| Amino acids are the building blocks of proteins, cellular sources of energy and precursors for the synthesis of many signaling molecules. Because of their hydrophilic nature, amino acids require transport proteins to facilitate their translocation across biological membranes. The discovery of amino acid transporters was facilitated by the human genome project and at least thirty-four amino acid transporter genes belonging to five gene families have been identified. This work describes the cloning, functional characterization and tissue distribution of a novel amino acid transporter, ATB0+, which is a member of the Na+/Cl- dependent/SLC6 neurotransmitter transporter gene family. Human ATB0+ is a Na+ and Cl- dependent A&barbelow;mino acid T&barbelow;ransporter that has B&barbelow;road substrate specificity and carries neutral (0) and cationic (+) amino acids. hATB0+ displays the highest affinity for hydrophobic amino acids and lower affinity for cationic and other neutral amino acids. hATB0+ mRNA was expressed in trachea, lung, fetal lung, mammary gland and salivary gland and lower levels were detected in pituitary gland, stomach, colon, prostate, uterus and testis. hATB0+ protein was present in human airway and distal lung as indicated by western blot. The transporter protein was detected in mouse trachea, bronchi, distal lung, stomach, colon, testis and blastocysts. ATB0+ is expressed at the apical membrane of ciliated airway epithelial cells, from trachea to bronchioles, and in alveolar type I cells of the mouse lung. The broad tissue distribution of ATB0+ implies a vital role for the transporter in many organs and systems. The specific localization in the lung suggests that ATB0+ may be involved in clearance of protein degradation products and in pathological conditions where protein accumulates in the alveolar space. The cloning, functional characterization, tissue distribution analysis of ATB0+ has provided the foundation for future investigation of the role of the transporter in physiology and disease. |
