Proton-dependent transporter of monocarboxylates such as L-lactate and pyruvate (PubMed:9632638, PubMed:10926847) (By similarity). Plays a predominant role in the transport of L-lactate efflux from highly glycolytic cells (PubMed:9632638, PubMed:10926847). (S)-lactate(in) + H(+)(in) = (S)-lactate(out) + H(+)(out) H(+)(out) + pyruvate(out) = H(+)(in) + pyruvate(in) Interacts with BSG; interaction mediates SLC16A3 targeting to the plasma membrane. Plasma membrane localization is dependent upon the BSG/MCT4 interaction. Basolateral sorting signals (BLSS) in C-terminal cytoplasmic tail ensure its basolateral expression in polarised epithelial cells. Detected in testis, small intestine, parotid gland, lung and brain. Small amounts are detected in heart, kidney and spleen (PubMed:10926847). Expressed in skeletal muscle (PubMed:9632638). Two basolateral sorting signals (BSS) in its C-terminal cytoplasmic tail are required to direct SLC16A3 to the basolateral membrane. Belongs to the major facilitator superfamily. Monocarboxylate porter (TC 2.A.1.13) family. Was initially thought to be considered to be a low affinity lactate transporter with negligible affinity for pyruvate (By similarity). However, it was later shown that SLC16A3 is a high affinity lactate transporter with physiologically relevant affinity for pyruvate (By similarity). Was initially assigned as monocarboxylate transporter 3 (MCT3) (PubMed:9632638). However, it was later shown that it corresponds to monocarboxylate transporter 4 (MCT4).