Nakanishi T, Sugawara M, Huang W, Martindale R G, Leibach F H, Ganapathy M E, Prasad P D, Ganapathy V
Department of Biochemistry and Molecular Biology, Medical College of Georgia, Augusta, Georgia, 30912, USA.
Biochem Biophys Res Commun. 2001 Mar;281(5):1343-8. doi: 10.1006/bbrc.2001.4504.
We have cloned a new subtype of the amino acid transport system N from a human liver cell line. This transporter, designated SN2, consists of 472 amino acids and exhibits 62% identity with human SN1 at the level of amino acid sequence. SN2-specific transcripts are expressed predominantly in the stomach, brain, liver, lung, and intestinal tract. The sizes of the transcripts vary in different tissues, indicating tissue-specific alternative splicing of the SN2 mRNA. In contrast, SN1 is expressed primarily in the brain and liver and there is no evidence for the presence of multiple transcripts of varying size for SN1. When expressed in mammalian cells, the cloned human SN2 mediates Na(+)-coupled transport of system N-specific amino acid substrates (glutamine, asparagine, and histidine). In addition, SN2 also transports serine, alanine, and glycine. Anionic amino acids, cationic amino acids, imino acids, and N-alkylated amino acids are not recognized as substrates by human SN2. The SN2-mediated transport process is Li(+)-tolerant and highly pH-dependent. The Michaelis-Menten constant for histidine uptake via human SN2 is 0.6 +/- 0.1 mM. The gene coding for SN2 is located on human chromosome Xp11.23. Successful cloning of SN2 provides the first molecular evidence for the existence of subtypes within the amino acid transport system N in mammalian tissues.
我们已从一种人类肝细胞系中克隆出氨基酸转运系统N的一种新亚型。这种转运体命名为SN2，由472个氨基酸组成，在氨基酸序列水平上与人类SN1有62%的同一性。SN2特异性转录本主要在胃、脑、肝、肺和肠道中表达。不同组织中转录本的大小有所不同，这表明SN2 mRNA存在组织特异性可变剪接。相比之下，SN1主要在脑和肝中表达，没有证据表明存在多种大小不同的SN1转录本。当在哺乳动物细胞中表达时，克隆的人类SN2介导系统N特异性氨基酸底物（谷氨酰胺、天冬酰胺和组氨酸）的Na⁺偶联转运。此外，SN2还转运丝氨酸、丙氨酸和甘氨酸。阴离子氨基酸、阳离子氨基酸、亚氨基酸和N-烷基化氨基酸不被人类SN2识别为底物。SN2介导的转运过程对Li⁺耐受且高度依赖pH值。通过人类SN2摄取组氨酸的米氏常数为0.6±0.1 mM。编码SN2的基因位于人类X染色体的p11.23区域。SN2的成功克隆为哺乳动物组织中氨基酸转运系统N内存在亚型提供了首个分子证据。
