I peptide loading obtained from the structure of a TAPBPR/MHC-I complex

Abstract The cell surface expression of MHC-I molecules displaying peptides derived from intracellular proteolytic processing is a critical requirement for T cell-mediated immunity. A complex series of steps, guided by the peptide loading machinery in the endoplasmic reticulum, ensures that only stably folded MHC-I molecules carrying peptides of proper length and high affinity are transported for display at the cell surface. TAP-binding protein, related (TAPBPR), has been recently shown to directly bind to certain MHC-I alleles and to facilitate peptide loading. To elucidate mechanistic details, we have examined the binding interaction between purified recombinant TAPBPR and MHC-I molecules loaded with truncated peptides that leave a portion of the peptide binding groove unoccupied. Such MHC-I molecules interact with TAPBPR with nanomolar affinities as measured by surface plasmon resonance and these TAPBPR/MHC-I complexes dissociate when offered a high affinity peptide. An extensive crystallization screen with purified TAPBPR/MHC-I complexes yielded crystals that diffracted to 3.5 Å. The structure of the TAPBPR/MHC-I complex, solved by molecular replacement, and refined at this resolution readily reveals the binding footprint and further structural details of the interaction. Data will be presented on the binding interaction and structural characterization of the TAPBPR/MHC-I complex that reveal insights into the mechanism of TAPBPR- and tapasin-mediated loading of peptides onto MHC-I molecules. (KN and JJ contributed equally to this work).