Abstract
A cDNA was isolated from the mouse brain that encodes a novel Na(+)-independent neutral amino acid transporter. The encoded protein, designated as Asc-1 (asc-type amino acid transporter 1), was found to be structurally related to recently identified mammalian amino acid transporters for the transport systems L, y(+)L, x(C)(-), and b(0,+), which are linked, via a disulfide bond, to the type II membrane glycoproteins, 4F2 heavy chain (4F2hc), or rBAT (related to b(0,+) amino acid transporter). Asc-1 required 4F2hc for its functional expression. In Western blot analysis in the nonreducing condition, a 118-kDa band, which seems to correspond to the heterodimeric complex of Asc-1 and 4F2hc, was detected in the mouse brain. The band shifted to 33 kDa in the reducing condition, confirming that Asc-1 and 4F2hc are linked via a disulfide bond. Asc-1-mediated transport was not dependent on the presence of Na(+) or Cl(-). Although Asc-1 showed a high sequence homology (66% identity at the amino acid level) to the Na(+)-independent broad scope neutral amino acid transporter LAT2 (Segawa, H., Fukasawa, Y., Miyamoto, K., Takeda, E., Endou, H., and Kanai, Y. (1999) J. Biol. Chem. 274, 19745-19751), Asc-1 also exhibited distinctive substrate selectivity and transport properties. Asc-1 preferred small neutral amino acids such as Gly, L-Ala, L-Ser, L-Thr, and L-Cys, and alpha-aminoisobutyric acid as substrates. Asc-1 also transported D-isomers of the small neutral amino acids, in particular D-Ser, a putative endogenous modulator of N-methyl-D-aspartate-type glutamate receptors, with high affinity. Asc-1 operated preferentially, although not exclusively, in an exchange mode. Asc-1 mRNA was detected in the brain, lung, small intestine, and placenta. The functional properties of Asc-1 seem to be consistent with those of a transporter subserving the Na(+)-independent small neutral amino acid transport system asc.
Highlights
A cDNA was isolated from the mouse brain that encodes a novel Na؉-independent neutral amino acid transporter
When expressed with 4F2 heavy chain (4F2hc) in Xenopus oocytes, Asc-1 mediates the transport of neutral amino acids, in particular, small amino acids without bulky or branched side chains, in a Naϩindependent manner, reminiscent of the Naϩ-independent small neutral amino acid transport system asc [1, 19]
Asc-1 is a member of the family of amino acid transporters associated with type II membrane glycoproteins (LAT family) [17]
Summary
4F2hc, 4F2 heavy chain; AIB, ␣-aminoisobutyric acid; MeAIB, a-(aminomethyl)isobutyric acid; BCH, 2-Aminobicyclo-(2, 2, 1)-heptane-2-carboxylic acid; MES, 4-morpholineethanesulfonic acid; PIPES, 1,4-piperazinediethanesulfonic acid. (related to b0,ϩ amino acid transporter) to form a system b0,ϩ amino acid transporter, thereby establishing a family of amino acid transporters associated with type II membrane glycoproteins (LAT family) [17, 18]. Because most of the hitherto identified members of the LAT family have been shown to function as Naϩ-independent amino acid transporters [3,4,5,6, 12,13,14,15,16,17], it may be reasonable to assume that the as yet unidentified Naϩ-independent amino acid transporters would belong to this family. We attempted to search for proteins structurally related to the 4F2hc-associated transporters and identified a novel transporter that exhibits functional properties apparently consistent with those of a transporter subserving system asc, a Naϩ-independent transport system for small neutral amino acids [1, 19]
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