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 and with isoform 2 of BSG (By similarity). Interacts with EMB (PubMed:15917240, PubMed:20695846). Detected in brain and kidney (at protein level). Belongs to the major facilitator superfamily. Monocarboxylate porter (TC 2.A.1.13) family.