Proton-coupled monocarboxylate transporter. Catalyzes the rapid transport across the plasma membrane of many monocarboxylates such as lactate, pyruvate, branched-chain oxo acids derived from leucine, valine and isoleucine, and the ketone bodies acetoacetate, beta-hydroxybutyrate and acetate. Functions as high-affinity pyruvate transporter. Interacts with GRID2IP (By similarity). Interacts with EMB (By similarity). Interacts with BSG (By similarity). Abundant on the surface of hepatocytes. Present on parietal cells of the oxyntic gland of the stomach, on the basolateral surface of epithelial cells in the collecting ducts of the kidney, on sperm tails throughout the epididymis. Expressed in mitochondria-rich skeletal muscle fibers and cardiac myocytes (at protein level). Belongs to the major facilitator superfamily. Monocarboxylate porter (TC 2.A.1.13) family.