Cloning and functional characterization of a Na+-independent, broad-specific neutral amino acid transporter from mammalian intestine

AbstractWe have isolated a cDNA from a rabbit intestinal cDNA library which, when co-expressed with the heavy chain of the human 4F2 antigen (4F2hc) in mammalian cells, induces system L-like amino acid transport activity. This protein, called LAT2, consists of 535 amino acids and is distinct from LAT1 which also interacts with 4F2hc to induce system L-like amino acid transport activity. LAT2 does not interact with rBAT, a protein with a significant structural similarity to 4F2hc. The 4F2hc/LAT2-mediated transport process differs from the 4F2hc/LAT1-mediated transport in substrate specificity, substrate affinity, tissue distribution, interaction with D-amino acids, and pH-dependence. The 4F2hc/LAT2-associated transport process has a broad specificity towards neutral amino acids with Kt values in the range of 100–1000 μM, does not interact with D-amino acids to any significant extent, and is stimulated by acidic pH. In contrast, the 4F2hc/LAT1-associated transport process has a narrower specificity towards neutral amino acids, but with comparatively higher affinity (Kt values in the range of 10–20 μM), interacts with some D-amino acids with high affinity, and is not influenced by pH. LAT2 is expressed primarily in the small intestine and kidney, whereas LAT1 exhibits a much broader tissue distribution

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

AbstractTo date, two different transporters that are capable of transporting α-(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 α-[14C](methylamino)isobutyric acid and [3H]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 K0.5 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