Feldman D H, Harvey W R, Stevens B R
Department of Physiology, University of Florida College of Medicine, Gainesville, Florida 32652, USA.
J Biol Chem. 2000 Aug 11;275(32):24518-26. doi: 10.1074/jbc.M907582199.
A new eukaryotic nutrient amino acid transporter has been cloned from an epithelium that is exposed to high voltages and alkaline pH. The full-length cDNA encoding this novel CAATCH1 (cation-anion-activated Amino acid transporter/channel) was isolated using a polymerase chain reaction-based strategy, and its expression product in Xenopus oocytes displayed a combination of several unique, unanticipated functional properties. CAATCH1 electrophysiological properties resembled those of Na(+),Cl(-)-coupled neurotransmitter amine transporters, although CAATCH1 was cloned from a gut absorptive epithelium rather than from an excitable tissue. Amino acids such as l-proline, l-threonine, and l-methionine elicited complex current-voltage relationships in alkaline pH-dependent CAATCH1 that were reminiscent of the behavior of the dopamine, serotonin, and norepinephrine transporters (DAT, SERT, NET) in the presence of their substrates and pharmacological inhibitors such as cocaine or antidepressants. These I-V relationships indicated a combination of substrate-associated carrier current plus an independent CAATCH1-associated leakage current that could be blocked by certain amino acids. However, unlike all structurally related proteins, CAATCH1 activity is absolutely independent of Cl(-). Unlike related KAAT1, CAATCH1 possesses a methionine-inhibitable constitutive leakage current and is able to switch its narrow substrate selectivity, preferring threonine in the presence of K(+) but preferring proline in the presence of Na(+).
一种新的真核生物营养氨基酸转运体已从暴露于高电压和碱性pH值的上皮细胞中克隆出来。使用基于聚合酶链反应的策略分离出了编码这种新型CAATCH1（阳离子-阴离子激活的氨基酸转运体/通道）的全长cDNA，其在非洲爪蟾卵母细胞中的表达产物表现出几种独特的、意想不到的功能特性的组合。CAATCH1的电生理特性类似于Na(+)、Cl(-)偶联的神经递质胺转运体，尽管CAATCH1是从肠道吸收上皮细胞而不是从可兴奋组织中克隆出来的。诸如L-脯氨酸、L-苏氨酸和L-甲硫氨酸等氨基酸在碱性pH依赖性CAATCH1中引发了复杂的电流-电压关系，这让人联想到多巴胺、5-羟色胺和去甲肾上腺素转运体（DAT、SERT、NET）在其底物和可卡因或抗抑郁药等药理学抑制剂存在时的行为。这些电流-电压关系表明底物相关的载体电流与独立的CAATCH1相关泄漏电流的组合，某些氨基酸可以阻断该泄漏电流。然而，与所有结构相关蛋白不同的是，CAATCH1的活性绝对独立于Cl(-)。与相关的KAAT1不同，CAATCH1具有甲硫氨酸可抑制的组成性泄漏电流，并且能够改变其狭窄的底物选择性，在K(+)存在时优先选择苏氨酸，而在Na(+)存在时优先选择脯氨酸。
