Effects of truncation of the COOH-terminal region of a Na+-independent neutral and basic amino acid transporter on amino acid transport in Xenopus oocytes

To determine the role of a neutral and basic amino acid transporter (NEAT) in amino acid transport, we microinjected several COOH-terminal deletion mutants of NEAT cRNA into Xenopus oocytes and measured transport activity for arginine, leucine, and cystine in the presence and absence of sodium, Wild type NEAT significantly stimulated the uptake of all three amino acids 10-20-fold compared with controls, On the other hand, no mutant, except a Delta 511-685 mutant, stimulated the uptake of these amino acids, The Delta 511-685 mutant significantly increased the uptake of arginine. In the presence of sodium, the Delta 511-685 mutant also increased the uptake of leucine, The Delta 511-685 mutant did not stimulate cystine uptake in the presence or absence of sodium, The stimulation of arginine uptake by the Delta 511-685 mutant was inhibited by a 100-fold excess of unlabeled leucine in the presence of sodium, Inhibition of L-arginine uptake by L-homoserine was seen only in the presence of sodium, and an increase in the inhibition of L-arginine uptake by L-histidine was seen when the extracellular pH was decreased, Furthermore, an inward current in oocytes injected with the Delta 511-685 mutant was recorded electrophysiologically when basic amino acids were applied, Homoserine was also taken up, but sodium was necessary for their transport, These properties of the Delta 511-685 mutant correspond to those of the y(+) amino acid transporter, If NEAT is a component of the b(0,+)-like amino acid transport system, it is unlikely that a mutant protein (Delta 511-685) is able to stimulate an endogenous y(+)-like transport system, These results suggest that NEAT functions as a activator of the amino acid transport system in Xenopus oocytes.