Babu Ellappan, Kanai Yoshikatsu, Chairoungdua Arthit, Kim Do Kyung, Iribe Yuji, Tangtrongsup Sahatchai, Jutabha Promsuk, Li Yuewei, Ahmed Nesar, Sakamoto Shinichi, Anzai Naohiko, Nagamori Seishi, Endou Hitoshi
Department of Pharmacology and Toxicology, Kyorin University School of Medicine, 6-20-2 Shinkawa, Mitaka, Tokyo 181-8611, Japan.
J Biol Chem. 2003 Oct 31;278(44):43838-45. doi: 10.1074/jbc.M305221200. Epub 2003 Aug 20.
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.
通过表达克隆技术，从FLC4人肝癌细胞中分离出一个编码新型非Na⁺依赖性中性氨基酸转运体的cDNA。当在非洲爪蟾卵母细胞中表达时，编码的蛋白质命名为LAT3（L型氨基酸转运体3）可转运中性氨基酸，如L-亮氨酸、L-异亮氨酸、L-缬氨酸和L-苯丙氨酸。LAT3介导的转运不依赖Na⁺，并被2-氨基双环[2.2.1]庚烷-2-羧酸抑制，这与系统L的特性一致。与已知的与4F2重链形成异二聚体复合物的系统L转运体LAT1和LAT2不同，LAT3在非洲爪蟾卵母细胞中自身具有功能。LAT3推导的氨基酸序列与报道为前列腺癌上调基因但功能未确定的POV1的基因产物相同，而与已鉴定的转运体没有显著相似性。LAT3介导的转运的伊迪-霍夫斯泰因图呈曲线状，而在生理血浆氨基酸浓度下低亲和力成分占主导。除了氨基酸底物外，LAT3还识别氨基酸醇。L-亮氨酸的转运是电中性的，由易化扩散介导。相反，L-亮氨醇、L-缬氨醇和L-苯丙氨醇在电压钳制下具有净正电荷并诱导内向电流，表明这些化合物由LAT3转运。LAT3介导的转运被N-乙基马来酰亚胺预处理抑制，这与最初在肝细胞原代培养中鉴定的系统L2的特性一致。基于底物选择性、亲和力和对N-乙基马来酰亚胺的敏感性，LAT3被认为是一个服务于系统L2的转运体。LAT3应代表一个新的有机溶质转运体家族。
