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 en codes a novel Na+-independent transporter for aromatic amino acids. When ex pressed in Xenopus oocytes, the encoded protein designated as TAT1 (T-type amino acid transporter I) exhibited Na+-independent and low-affinity transp ort of aromatic amino acids such as tryptophan, tyrosine, and phenylalanine (K-m values: similar to5 mM), consistent with the properties of classical amino acid transport system T, TATI accepted some variations of aromatic si de 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-der ivatives of aromatic amino acids but did not accept their methylesters, it is proposed that TAT1 recognizes amino acid substrates as anions, Consisten t with this, TAT1 exhibited sequence similarity (similar to 30% identity at the amino acid level) to H+/monocarboxylate transporters. Distinct from H/monocarboxylate transporters, however, TAT1 was not coupled with the H+ tr ansport but it mediated an electroneutral facilitated diffusion. TATI mRNA was strongly expressed in intestine, placenta, and liver. In rat small inte stine TAT1 immunoreactivity was detected in the basolateral membrane of the epithelial cells suggesting its role in the transepithelial transport of a romatic 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 insi ghts into the mechanisms of substrate recognition of organic solute transpo rters.