A novel human amino acid transporter, hNAT3: cDNA cloning, chromosomal mapping, genomic structure, expression, and functional characterization

Amino acid transporters are proteins that transport amino acids across the membrane. We report here the isolation and characterization of a novel huma n cDNA clone encoding a protein of 547 amino acids. This protein shares sim ilar to 50% amino acid sequence homology with the amino acid transporters m ouse mNAT and its orthologs, rat, SN1 and human g17, and mouse GlnT/ATA1 an d ATA2. Expression of this cRNA in Xenopus oocytes revealed that the strong est transport activities were specific for L-alanine. In addition, hNAT3 is a Na+- and pH-dependent, low-affinity transporter and partially tolerates substitution of Na+ by Li+. Since this protein has sequence and functional similarities to the previously identified system N amino acid transporters, we named this protein hNAT3. The genomic DNA sequence encoding the transcri pt of hNAT3 spans over 14 kb with 16 exons and 15 introns. Using fluorescen ce insitu hybridization, we mapped the hNAT3 gene to human chromosome 12q12 -q13. By RT-PCR of embryonic and adult human tissues, hNAT3 was detected to be predominately expressed in the Liver and to a much lesser extent in the muscle, kidney, and pancreas. The data obtained in this study are likely t o offer critical clues for identification of amino acid transporter-associa ted diseases. (C) 2001 Academic Press.