Antigen-presenting molecule specialized in presenting microbial vitamin B metabolites. Involved in the development and expansion of a small population of T-cells expressing an invariant T-cell receptor alpha chain called mucosal-associated invariant T-cells (MAIT). MAIT lymphocytes are preferentially located in the gut lamina propria and therefore may be involved in monitoring commensal flora or serve as a distress signal. Expression and MAIT cell recognition seem to be ligand-dependent (By similarity). Heterodimerizes with B2M, this interaction is required for surface expression. Associated with the peptide-loading complex, TAPBP, CALR and PDIA3 (By similarity). The alpha-3 region and to a lesser extent the transmembrane and cytosolic domains regulate surface expression. The alpha-3 region mediates interaction with B2M (By similarity). The ligand-binding groove is ideally suited to present small organic compounds that can originate from vitamins rather than antigenic peptides. N-glycosylated. MR1 is detected in an open versus folded conformation. Only the folded MR1 conformer activates MAIT cells (By similarity). Belongs to the MHC class I family.