Department of Biological Sciences, Indian Institute of Science Education and Research (IISER) Mohali, SAS Nagar, Punjab, India.
PLoS Genet. 2020 Oct 20;16(10):e1009081. doi: 10.1371/journal.pgen.1009081. eCollection 2020 Oct.
The envelope of gram-negative bacteria serves as the first line of defense against environmental insults. Therefore, its integrity is continuously monitored and maintained by several envelope stress response (ESR) systems. Due to its oxidizing environment, the envelope represents an important site for disulfide bond formation. In Escherichia coli, the periplasmic oxidoreductase, DsbA introduces disulfide bonds in substrate proteins and transfers electrons to the inner membrane oxidoreductase, DsbB. Under aerobic conditions, the reduced form of DsbB is re-oxidized by ubiquinone, an electron carrier in the electron transport chain (ETC). Given the critical role of ubiquinone in transferring electrons derived from the oxidation of reduced cofactors, we were intrigued whether metabolic conditions that generate a large number of reduced cofactors render ubiquinone unavailable for disulfide bond formation. To test this, here we investigated the influence of metabolism of long-chain fatty acid (LCFA), an energy-rich carbon source, on the redox state of the envelope. We show that LCFA degradation increases electron flow in the ETC. Further, whereas cells metabolizing LCFAs exhibit characteristics of insufficient disulfide bond formation, these hallmarks are averted in cells exogenously provided with ubiquinone. Importantly, the ESR pathways, Cpx and σE, are activated by envelope signals generated during LCFA metabolism. Our results argue that Cpx is the primary ESR that senses and maintains envelope redox homeostasis. Amongst the two ESRs, Cpx is induced to a greater extent by LCFAs and senses redox-dependent signal. Further, ubiquinone accumulation during LCFA metabolism is prevented in cells lacking Cpx response, suggesting that Cpx activation helps maintain redox homeostasis by increasing the oxidizing power for disulfide bond formation. Taken together, our results demonstrate an intricate relationship between cellular metabolism and disulfide bond formation dictated by ETC and ESR, and provide the basis for examining whether similar mechanisms control envelope redox status in other gram-negative bacteria.
革兰氏阴性菌的外膜作为抵御环境侵袭的第一道防线。因此，有几个外膜应激反应（ESR）系统持续监测和维持其完整性。由于其氧化环境，外膜是形成二硫键的重要部位。在大肠杆菌中，周质氧化还原酶 DsbA 在底物蛋白中引入二硫键，并将电子转移到内膜氧化还原酶 DsbB。在需氧条件下，电子传递链（ETC）中的电子载体泛醌将 DsbB 的还原形式重新氧化。鉴于泛醌在传递来自还原辅因子氧化的电子方面的关键作用，我们想知道是否会产生大量还原辅因子的代谢条件会使泛醌无法用于形成二硫键。为了检验这一点，我们在这里研究了长链脂肪酸（LCFA）代谢对外膜氧化还原状态的影响。我们表明，LCFA 降解增加了 ETC 中的电子流。此外，代谢 LCFAs 的细胞表现出二硫键形成不足的特征，但这些特征在细胞外源性提供泛醌时得到避免。重要的是，Cpx 和 σE 等 ESR 途径被 LCFA 代谢过程中产生的外膜信号激活。我们的结果表明，Cpx 是感应和维持外膜氧化还原平衡的主要 ESR。在这两个 ESR 中，LCFA 诱导 Cpx 的程度更大，并感应依赖于氧化还原的信号。此外，在缺乏 Cpx 反应的细胞中，LCFA 代谢过程中泛醌的积累被阻止，这表明 Cpx 激活通过增加形成二硫键的氧化能力来帮助维持氧化还原平衡。总之，我们的结果表明，ETC 和 ESR 决定的细胞代谢与二硫键形成之间存在复杂的关系，并为研究其他革兰氏阴性菌的外膜氧化还原状态是否存在类似的机制提供了基础。
