RBAT IS AN AMINO-ACID EXCHANGER WITH VARIABLE STOICHIOMETRY

The rBAT protein, when expressed in Xenopus oocytes, was previously sh own to reproduce the selectivity of the Na+-independent neutral and ba sic amino acid transport system called b(o,+). More recently, the capa city of rBAT to generate a transmembrane current was demonstrated when addition of neutral amino acids stimulated the efflux of cations (pre sumably basic amino acids) in rBAT-injected oocytes. In the present pa per, aminoisobutyric acid (AIB), a neutral amino acid analogue, was sh own to induce outward currents (efflux of basic amino acids) through r BAT similar to those caused by alanine in terms of affinity, maximal c urrents and I-V curves. Despite generating similar currents, the AIB t ransport rate was more than 30 times lower than that of alanine, thus challenging the assumption that rBAT functions as a classical exchange r. Experiments using a cut-open oocyte voltage clamp demonstrated that AIB was capable of stimulating rBAT-mediated currents from either sid e of the membrane. AIB, like alanine, was able to stimulate the efflux of radiolabeled alanine and arginine while no rBAT-mediated efflux wa s measurable in the absence of external rBAT substrates. These results demonstrate that (i) the presence of amino acids is required on both sides of the membrane for rBAT to mediate amino acid flux and thus rBA T must be some type of exchanger but (ii) rBAT-mediated amino acid inf lux is not stoichiometrically related to the efflux. A model of a ''do uble gated pore'' is proposed to account for these properties of rBAT, which contravene standard models of exchangers and other transporters .