Identification of a novel system L amino acid transporter structurally distinct from heterodimeric amino acid transporters.
A cDNA that encodes a novel Na+-independent neutral amino acid transporter was isolated from FLC4 human hepatocarcinoma cells by expression cloning. When expressed in Xenopus oocytes, the encoded protein designated LAT3 (L-type amino acid transporter 3) transported neutral amino acids such as l-leucine, l-isoleucine, l-valine, and l-phenylalanine. The LAT3-mediated transport was Na+-independent and inhibited by 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid, consistent with the properties of system L. Distinct from already known system L transporters LAT1 and LAT2, which form heterodimeric complex with 4F2 heavy chain, LAT3 was functional by itself in Xenopus oocytes. The deduced amino acid sequence of LAT3 was identical to the gene product of POV1 reported as a prostate cancer-up-regulated gene whose function was not determined, whereas it did not exhibit significant similarity to already identified transporters. The Eadie-Hofstee plots of LAT3-mediated transport were curvilinear, whereas the low affinity component is predominant at physiological plasma amino acid concentration. In addition to amino acid substrates, LAT3 recognized amino acid alcohols. The transport of l-leucine was electroneutral and mediated by a facilitated diffusion. In contrast, l-leucinol, l-valinol, and l-phenylalaninol, which have a net positive charge induced inward currents under voltage clamp, suggesting these compounds are transported by LAT3. LAT3-mediated transport was inhibited by the pretreatment with N-ethylmaleimide, consistent with the property of system L2 originally characterized in hepatocyte primary culture. Based on the substrate selectivity, affinity, and N-ethylmaleimide sensitivity, LAT3 is proposed to be a transporter subserving system L2. LAT3 should denote a new family of organic solute transporters.
Babu
Ellappan
E
Department of Pharmacology and Toxicology, Kyorin University School of Medicine, 6-20-2 Shinkawa, Mitaka, Tokyo 181-8611, Japan.
Kanai
Yoshikatsu
Y
Chairoungdua
Arthit
A
Kim
Do Kyung
DK
Iribe
Yuji
Y
Tangtrongsup
Sahatchai
S
Jutabha
Promsuk
P
Li
Yuewei
Y
Ahmed
Nesar
N
Sakamoto
Shinichi
S
Anzai
Naohiko
N
Nagamori
Seishi
S
Endou
Hitoshi
H
D027062:Amino Acid Transport System L / Q000737:chemistry* / Q000502:physiology*; D026923:Amino Acid Transport Systems, Basic / Q000737:chemistry* / Q000235:genetics*; D000818:Animals; D045744:Cell Line, Tumor; D002478:Cells, Cultured; D003001:Cloning, Molecular; D018076:DNA, Complementary / Q000378:metabolism; D019281:Dimerization; D004305:Dose-Response Relationship, Drug; D004594:Electrophysiology; D005033:Ethylmaleimide / Q000494:pharmacology; D022781:Hepatocytes / Q000378:metabolism; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D007930:Leucine / Q000737:chemistry; D008969:Molecular Sequence Data; D009865:Oocytes / Q000378:metabolism; D013379:Substrate Specificity; D013997:Time Factors; D014018:Tissue Distribution; D015854:Up-Regulation; D014981:Xenopus / Q000378:metabolism
D027062:Amino Acid Transport System L; D026923:Amino Acid Transport Systems, Basic; D018076:DNA, Complementary; C469951:SLC7A6 protein, human; D007930:Leucine; D005033:Ethylmaleimide