H. Segawa et al., Identification and functional characterization of a Na+-independent neutral amino acid transporter with broad substrate selectivity, J BIOL CHEM, 274(28), 1999, pp. 19745-19751
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.