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1.
The study of glutathione status in aerobically grown Escherichia coli cultures showed that the total intracellular glutathione (GSHin + GSSGin) level falls by 63% in response to a rapid downshift in the extracellular pH from 6.5 to 5.5. The incubation of E. coli cells in the presence of 50 mM acetate or 10 g/ml gramicidin S decreased the total intracellular glutathione level by 50 and 25%, respectively. The fall in the total intracellular glutathione level was accompanied by a significant decrease in the (GSHin : GSSGin) ratio. The most profound effect on the extracellular glutathione level was exerted by gramicidin S, which augmented the total glutathione level by 1.8 times and the (GSHout : GSSGout) ratio by 2.1 times. The gramicidin S treatment and acetate stress inhibited the growth of mutant E. coli cells defective in glutathione synthesis 5 and 2 times more severely than the growth of the parent cells. The pH downshift and the exposure of E. coli cells to gramicidin S and 50 mM acetate enhanced the expression of the sodA gene coding for superoxide dismutase SodA. 相似文献
2.
Extended exposure of Escherichia coli to temperatures above and below their growth optimum led to significant changes in oxidant production and antioxidant defense. At 20 °C an increase in the intracellular H2O2 concentration and oxidized glutathione (GSSG) level was observed against a background of low levels of reduced glutathione (GSH) and decreased catalase and glutathione reductase (GOR) activities. The intracellular H2O2 and GSSG concentrations had minimal values at 30 and 37 °C, but rose again at 42 °C, suggesting that oxidative processes were intensified at high temperatures. An increase in temperature from 20 to 42 °C led to an elevation in the oxygen respiration rate and superoxide production; a 5-fold increase in the intracellular GSH concentration and in the GSH:GSSG ratio occurred simultaneously. Catalase HPI and GOR activities were elevated 4.4- and 1.5-fold, respectively. Prolonged exposure to sublethal temperatures facilitated an adaptation to subsequent oxidative stress produced by the addition of H2O2. 相似文献
3.
The oxidative stress-inducible cystine/glutamate exchange system, system xc−, transports one molecule of cystine, the oxidized form of cysteine, into cells and thereby releases one molecule of glutamate
into the extracellular space. It consists of two protein components, the 4F2 heavy chain, necessary for membrane location
of the heterodimer, and the xCT protein, responsible for transport activity. Previously, system xc− has been regarded to be a mere supplier of cysteine to cells for the synthesis of proteins and the antioxidant glutathione
(GSH). In that sense, oxygen, electrophilic agents, and bacterial lipopolysaccharide trigger xCT expression to accommodate
with increased oxidative stress by stimulating GSH biosynthesis. However, emerging evidence established that system xc− may act on its own as a GSH-independent redox system by sustaining a redox cycle over the plasma membrane. Hallmarks of this
cycle are cystine uptake, intracellular reduction to cysteine and secretion of the surplus of cysteine into the extracellular
space. Consequently, increased levels of extracellular cysteine provide a reducing microenvironment required for proper cell
signaling and communication, e.g. as already shown for the mechanism of T cell activation. By contrast, the enhanced release
of glutamate in exchange with cystine may trigger neurodegeneration due to glutamate-induced cytotoxic processes. This review
aims to provide a comprehensive picture from the early days of system xc− research up to now. 相似文献
4.
Nkabyo YS Ziegler TR Gu LH Watson WH Jones DP 《American journal of physiology. Gastrointestinal and liver physiology》2002,283(6):G1352-G1359
Cellular redox, maintained by the glutathione (GSH)- and thioredoxin (Trx)-dependent systems, has been implicated in the regulation of a variety of biological processes. The redox state of the GSH system becomes oxidized when cells are induced to differentiate by chemical agents. The aim of this study was to determine the redox state of cellular GSH/glutathione disulfide (GSH/GSSG) and Trx as a consequence of progression from proliferation to contact inhibition and spontaneous differentiation in colon carcinoma (Caco-2) cells. Results showed a significant decrease in GSH concentration, accompanied by a 40-mV oxidation of the cellular GSH/GSSG redox state and a 28-mV oxidation of the extracellular cysteine/cystine redox state in association with confluency and increase in differentiation markers. The redox state of Trx did not change. Thus the two central cellular antioxidant and redox-regulating systems (GSH and Trx) were independently controlled. According to the Nernst equation, a 30-mV oxidation is associated with a 10-fold change in the reduced/oxidized ratio of a redox-sensitive dithiol motif. Therefore, the measured 40-mV oxidation of the cellular GSH/GSSG couple or the 28-mV oxidation of the extracellular cysteine/cystine couple should be sufficient to function in signaling or regulation of differentiation in Caco-2 cells. 相似文献
5.
