Catalyzes the exchange of L-carnitine for gamma-butyrobetaine (PubMed:12163501, PubMed:20829798, PubMed:30846799). Can also transport D-carnitine and, with lower efficiency, acetyl-L-carnitine and glycine betaine (PubMed:12163501). (R)-carnitine(out) + 4-(trimethylamino)butanoate(in) = (R)-carnitine(in) + 4-(trimethylamino)butanoate(out) Inhibited by the sulfhydryl reagent N-ethylmaleimide, but not by the ionophore carbonyl cyanide m-chlorophenylhydrazone (CCCP). kcat is 279 min(-1) with L-carnitine as substrate. Amine and polyamine metabolism; carnitine metabolism. Homotrimer (PubMed:16365043, PubMed:20357772, PubMed:20829798, PubMed:30846799). The monomer is stable in the membrane and is fully functional, but physical contact between the protomers of a trimer may facilitate conformational alterations associated with substrate binding (PubMed:30846799). Contains two carnitine binding sites, a primary binding site at the center of the protein and a secondary binding site at the bottom of the intracellular vestibule (PubMed:20357772). Contains two gamma-butyrobetaine binding sites, one in the central transport site, the other in an extracellular binding pocket (PubMed:20829798). Binding of both butyrobetaine molecules is cooperative, indicating that the extracellular site is regulatory (PubMed:20829798). The occupied regulatory site may increase the binding affinity of the transport site and initiates substrate translocation (PubMed:20829798). Both L-carnitine and gamma-butyrobetaine are able to dissociate completely from their primary site into the cytoplasm (PubMed:22843728). Substrate molecules initially located at the secondary sites dissociate even faster into the extra- or intracellular regions (PubMed:22843728). Essential, it cannot be deleted. Belongs to the BCCT transporter (TC 2.A.15) family. CaiT subfamily.