Identification of an amino acid transporter associated with the cystinuria-related type II membrane glycoprotein

We identified an amino acid transporter that is associated with the cystinuria-related type II membrane glycoprotein, rBAT (related to b^0,+ amino acid transporter). The transporter designated BAT1 (b^0,+-type amino acid transporter 1) from rat kidney was found to be structurally related to recently identified amino acid transporters for system L, system y⁺L, and system x^-C, which are linked, via a disulfide bond, to the other type II membrane glycoprotein, 4F2hc (4F2 heavy chain). In the non-reducing condition, a 125-kDa band, which seems to correspond to the heterodimeric complex of BAT1 and rBAT, was detected in rat kidney with anti-BAT1 antibody. The band was shifted to 41 kDa in the reducing condition, confirming that BAT1 and rBAT are linked via a disulfide bond. The BAT1 and rBAT proteins were shown to be colocalized in the apical membrane of the renal proximal tubules where massive cystine transport had been proposed. When expressed in COS-7 cells with rBAT, but not with 4F2hc, BAT1 exhibited a Na⁺-independent transport of cystine as well as basic and neutral amino acids with the properties of system b^0,+. The results from the present investigation were used to establish a family of amino acid transporters associated with type II membrane glycoproteins.