Transport properties of a system y⁺L neutral and basic amino acid transporter. Insights into the mechanisms of substrate recognition

The properties of system y⁺L-mediated transport were investigated on rat system y⁺L transporter, ry⁺LAT1, coexpressed with the heavy chain of cell surface antigen 4F2 in Xenopus oocytes. ry⁺LAT1-mediated transport of basic amino acids was Na⁺-independent, whereas that of neutral amino acids, although not completely, was dependent on Na⁺, as is typical of system y⁺L- mediated transport. In the absence of Na⁺, lowering of pH increased leucine transport, without affecting lysine transport. Therefore, it is proposed that H⁺, besides Na⁺ and Li⁺, is capable of supporting neutral amino acid transport. Na⁺ and H⁺ augmented leucine transport by decreasing the apparent K(m) values, without affecting the V(max) values. We demonstrate that although ry⁺LAT1-mediated transport of [¹⁴C]L-leucine was accompanied by the cotransport of ²²Na⁺, that of [¹⁴C]L-lysine was not. The Na⁺ to leucine coupling ratio was determined to be 1:1 in the presence of high concentrations of Na⁺. ry⁺LAT1-mediated leucine transport, but not lysine transport, induced intracellular acidification in chinese hamster ovary cells coexpressing ry⁺LAT1 and 4F2 heavy chain in the absence of Na⁺, but not in the presence of physiological concentrations of Na⁺, indicating that cotransport of H⁺ with leucine occurred in the absence of Na⁺. Therefore, for the substrate recognition by ry⁺LAT1, the positive charge on basic amino acid side chains or that conferred by inorganic monovalent cations such as Na⁺ and H⁺, which are cotransported with neutral amino acids, is presumed to be requited. We further demonstrate that ry⁺LAT1, due to its peculiar cation dependence, mediates a heteroexchange, wherein the influx of substrate amino acids is accompanied by the efflux of basic amino acids.