Antigen-presenting molecule specialized in displaying microbial pyrimidine-based metabolites to alpha-beta T cell receptors (TCR) on innate-type mucosal-associated invariant T (MAIT) cells (PubMed:23051753, PubMed:26795251, PubMed:12794138, PubMed:19416870, PubMed:22692454, PubMed:23846752). In complex with B2M preferentially presents riboflavin-derived metabolites to semi-invariant TRAV1-2 TCRs on MAIT cells, guiding immune surveillance of the microbial metabolome at mucosal epithelial barriers (PubMed:26795251, PubMed:24695216, PubMed:20581831). Signature pyrimidine-based microbial antigens are generated via non-enzymatic condensation of metabolite intermediates of the riboflavin pathway with by-products arising from other metabolic pathways such as glycolysis. Typical potent antigenic metabolites are 5-(2-oxoethylideneamino)-6-D-ribitylaminouracil (5-OE-RU) and 5-(2-oxopropylideneamino)-6-D-ribitylaminouracil (5-OP-RU), products of condensation of 5-amino-6-D-ribityaminouracil (5-A-RU) with glyoxal or methylglyoxal by-products, respectively (PubMed:24695216). May present microbial antigens to various TRAV1-2-negative MAIT cell subsets, providing for unique recognition of diverse microbes, including pathogens that do not synthesize riboflavin (PubMed:27527800, PubMed:31113973). Upon antigen recognition, elicits rapid innate-type MAIT cell activation to eliminate pathogenic microbes by directly killing infected cells (PubMed:24695216, PubMed:27527800, PubMed:23846752). During T cell development, drives thymic selection and post-thymic terminal differentiation of MAIT cells in a process dependent on commensal microflora (By similarity). Acts as an immune sensor of cancer cell metabolome (PubMed:31959982). May present a tumor-specific or -associated metabolite essential for cancer cell survival to a pan-cancer TCR consisting of TRAV38.2-DV8*TRAJ31 alpha chain paired with a TRBV25.1*TRBJ2.3 beta chain on a non-MAIT CD8-positive T cell clone (MC.7.G5), triggering T cell-mediated killing of a wide range of cancer cell types (PubMed:31959982). Usually inhibited by pterin-based metabolites such as 6-formylpterin (6-FP, a product of folic acid photodegradation). 6-FP competitively inhibits MAIT cell activation by 5-OP-RU (PubMed:28166217). Modulated by commonly prescribed anti-inflammatory drug metabolites. Inhibited by salicilates such as 3-formylsalicylic and 5-formylsalicylic acids. Activated by diclofenac and/or its hydroxy metabolites (PubMed:28166217). Heterotrimer that consists of MR1, B2M and a metabolite antigen (PubMed:27043408, PubMed:23051753, PubMed:23846752, PubMed:24695216). Forms reversible covalent Schiff base complexes with the microbial metabolite, which serves as a molecular switch triggering complete folding, stable association with B2M and translocation of the ternary complex from endoplasmic reticulum to the plasma membrane (PubMed:27043408, PubMed:23051753, PubMed:23846752, PubMed:24695216). On antigen-presenting cells, the ternary complex interacts with TCR on CD8-positive T cells (PubMed:23846752, PubMed:24695216, PubMed:26795251). The molecular machinery involved in antigen processing remains unknown, but appears to be TAP1-TAP2 and proteasome-independent. Structurally, MR1-B2M heterodimer adopts a topology similar to classical MHC class I molecules, with alpha-1 and alpha-2 domains of MR1 forming the antigen-binding cleft composed of two alpha-helices resting on a floor of 7-stranded anti-parallel beta-pleated sheet (PubMed:23846752, PubMed:24695216, PubMed:26795251). The ribityl moiety of pyrimidine-based antigens is recognized by Tyr-95 residue in the CDR3 alpha loop of the invariant TRAV1-2 TCR (PubMed:23846752, PubMed:24695216, PubMed:26795251). Homodimerizes and does not associate with B2M. In the absence of antigen remains within the endoplasmic reticulum where it acts as a metabolite sensor. Antigen binding triggers trafficking of the ternary complex to the plasma membrane. After presentation, most of these complexes are rapidly internalized and degraded via endocytosis. A small subset recycles via endosomes back to the plasma membrane and may thus acquire and present new antigens that do not efficiently reach the endoplasmic reticulum. The larger proportion remains in the ER in an immature state. The subset that reach cell surface does it through a B2M-independent pathway. Ubiquitous (PubMed:7624800, PubMed:9780177). Low expression is detected in peripheral blood B cells, T cells, monocytes and in bronchial epithelial cells (at protein level) (PubMed:27043408). Expressed in plasmablasts or plasma B cells in the lamina propria of ileum, appendix and colon (at protein level) (PubMed:19760593). Highly expressed on a subset of CD45-positive CD3-positive thymocytes (at protein level) (PubMed:22692454). The alpha-1 domain is a structural part of antigen-binding cleft. The alpha-2 domain is a structural part of antigen-binding cleft. N-glycosylated. Several individuals from different ethnic background were analyzed for polymorphism. MR1 was identical in all individuals analyzed, except one. MR1 is not polymorphic. Belongs to the MHC class I family. Reported to be associated with components of the peptide-loading complex, TAPBP, CALR, CANX and PDIA3 (PubMed:23457030, PubMed:12794138). This association in primary cells and its functional relevance is disputable, given that antigen presentation and MAIT cell activation is shown to be TAP1-TAP2 and proteasome-independent. Truncated N-terminus.