A schematic for the dominant pathway of metabolite presentation by MR1. MR1 (pink) resides in the ER in a ligand-receptive form. Extracellular or intracellular VitBAg (orange) can load onto MR1 within ER (1). MR1–VitBAg complexes traffic through the Golgi apparatus to the plasma membrane (PM) for presentation (2). Complexes are recognized by the AP2 complex (dark blue) at the cytoplasmic tail and internalized to the early endosomes (EE; 3). The majority are degraded in the late endosomes/lysosomes (LEL; 4) with some recycling back to the cell surface (5). Surface MR1 can exchange its VitBAg for another type (green) at the cell surface (6).