Endoplasmic reticulum chaperones stabilize ligand-receptive MR1 molecules for efficient presentation of metabolite antigens
Hamish E. G. McWilliam, Jeffrey Y. W. Mak, Wael Awad, Matthew Zorkau, Sebastián Cruz-Gómez, Hui Jing Lim, Yuting Yan, Sam Wormald, Laura F. Dagley, Sidonia B. G. Eckle, Alexandra J. Corbett, Haiyin Liu, Shihan Li, Scott J. J. Reddiex, Justine D. Mintern, Ligong Liu, James McCluskey, Jamie Rossjohn, David P. Fairlie, Jóse A. Villadangos
Abstract
synthesis to activate mucosal-associated invariant T (MAIT) cells. Key aspects of this evolutionarily conserved pathway remain uncharacterized, including where MR1 acquires ligands and what accessory proteins assist ligand binding. We answer these questions by using a fluorophore-labeled stable MR1 antigen analog, a conformation-specific MR1 mAb, proteomic analysis, and a genome-wide CRISPR/Cas9 library screen. We show that the endoplasmic reticulum (ER) contains a pool of two unliganded MR1 conformers stabilized via interactions with chaperones tapasin and tapasin-related protein. This pool is the primary source of MR1 molecules for the presentation of exogenous metabolite antigens to MAIT cells. Deletion of these chaperones reduces the ER-resident MR1 pool and hampers antigen presentation and MAIT cell activation. The MR1 antigen-presentation pathway thus co-opts ER chaperones to fulfill its unique ability to present exogenous metabolite antigens captured within the ER.