Physiological Roles and Adverse Effects of the Two Cystine Importers of Escherichia coli
Physiological Roles and Adverse Effects of the Two Cystine Importers of Escherichia coli
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DOI：
10.1128/jb.00277-15
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发表时间：
2015-12-01
影响因子：
3.2
通讯作者：
Imlay, James A.
中科院分区：
文献类型：
--
作者：
Imlay, Karin R. Chonoles;Korshunov, Sergey;Imlay, James A.
当将胱氨酸添加到大肠杆菌中时，细菌对过氧化氢变得非常敏感。这种效应是由于细胞内半胱氨酸池扩大所致，半胱氨酸池可以驱动芬顿化学。遗传分析将敏感性与 YdjN 联系起来，YdjN 是一种次级转运蛋白，与 FliY-YecSC ABC 系统一起负责胱氨酸的摄取。 FliY-YecSC 具有纳摩尔 K-m，对于痕量胱氨酸的输入至关重要，而 YdjN 具有微摩尔 K-m，当胱氨酸更丰富时，它是主要输入者。奇怪的是，这两个系统都是由 CysB 对硫缺乏的反应强烈诱导的。 FliY-YecSC系统可以导入多种生物分子，包括二氨基庚二酸；因此，它很容易受到竞争性抑制，大概需要在低硫条件下诱导 YdjN。但结果是，如果微摩尔胱氨酸变得可用，丰富的 YdjN 会大量过量导入它，其量> 细胞总硫需求量的 30 倍。输入的胱氨酸在谷胱甘肽依赖性过程中迅速还原为半胱氨酸。这一作用避免了二硫键应激的危险，但它排除了胱氨酸对 YdjN 的反馈抑制。我们推测 YdjN 不具有半胱氨酸变构位点，因为同构氨基酸丝氨酸可能不适当地结合在其位置上。相反，细胞通过立即排出半胱氨酸来部分解决半胱氨酸的过度积累，完成消耗大量细胞能量的徒劳的导入/减少/导出循环。胱氨酸代谢的这些独特、浪费和危险的特征可以由其他细菌复制。我们建议将 ydjN 重命名为 tcyP，将 fliY-yecSC 重命名为 tcyJLN。 重要性 一般来说，通过转录和变构控制的复杂组合，细胞内代谢物库保持在稳定、无毒的水平。令人惊讶的是，在大肠杆菌中，胱氨酸的主要输入者完全不存在变构控制。这一缺陷导致胱氨酸大量过量输入，从而导致对氧化应激的严重脆弱性，并且只能通过浪费的半胱氨酸流出来补救。缺乏进口管制可能是由于半胱氨酸本身的不寻常特性而合理化的。这种现象证明了反补贴半胱氨酸输出系统的存在是合理的，否则其目的很难理解。它还强调了硫代谢和氧化损伤之间意想不到的联系。尽管这项研究的重点是大肠杆菌，但实验证实类似的现象也发生在其他物种中。
When cystine is added to Escherichia coli, the bacterium becomes remarkably sensitive to hydrogen peroxide. This effect is due to enlarged intracellular pools of cysteine, which can drive Fenton chemistry. Genetic analysis linked the sensitivity to YdjN, a secondary transporter that along with the FliY-YecSC ABC system is responsible for cystine uptake. FliY-YecSC has a nanomolar K-m and is essential for import of trace cystine, whereas YdjN has a micromolar K-m and is the predominant importer when cystine is more abundant. Oddly, both systems are strongly induced by the CysB response to sulfur scarcity. The FliY-YecSC system can import a variety of biomolecules, including diaminopimelate; it is therefore vulnerable to competitive inhibition, presumably warranting YdjN induction under low-sulfur conditions. But the consequence is that if micromolar cystine then becomes available, the abundant YdjN massively overimports it, at > 30 times the total sulfur demand of the cell. The imported cystine is rapidly reduced to cysteine in a glutathione-dependent process. This action avoids the hazard of disulfide stress, but it precludes feedback inhibition of YdjN by cystine. We conjecture that YdjN possesses no cysteine allosteric site because the isostructural amino acid serine might inappropriately bind in its place. Instead, the cell partially resolves the overaccumulation of cysteine by immediately excreting it, completing a futile import/reduction/export cycle that consumes a large amount of cellular energy. These unique, wasteful, and dangerous features of cystine metabolism are reproduced by other bacteria. We propose to rename ydjN as tcyP and fliY-yecSC as tcyJLN.IMPORTANCEIn general, intracellular metabolite pools are kept at steady, nontoxic levels by a sophisticated combination of transcriptional and allosteric controls. Surprisingly, in E. coli allosteric control is utterly absent from the primary importer of cystine. This flaw allows massive overimport of cystine, which causes acute vulnerability to oxidative stress and is remedied only by wasteful cysteine efflux. The lack of import control may be rationalized by the unusual properties of cysteine itself. This phenomenon justifies the existence of countervailing cysteine export systems, whose purpose is otherwise hard to understand. It also highlights an unexpected link between sulfur metabolism and oxidative damage. Although this investigation focused upon E. coli, experiments confirmed that similar phenomena occur in other species.