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. In complex with B2M preferentially presents riboflavin-derived metabolites to semi-invariant TCRs on MAIT cells, guiding immune surveillance of the microbial metabolome at mucosal epithelial barriers (By similarity). 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 (By similarity). May present microbial antigens to various MAIT cell subsets, providing for unique recognition of diverse microbes, including pathogens that do not synthesize riboflavin. Upon antigen recognition, elicits rapid innate-type MAIT cell activation to eliminate pathogenic microbes by directly killing infected cells (By similarity). 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. May present a tumor-specific or -associated metabolite essential for cancer cell survival to a pan-cancer TCR on a non-MAIT CD8-positive T cell clone, triggering T cell-mediated killing of a wide range of cancer cell types (By similarity). Heterotrimer that consists of MR1, B2M and a metabolite antigen. 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. On antigen-presenting cells, the ternary complex interacts with TCR on CD8-positive T cells. 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. The ribityl moiety of pyrimidine-based antigens is recognized by Tyr-95 residue in the CDR3 alpha loop of the invariant TRAV1-2 TCR. 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. Expressed in kidney, liver, testis, spleen, thymus, brain, and heart. 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. Does not appear to be polymorphic. MR1 is detected in an open versus folded conformation. Only the folded MR1 conformer activates MAIT cells (By similarity).