Expression of the amino acid transporter ATB0+ in lung: possible role in luminal protein removal

Normal lung function requires transepithelial clearance of luminal proteins; however, little is known about the molecular mechanisms of protein transport. Protein degradation followed by transport of peptides and amino acids may play an important role in this process. We previously cloned and functionally characterized the neutral and cationic amino acid transporter ATB0+ and showed expression in the lung by mRNA analysis. In this study, the tissue distribution, subcellular localization, and function of the transporter in native tissue were investigated. Western blots showed expression of the ATB0+protein in mouse lung, stomach, colon, testis, blastocysts, and human lung. Immunohistochemistry revealed that ATB0+ is predominantly expressed on the apical membrane of ciliated epithelial cells throughout mouse airways from trachea to bronchioles and in alveolar type I cells. Electrical measurements from mouse trachea preparations showed Na+- and Cl−-dependent, amino acid-induced short-circuit current consiste...