Human cystinuria-related transporter: Localization and functional characterization

Background. Cystinuria has been proposed to be an inherited defect of apical membrane transport systems for cystine and basic amino acids in renal proximal tubules. Although the mutations of the recently identified transporter BAT1/b^0,+AT have been related to nontype I cystinuria, the function and localization of human BAT1 (hBAT1)/b^0,+AT have not been well characterized. Methods. The cDNA encoding hBAT1 was isolated from human kidney. Fluorescence in situ hybridization was performed to map the hBAT1 gene on human chromosomes. Tissue distribution and localization of expression were examined by Northern blot and immunohistochemical analyses. hBAT1 cDNA was transfected to COS-7 cells with rBAT cDNA, and the uptake and efflux of ¹⁴C-labeled amino acids were measured to determine the functional properties. The roles of protein kinase-dependent phosphorylation were investigated using inhibitors or activators of protein kinases. Results. The hBAT1 gene was mapped to 19q12-13.1 on the human chromosome, which is the locus of nontype I cystinuria. hBAT1 message was expressed predominantly in kidney. hBAT1 protein was localized in the apical membrane of proximal tubules in human kidney. When expressed in COS-7 cells with a type II membrane glycoprotein rBAT (related to b^0,+-amino acid transporter), hBAT1 exhibited the transport activity with the properties of amino acid transport system b^0,+, which transported cystine as well as basic and neutral amino acids presumably via a substrate exchange mechanism. BATl-mediated transport was reduced by the protein kinase A activator and enhanced by the tyrosine kinase inhibitor. Conclusions. hBAT1 exhibited the properties expected for a transporter subserving the high-affinity cystine transport system in renal proximal tubules. The hBAT1 gene was mapped to the locus of nontype I cystinuria, confirming the involvement of hBAT1 in cystinuria.