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. Depending on the tissue and on cicumstances, mediates the import or export of lactic acid and ketone bodies. Required for normal nutrient assimilation, increase of white adipose tissue and body weight gain when on a high-fat diet. Plays a role in cellular responses to a high-fat diet by modulating the cellular levels of lactate and pyruvate, small molecules that contribute to the regulation of central metabolic pathways and insulin secretion, with concomitant effects on plasma insulin levels and blood glucose homeostasis. Interacts with BSG; interaction mediates SLC16A1 targeting to the plasma membrane (By similarity). Interacts with EMB; interaction mediates SLC16A1 targeting to the plasma membrane (By similarity). Expressed at abundant levels in erythrocytes, cardiac muscle, basolateral intestinal epithelium, skeletal muscle myocytes, testis and at low levels in liver. In the stomach and in the proximal tubules of the kidney, present on the basolateral surfaces of epithelial cells. Expressed on sperm heads in the testis and proximal epididymis. In the distal epididymis, it disappeared from sperm and appeared on the microvillar surface of the lining epithelium. Also expressed in lung, cecum and eye. Not glycosylated. Belongs to the major facilitator superfamily. Monocarboxylate porter (TC 2.A.1.13) family.