Masaya Satoh Yoshinobu Hirachi Ai Yoshioka Maki Kobayashi Yasuo Oyama 《Journal of phycology》2002,38(5):983-990
We determined the intracellular contents and concentrations of cysteine and glutathione in five species of marine phytoplankton, Tetraselmis tetrathele (West) Butcher (Prasinophyceae), Porphyridium purpureum (Bory) Drew et Ross (Rhodophyceae), Pavlova sp. (Haptophyceae), Isochrysis sp. (Haptophyceae), and Pleurochrysis carterae (Braarud et Fagerl) Christensen (Haptophyceae), and examined relationships to mercury susceptibility. Intracellular contents (concentrations) of nonprotein thiols in the five species ranged from 119 to 1210 amol (0.66–12.0 mM) for cysteine, 78 to 719 amol (0.65–2.52 mM) for cystine, 31 to 677 amol (0.13–1.25 mM) for reduced glutathione (GSH), and 12 to 123 amol (0.15–0.26 mM) for oxidized glutathione (GSSG). The intracellular contents of the nonprotein thiols were not proportional to the intracellular concentrations because the cell sizes differed. Oxidation ratios of cysteine:cystine and GSH:GSSG were also wide ranging in the five species, and the higher the concentration of the reduced form of nonprotein thiols, the less they tended to be oxidized. Flow cytometric analyses with fluorescein diacetate were used to monitor the effect of HgCl2 on esterase, and the 50% effect concentrations (EC50) were compared in the five species. The EC50 after 3 h exposure to HgCl2 correlated well with the GSH concentrations but not with those of cysteine. These results indicate that the intracellular concentrations of the nonprotein thiols reflect antioxidant activity and susceptibility to heavy metals. 相似文献
6.
Glutathione (γ-glutamylcysteinylglycine, GSH and oxidized glutathione, GSSG), may function as a neuromodulator at the glutamate
receptors and as a neurotransmitter at its own receptors. We studied now the effects of GSH, GSSG, glutathione derivatives
and thiol redox agents on the spontaneous, K+- and glutamate-agonist-evoked releases of [3H]dopamine from mouse striatal slices. The release evoked by 25 mM K+ was inhibited by GSH, S-ethyl-, -propyl-, -butyl- and pentylglutathione and glutathione sulfonate. 5,5′-Dithio-bis-2-nitrobenzoate (DTNB) and l-cystine were also inhibitory, while dithiothreitol (DTT) and l-cysteine enhanced the K+-evoked release. Ten min preperfusion with 50 μM ZnCl2 enhanced the basal unstimulated release but prevented the activation of K+-evoked release by DTT.
Kainate and 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) evoked dopamine release but the other glutamate receptor
agonists N-methyl-d-aspartate (NMDA), glycine (1 mM) and trans-1-aminocyclopentane-1,3-dicarboxylate (t-ACPD, 0.5 mM), and the modulators GSH, GSSG, glutathione sulfonate, S-alkyl-derivatives of glutathione, DTNB, cystine, cysteine and DTT (all 1 mM) were without effect. The release evoked by 1 mM
glutamate was enhanced by 1 mM GSH, while GSSG, glutathionesulfonate and S-alkyl derivatives of glutathione were generally without effect or inhibitory. NMDA (1 mM) evoked release only in the presence
of 1 mM GSH but not with GSSG, other peptides or thiol modulators. l-Cysteine (1 mM) enhanced the glutamate-evoked release similarly to GSH. The activation by 1 mM kainate was inhibited by S-ethyl-, -propyl-, and -butylglutathione and the activation by 0.5 mM AMPA was inhibited by S-ethylglutathione but enhanced by GSSG.
Glutathione alone does not directly evoke dopamine release but may inhibit the depolarization-evoked release by preventing
the toxic effects of high glutamate, and by modulating the cysteine–cystine redox state in Ca2+ channels. GSH also seems to enhance the glutamate-agonist-evoked release via both non-NMDA and NMDA receptors. In this action,
the γ-glutamyl and cysteinyl moieties of glutathione are involved. 相似文献
7.
8.
