Identification and functional characterization of a Na+-independent neutral amino acid transporter with broad substrate selectivity

We have isolated a cDNA from rat small intestine that encodes a novel Na+-i ndependent neutral amino acid transporter with distinctive characteristics in substrate selectivity and transport property. The encoded protein, desig nated L-type amino acid transporter-2 (LAT-2), shows amino acid sequence si milarity to the system L Na+-independent neutral amino acid transporter LAT -1 (Kanai, Y., Segawa, H., Miyamoto, K., Uchino, H., Takeda, E., and Endou, H.(1998) J. Biol. Chem. 273, 23629-23632) (50% identity) and the system y( +)L transporters y(+)LAT-1 (47%) and KIAA0245/y(+)LAT-2 (45%) (Torrents, D. , Estevez, R., Pineda, M., Fernandez, E., Lloberas, J., Shi, Y.-B., Zorzano , A. and Palacin, M. (1998) J. Biol. Chem. 273, 32437-32445). LAT-2 is a no nglycosylated membrane protein. It requires 4F2 heavy chain, a type II memb rane glycoprotein, for its functional expression in Xenopus oocytes. LAT-2- mediated transport is not dependent on Na+ or Cl- and is inhibited by a sys tem L-specific inhibitor, 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (BCH), indicating that LAT-2 is a second isoform of the system L transporte r. Compared with LAT-1, which prefers large neutral amino acids with branch ed or aromatic side chains, LAT-2 exhibits remarkably broad substrate selec tivity. It transports all of the L-isomers of neutral cu-amino acids. LAT-2 exhibits higher affinity (K-m = 30-50 mu M) to Tyr, Phe, Trp, Thr, Asn, Il e, Cys, Ser, Leu, Val, and Gin and relatively lower affinity (K-m = 180-300 mu M) to His, Ale, Met, and Gly. In addition, LAT-P mediates facilitated d iffusion of substrate amino acids, as distinct from LAT-1, which mediates a mino acid exchange. LAT-2-mediated transport is increased by lowering the p H level, with peak activity at pH 6.25, because of the decrease in the K-m value without changing the V-max value. Because of these functional propert ies and a high level of expression of LAT-S in the small intestine, kidney, placenta, and brain, it is suggested that the heterodimeric complex of LAT -2 and 4F2 heavy chain is involved in the trans-cellular transport of neutr al amino acids in epithelia and blood-tissue barriers.