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 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.

抗原呈递分子MR1（MHC I类相关蛋白1）呈递源自微生物维生素B _2的代谢物抗原合成激活粘膜相关的不变T（MAIT）细胞。该进化保守途径的关键方面仍未鉴定，包括MR1在何处获得配体以及哪些辅助蛋白协助配体结合。我们通过使用荧光团标记的稳定MR1抗原类似物，构象特异性MR1 mAb，蛋白质组学分析和全基因组CRISPR / Cas9文库筛选来回答这些问题。我们显示，内质网（ER）包含两个未配体的MR1构象体池，该构象体通过与伴侣塔帕蛋白和塔帕蛋白相关蛋白的相互作用而稳定化。该库是将外源代谢物抗原呈递给MAIT细胞的MR1分子的主要来源。这些伴侣的删除减少了居民驻地MR1库和阻碍抗原呈递和MAIT细胞活化。