M. López-Torres R. Pérez-Campo G. Barja de Quiroga 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1991,160(6):655-661
Summary A study of the physiological role of oxygen free radicals in relation to the ageing process was performed using the liver ofRana perezi, an animal with a moderate rate of oxygen consumption and a life span substantially longer than that of laboratory rodents.Among the five different antioxidant enzymes only superoxide dismutase (SOD) showed an age-dependent decrease. Cytochrome oxidase (COX), glutathione status, in vivo and in vitro liver peroxidation, and metabolic rate did not vary as a function of age.Long-term (2.5 months) treatment with aminotriazole and diethyldithiocarbamate depleted catalase (CAT) activity and did not change both glutathione peroxidases (GPx), COX, reduced (GSH) and oxidized (GSSG) glutathione, or metabolic rate. This treatment resulted in great compensatory increases in SOD (to 250–460% of controls) and glutathione reductase (GR) (to 200%) which are possibly responsible for the lack of increase of in vivo and in vitro liver peroxidation and for the absence of changes in survival rate.The comparison of these results with previous data from other species suggests the possibility that decreases in antioxidant capacity in old age are restricted to animal species with high metabolic rates. Nevertheless, ageing can still be due to the continuous presence of small concentrations of O2 radicals in the tissues throughout life in animals with either high or low metabolic rates, because radical scavenging can not be 100% effective. Compensatory homeostasis among antioxidants seems to be a general phenomenon in different species.Abbreviations
AT
3-amino-1,2,4 triazole
-
CAT
catalase
-
COX
cytochrome c oxidase
-
DDC
diethyldithiocarbamate
-
GPx
glutathione peroxidase
-
GR
glutathione reductase
-
GSH
reduced glutathione
-
GSSG
oxidized glutathione
-
MDA
malondialdehyde
-
SOD
superoxide dismutase
-
TBA-RS
thiobarbituric acid-reacting substances
-
VO
2
oxygen consumption 相似文献
9.
Summary It has previously been shown that radioinduced organic radicals can be repaired by hydrogen donation from glutathione (GSH) and this repair is in competition with oxygen (damage fixation).In this paper the influence of exogenous glutathione on the radiation response of the enzyme alcoholdehydrogenase (YADH), DNA in vitro, andE. coli B/r cells has been investigated.GSH is observed to protect YADH essentially by free radical scavenging mechanisms in both presence or absence of oxygen. The same mechanism seems operate in the radioprotection afforded by GSH to DNA in vitro.E. coli B/r cells are protected at higher extent by GSH than its oxidized form (GSSG); the possibility that GSH penetrate into bacterial cells more easily that GSSG can explain their different behaviour.None of the three systems studied has provided definitive support for the occurrence of the hydrogen donation reaction in the radioprotective mechanisms of GSH versus biomolecules and bacterial cells. 相似文献
10.
Il-Sup Kim Sun-Young Shin Young-Saeng Kim Hyun-Young Kim Ho-Sung Yoon 《Molecules and cells》2009,28(5):479-487
Glutathione reductase (GR) is an enzyme that recycles a key cellular antioxidant molecule glutathione (GSH) from its oxidized
form (GSSG) thus maintaining cellular redox homeostasis. A recombinant plasmid to overexpress a GR of Brassica rapa subsp. pekinensis (BrGR) in E. coli BL21 (DE3) was constructed using an expression vector pKM260. Expression of the introduced gene was confirmed by semiquantitative
RT-PCR, immunoblotting and enzyme assays. Purification of the BrGR protein was performed by IMAC method and indicated that the BrGR was a dimmer. The BrGR required NADPH as a cofactor and specific activity was approximately 458 U. The BrGR-expressing E. coli cells showed increased GR activity and tolerance to H2O2, menadione, and heavy metal (CdCl2, ZnCl2 and AlCl2)-mediated growth inhibition. The ectopic expression of BrGR provoked the co-regulation of a variety of antioxidant enzymes including catalase, superoxide dismutase, glutathione peroxidase,
and glucose-6-phosphate dehydrogenase. Consequently, the transformed cells showed decreased hydroperoxide levels when exposed
to stressful conditions. A proteomic analysis demonstrated the higher level of induction of proteins involved in glycolysis,
detoxification/oxidative stress response, protein folding, transport/binding proteins, cell envelope/porins, and protein translation
and modification when exposed to H2O2 stress. Taken together, these results indicate that the plant GR protein is functional in a cooperative way in the E. coli system to protect cells against oxidative stress. 相似文献
11.
