biostudies-literatureMcWilliam HEGNIAID NIH HHSDepartment of Health | National Health and Medical Research CouncilAustralian Research Council24974-24985https://www.ebi.ac.uk/biostudies/studies/S-EPMC7547156biostudies-literatureUnknown117(40)The antigen-presenting molecule MR1 (MHC class I-related protein 1) presents metabolite antigens derived from microbial vitamin B₂ 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 presentationProceedings of the National Academy of Sciences of the United States of AmericaEndoplasmic reticulum chaperones stabilize ligand-receptive MR1 molecules for efficient presentation of metabolite antigens.PMC75471561113293DE1701005751058193R01 AI148407FT160100083CE140100011170102471DE1701004071117017Liu HVilladangos JAMcWilliam HEGReddiex SJJLiu LMak JYWRossjohn JYan YWormald SMcCluskey JFairlie DPLi SMintern JDAwad WEckle SBGDagley LFZorkau MCruz-Gomez SLim HJCorbett AJfalseEndoplasmic reticulum chaperones stabilize ligand-receptive MR1 molecules for efficient presentation of metabolite antigens.The antigen-presenting molecule MR1 (MHC class I-related protein 1) presents metabolite antigens derived from microbial vitamin B₂ 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 presentation2020-01-01T00:00:00Z2020 Oct2026-05-01T17:34:40.631Z2025-04-06T07:58:49.28ZS-EPMC75471563295863710.1073/pnas.2011260117