Amino acid transport system A resembles system N in sequence but differs in mechanism

Evidence for the transport of neutral as well as cationic amino acids by ATA3, a novel and liver-specific subtype of amino acid transport system A

We report here on the cloning and functional characterization of the third subtype of amino acid transport system A, designated ATA3 (amino acid transporter A3), from a human liver cell line. This transporter consists of 547 amino acids and is structurally related to the members of the glutamine transporter family. The human ATA3 (hATA3) exhibits 88% identity in amino acid sequence with rat ATA3. The gene coding for hATA3 contains 16 exons and is located on human chromosome 12q13. It is expressed almost exclusively in the liver. hATA3 mediates the transport of neutral amino acids including alpha-(methylamino)isobutyric acid (MeAIB), the model substrate for system A, in a Na(+)-coupled manner and the transport of cationic amino acids in a Na(+)-independent manner. The affinity of hATA3 for cationic amino acids is higher than for neutral amino acids. The transport function of hATA3 is thus similar to that of system y(+)L. The ability of hATA3 to transport cationic amino acids with high affinity is unique among the members of the glutamine transporter family. hATA1 and hATA2, the other two known members of the system A subfamily, show little affinity toward cationic amino acids. hATA3 also differs from hATA1 and hATA2 in exhibiting low affinity for MeAIB. Since liver does not express any of the previously known high-affinity cationic amino acid transporters, ATA3 is likely to provide the major route for the uptake of arginine in this tissue.

The adaptive regulation of amino acid transport system A is associated to changes in ATA2 expression

The activity of transport system A for neutral amino acids is adaptively stimulated upon amino acid starvation. In cultured human fibroblasts this treatment causes an increase in the expression of the ATA2 system A transporter gene. ATA2 mRNA increase and transport stimulation are suppressed by system A substrates, but they are unaffected by other amino acids. Supplementation of amino acid-starved cells with substrates of system A causes a decrease in both ATA2 mRNA and system A transport activity. These results suggest a direct relationship between ATA2 expression and system A transport activity.

The role of system A for neutral amino acid transport in the regulation of cell volume

System A is a secondary active, sodium dependent transport system for neutral amino acids. Strictly coupled with Na,K-ATPase, its activity determines the size of the intracellular amino acid pool, through a complex network of metabolic reaction and exchange fluxes. Many hormones and drugs affect system A activity in specific cell models or tissues. In all the cell models tested thus far the activity of the system is stimulated by amino acid starvation, cell cycle progression, and the incubation under hypertonic conditions. These three conditions produce marked alterations of cell volume. The stimulation of system A activity plays an important role in cell volume restoration, through an expansion of the intracellular amino acid pool. Under normal conditions, system A substrates represent a major fraction of cell compatible osmolytes, organic compounds that exert a protein stabilizing effect. It is, therefore, likely that the activation of system A represents a portion of a more complex response triggered by exposure to stresses of various nature. Since system A transporters have been recently cloned, the molecular bases of these regulatory mechanisms will probably be elucidated in a short time.

Structure and function of ATA3, a new subtype of amino acid transport system A, primarily expressed in the liver and skeletal muscle

To date, two different transporters that are capable of transporting alpha-(methylamino)isobutyric acid, the specific substrate for amino acid transport system A, have been cloned. These two transporters are known as ATA1 and ATA2. We have cloned a third transporter that is able to transport the system A-specific substrate. This new transporter, cloned from rat skeletal muscle and designated rATA3, consists of 547 amino acids and has a high degree of homology to rat ATA1 (47% identity) and rat ATA2 (57% identity). rATA3 mRNA is present only in the liver and skeletal muscle. When expressed in Xenopus laevis oocytes, rATA3 mediates the transport of alpha-[(14)C](methylamino)isobutyric acid and [(3)H]alanine. With the two-microelectrode voltage clamp technique, we have shown that exposure of rATA3-expressing oocytes to neutral, short-chain aliphatic amino acids induces inward currents. The amino acid-induced current is Na(+)-dependent and pH-dependent. Analysis of the currents with alanine as the substrate has shown that the K(0. 5) for alanine (i.e., concentration of the amino acid yielding half-maximal current) is 4.2+/-0.1 mM and that the Na(+):alanine stoichiometry is 1:1.

Posttranscriptional regulation of ATA2 transport during liver regeneration

The recent cloning of ATA2, a cDNA displaying characteristics identical to the System A transporter, has provided the first molecular tool for study of System A-mediated amino acid transport in liver. Despite the 233 +/- 9 and 472 +/- 11% increase in System A transport activity following partial hepatectomy at 6 and 12 h, respectively, the steady-state level of ATA2 mRNA did not show a corresponding marked increase. Examination of the kinetic properties of System A following partial hepatectomy revealed a K(m) of 0.26 +/- 0.04 mM which is consistent with the reported K(m) for ATA2. These results indicate that a System A transporter present in regenerating liver and ATA2 are identical, but that the increase in System A activity following partial hepatectomy does not result from an increase in steady-state levels of ATA2 mRNA. These observations suggest that ATA2-mediated transport of amino acids is regulated at the posttranscriptional level.

Uptake of glutamate into synaptic vesicles by an inorganic phosphate transporter

Previous work has identified two families of proteins that transport classical neurotransmitters into synaptic vesicles, but the protein responsible for vesicular transport of the principal excitatory transmitter glutamate has remained unknown. We demonstrate that a protein that is unrelated to any known neurotransmitter transporters and that was previously suggested to mediate the Na(+)-dependent uptake of inorganic phosphate across the plasma membrane transports glutamate into synaptic vesicles. In addition, we show that this vesicular glutamate transporter, VGLUT1, exhibits a conductance for chloride that is blocked by glutamate.