The potent pro‐oxidant activity of rhododendrol–eumelanin induces cysteine depletion in B16 melanoma cells 下载免费PDF全文
Shosuke Ito Masae Okura Kazumasa Wakamatsu Toshiharu Yamashita 《Pigment cell & melanoma research》2017,30(1):63-67
RS‐4‐(4‐Hydroxyphenyl)‐2‐butanol (rhododendrol, RD), a skin‐whitening agent, is known to induce leukoderma in some people. To explore the mechanism underlying this effect, we previously showed that the oxidation of RD with mushroom or human tyrosinase produces cytotoxic quinone oxidation products. We then examined the metabolism of RD in B16F1 melanoma cells in vitro and detected RD‐pheomelanin and RD‐quinone bound to non‐protein and protein thiols. In this study, we examined the changes in glutathione (GSH) and cysteine in B16 cells exposed to RD for up to 24 h. We find that the levels of cysteine, but not those of GSH, decrease during 0.5‐ to 3‐h exposure, due to oxidation to cystine. This pro‐oxidant activity was then examined using synthetic melanins. Indeed, we find that RD‐eumelanin exerts a pro‐oxidant activity as potent as Dopa‐pheomelanin. GSH, cysteine, ascorbic acid, and NADH were oxidized by RD‐eumelanin with a concomitant production of H2O2. We propose that RD‐eumelanin induces cytotoxicity through its potent pro‐oxidant activity. 相似文献
12.
Jennifer M. Hsieh Gabriel Mercado Besserer M. Gregor Madej Ha‐Quyen Bui Seunghyug Kwon Jeff Abramson 《Protein science : a publication of the Protein Society》2010,19(4):868-880
Low expression and instability during isolation are major obstacles preventing adequate structure‐function characterization of membrane proteins (MPs). To increase the likelihood of generating large quantities of protein, C‐terminally fused green fluorescent protein (GFP) is commonly used as a reporter for monitoring expression and evaluating purification. This technique has mainly been restricted to MPs with intracellular C‐termini (Cin) due to GFP's inability to fluoresce in the Escherichia coli periplasm. With the aid of Glycophorin A, a single transmembrane spanning protein, we developed a method to convert MPs with extracellular C‐termini (Cout) to Cin ones providing a conduit for implementing GFP reporting. We tested this method on eleven MPs with predicted Cout topology resulting in high level expression. For nine of the eleven MPs, a stable, monodisperse protein‐detergent complex was identified using an extended fluorescence‐detection size exclusion chromatography procedure that monitors protein stability over time, a critical parameter affecting the success of structure‐function studies. Five MPs were successfully cleaved from the GFP tag by site‐specific proteolysis and purified to homogeneity. To address the challenge of inefficient proteolysis, we explored expression and purification conditions in the absence of the fusion tag. Contrary to previous studies, optimal expression conditions established with the fusion were not directly transferable for overexpression in the absence of the GFP tag. These studies establish a broadly applicable method for GFP screening of MPs with Cout topology, yielding sufficient protein suitable for structure‐function studies and are superior to expression and purification in the absence GFP fusion tagging. 相似文献
13.
Glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.13; GAPDH) from the cyanobacteriumAnacystis nidulans was activated up to five-fold by reduced glutathione (GSH) in the physiological concentration range (0.1–2 mM GSH). Non-physiological reductants, like dithiothreitol (DTT) and -mercaptoethanol, also activated the enzyme. Oxidized glutathione (GSSG) had no effect on the cyanobacterial GAPDH but treatment with H2O2 led to a rapid, reversible deactivation of both untreated and GSH-treated enzyme preparations. GSH reversed the inhibition induced by H2O2. An oligomeric form of the enzyme (apparentM
r440,000) was dissociated by GSH into a lower-M
r, more active enzyme form (M
r200,000). The enzyme was shown to obey regular Michaelis-Menten kinetics. The activation of GAPDH by GSH was associated with a decrease inK
m and an increase inV
max values of the enzyme for 3-phosphoglycerate. GSH had virtually no effect on a GAPDH preparation isolated from corn chloroplasts and studied for comparison.Abbreviations GAPDH
glyceraldehyde-3-phosphate dehydrogenase
- GSH
reduced glutathione
- GSSG
oxidized glutathione
- DTT
dithiothreitol 相似文献
14.
