Identification and functional characterization of all the genes of heterodimeric amino acid transporter family by means of whole genome information

异二聚体氨基酸转运蛋白是由十二个跨膜蛋白(催化亚基)和一个跨膜蛋白(调节亚单位)通过二硫键结合而成。我们以LAT1、BAT1和CAT1的氨基酸序列为查询，对人类和小鼠基因组数据库进行了BLAST搜索。我们已经发现了5个新基因，SLC7-N1~5，它们是SLC7家族的新成员。通过检索表达序列标签(EST)数据库，我们得出结论：其中两个(SLC7-N4和SLC7-N5)是伪基因，因为它们从未出现在EST数据库中。SLC7-N1具有12个跨膜区，被认为是异二聚体氨基酸转运蛋白的催化亚单位。我们已将其命名为AGT-1。AGT-1蛋白定位于小鼠肾脏近、远端小管的基侧膜上。由于AGT-1被认为与未知的调节亚基相连，我们已经构建了AGT-1与已知调节亚基4F2hc或rBAT相连的融合蛋白，并在非洲爪哇卵母细胞中表达了它们。融合蛋白出现在质膜上，并表现出酸性氨基酸的运输。SLC7-N2具有14个跨膜结构域，对精氨酸表现出特别高的亲和力，因此我们将其命名为CAT(阳离子氨基酸转运体5)。CAT在小鼠胎盘合体滋养层细胞中表达，被认为是介导氨基酸通过胎盘屏障的运输，并作为NO合成的前体向胎盘细胞供应精氨酸。SLC7-N3还具有14个跨膜结构域，运输小的中性氨基酸和小的碱性氨基酸，因此我们将其命名为y^+ASC-1。本研究揭示了异二聚体氨基酸转运蛋白家族的所有成员的功能。这项研究的结果将为疾病或病理状况的全基因组分析奠定基础，并促进针对感兴趣的转运蛋白的药物设计。

Heterodimeric amino acid transporters are formed by disulfide bonding of a twelve-membrane spanning protein (catalytic subunit) and a single membrane spanning protein (regulatory subunit). We have performed BLAST searches of human and mouse genome data bases using amino acid sequences of LAT1, BAT1 and CAT1 as queries. We have found 5 new genes, SLC7-N1〜5, as new members of SLC7 family. By searching EST (expressed sequence tag) data bases, we have concluded that two of them (SLC7-N4 and SLC7-N5) are pseudogenes, because they never appear in EST data bases. SLC7-N1 possesses 12 transmembrane domains and is supposed to be a catalytic subunit of heterodimeric amino acid transporters. We have named it AGT-1. AGT-1 protein is found on the basolateral membrane of proximal and distal tubules of mouse kidney. Because AGT-1 is supposed to be linked with unknown regulatory subunit, we have generated fusion proteins in which AGT-1 is connected with known regulatory subunits 4F2hc or rBAT and expressed them in Xenopus oocytes. The fusion proteins appear on the plasma membrane and exhibit the transport of acidic amino acids. SLC7-N2 possesses 14 transmembrane domains and exhibits particularly high affinity to arginine, thus we named it CATS (cationic amino acid transporter 5). CATS is expressed in the syncytio-trophoblast of mouse placenta and supposed to mediate transport of amino acids through the placental barrier as well as to supply placental cells with arginine as a precursor of NO synthesis. SLC7-N3 also possesses 14 transmembrane domains and transports small neutral amino acids as well as small basic amino acids, thus we have named it y^+Asc-1. In this study, the function of all the members of heterodimeric amino acid transporter family has been revealed. The results of this study would provide essential information to establish the basis for the genome-wide analyses of diseases or pathological conditions and facilitate drug design targeting the transporters of interest.