Functional properties of a newly cloned fish ortholog of the neutral amino acid transporter B0AT1 (SLC6A19)

The functional properties of an ortholog of the B0AT1 (SLC6A19) amino acid transporter, cloned from the intestine of the sea bass Dicentrachus labrax, were investigated. The two-electrode voltage-clamp technique was applied to Xenopus laevis oocytes heterologously expressing the transporter in order to measure the currents associated with the transport process in different conditions. In particular the substrate specificity, the ionic requirements, and possible effects of pH were examined. Among the organic substrates, leucine, glycine, serine and valine generated the largest transport currents with apparent affinities in the lower millimolar range. The importance of Na+ as the driver ion in the transport process is confirmed, although Li+ is also capable to sustain transport, while K+ is not. No evidence of a relevant role of Cl- in the transport activity was found. Concerning the other two kinds of currents commonly found in electrogenic transporters, very fast presteady-state currents were detected in the absence of organic substrate, while lithium-specific leak currents were not observed. The comparison of these properties with those of the mammalian and insect orthologs may give interesting indication for future structure-function studies in this transporter subfamily.