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 Ii, values, without affecting the V-max values. We demonstrate that although ry(+)LAT1-mediated transport of [C-14]L-leucine was accompanied by the cotransport of Na-22(+), that of [C-14]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 HC 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 required. 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.