Apical reabsorption of dibasic amino acids and cystine in kidney is mediated by the heteromeric amino acid antiporter rBAT/b0,+AT (system b0,+). Mutations in rBAT cause cystinuria type A, whereas mutations in b0,+AT cause cystinuria type B. b0,+AT is the catalytic subunit, whereas it is believed that rBAT helps the routing of the rBAT/b0,+AT heterodimeric complex to the plasma membrane. In the present study, we have functionally characterized the cystinuria-specific R365W (Arg365→Trp) mutation of human rBAT, which in addition to a trafficking defect, alters functional properties of the b0,+ transporter. In oocytes, where human rBAT interacts with the endogenous b0,+AT subunit to form an active transporter, the rBAT(R365W) mutation caused a defect of arginine efflux without altering arginine influx or apparent affinities for intracellular or extracellular arginine. Transport of lysine or leucine remained unaffected. In HeLa cells, functional expression of rBAT(R365W)/b0,+AT was observed only at the permissive temperature of 33 °C. Under these conditions, the mutated transporter showed 50% reduction of arginine influx and a similar decreased accumulation of dibasic amino acids. Efflux of arginine through the rBAT(R365W)/b0,+AT holotransporter was completely abolished. This supports a two-translocation-pathway model for antiporter b0,+, in which the efflux pathway in the rBAT(R365W)/b0,+AT holotransporter is defective for arginine translocation or dissociation. This is the first direct evidence that mutations in rBAT may modify transport properties of system b0,+.

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