Expression Cloning of a Na⁺-independent Aromatic Amino Acid Transporter with Structural Similarity to H⁺/Monocarboxylate Transporters

A cDNA was isolated from rat small intestine by expression cloning which encodes a novel Na⁺-independent transporter for aromatic amino acids. When expressed in Xenopus oocytes, the encoded protein designated as TAT1 (T-type amino acid transporter 1) exhibited Na⁺-independent and low-affinity transport of aromatic amino acids such as tryptophan, tyrosine, and phenylalanine (K_m values: ∼5 mM), consistent with the properties of classical amino acid transport system T. TAT1 accepted some variations of aromatic side chains because it interacted with amino acid-related compounds such as L-DOPA and 3-O-methyl-DOPA. Because TAT1 accepted N-methyl- and N-acetyl-derivatives of aromatic amino acids but did not accept their methylesters, it is proposed that TAT1 recognizes amino acid substrates as anions. Consistent with this, TAT1 exhibited sequence similarity (∼30% identity at the amino acid level) to H⁺/monocarboxylate transporters. Distinct from H⁺/monocarboxylate transporters, however, TAT1 was not coupled with the H⁺ transport but it mediated an electroneutral facilitated diffusion. TAT1 mRNA was strongly expressed in intestine, placenta, and liver. In rat small intestine TAT1 immunoreactivity was detected in the basolateral membrane of the epithelial cells suggesting its role in the transepithelial transport of aromatic amino acids. The identification of the amino acid transporter with distinct structural and functional characteristics will not only facilitate the expansion of amino acid transporter families but also provide new insights into the mechanisms of substrate recognition of organic solute transporters.