Shigeru Kubo Kyoko Tanimura Hideki Nishio Naoyoshi Chino Tadashi Teshima Terutoshi Kimura Yuji Nishiuchi 《International journal of peptide research and therapeutics》2008,14(4):341-349
The optimal conditions were determined for oxidative folding of the reduced human α-defensins, HNP1, HNP2, HNP3 and HD5, preferentially
into their native disulfide structures. Since the human α-defensin-molecule in both reduced and oxidized forms raised a solubility
problem arising from its basic and hydrophobic compositions, buffer concentration had to be lowered and cosolvent, such as
CH3CN, had to be added to the folding medium in the presence of reduced and oxidized gluthathione (GSH/GSSG) to prevent aggregation
and also to realize predominant formation of the native conformer. The four synthetic human α-defensins of high homogeneity
were confirmed to exhibit the same antimicrobial potencies against E. coli as those reported for the natural products. All these peptides were shown to possess the native disulfide structure by sequence
analyses and mass measurements with cystine segments obtained by enzymatic digestion. Edman degradation allowed for disulfide
assignment of cystine segments involving adjacent Cys residues composed of three peptide chains, for which two possible disulfide
modes could be considered, with the guidance of the cycles detecting diPTH cystine. As for HNP1, HNP2 and HNP3, however, diPTH
cystine was expected at the same cycles in both structures, which would have resulted in not being able to distinguish between
the two alternative modes. To avoid this, it was necessary to provide an acetyl tag for the specific peptide chain originating
from the N-terminus. Edman degradation of cystine segments tagged with the acetyl group would be a practical procedure for analyzing
disulfide structures involving adjacent Cys residues. 相似文献
15.
The role of glutathione (GSH) in the differentiated state of insulin-secreting cells was studied using 2-mercaptoethanol as a means of varying intracellular GSH levels. 2-Mercaptoethanol (50 microM) caused a marked increase of GSH in two rat insulinoma cell lines, RINm5F and INS-1, the latter being dependent on the presence of 2-mercaptoethanol for survival in tissue culture. The effect of 2-mercaptoethanol on GSH was shared by other thiol compounds. Since in other cell types 2-mercaptoethanol is thought to act on cystine transport, thereby increasing the supply of cysteine for GSH synthesis, we have studied [35S]cystine-uptake in INS-1 cells. At equimolar concentrations to cystine, 2-mercaptoethanol caused stimulation of [35S]cystine-uptake. The effect persisted in the absence of extracellular Na+, probably suggesting the involvement of the Xc- carrier system. INS-1 cells with a high GSH level, cultured 48 h with 2-mercaptoethanol, displayed a lower cystine uptake than control cells with a low GSH content. The effect of variations of the GSH levels on short-term insulin release was studied. No alteration of glyceraldehyde-induced or KCl-induced insulin release in RINm5F cells was detected. In contrast, both in islets and in INS-1 cells, a high GSH level was associated with a slightly lower insulin release. In INS-1 cells the effect was more marked at low glucose concentrations, resulting in an improved stimulation of insulin secretion. On the other hand, in islets, a decrease in the incremental insulin release evoked by glucose was seen. As in other cell types, oxidized glutathione (GSSG) was less than 5% of total GSH, and in INS-1 cells no change in the GSH/GSSG ratio was detected during glucose-induced or 3-isobutyl-1-methylxanthine-induced insulin release. In conclusion, 2-mercaptoethanol-dependent INS-1 cells, as well as RINm5F cells and islets of Langerhans, display a low capacity in maintaining intracellular levels of GSH in tissue culture without extracellular thiol supplementation; 2-mercaptoethanol possibly acts by promoting cyst(e)ine transport; changes in GSH levels caused a moderate effect on the differentiated function of insulin-secreting cells. 相似文献
16.
The effect of glutathione (GSH) on the ultraviolet (UV) induction of lambda prophage was investigated in lysogenic Escherichia coli. The data showed that extracellular GSH could inhibit the UV induction of lambda prophage. The inhibitory rates were concentration
dependent, and the maximal rate obtained was 94% with 3.0 M GSH. The effect was also measured in three different lambda lysogens:
a wild-type strain (wt), an isogenic GSH-deficient strain, and an isogenic strain producing increased amounts of GSH. The
result showed that when subjected to UV irradiation (254 nm, 60 J m−2), GSH-deficient strain was approximately fivefold more sensitive to be lysed than wt, whereas the strain with higher intracellular
GSH levels was only 28% susceptible to be lysed. With electron spin resonance and spin trapping techniques, we observed that
free radical signals occurred in the suspensions of UV irradiated lysogenic cells and the intensity of signals was influenced
by GSH levels. These results indicate that GSH can significantly inhibit the UV induction of lambda prophage, and that this
effect is correlated to its capacity to scavenge free radicals generated after UV irradiation. 相似文献
17.
Ascorbate and glutathione homeostasis in vascular smooth muscle cells: cooperation with endothelial cells 总被引:1,自引:0,他引:1
Voskoboinik Ilia; Soderholm Karin; Cotgreave Ian A. 《American journal of physiology. Cell physiology》1998,275(4):C1031
Human umbilical vein smooth muscle cells (HUVSMCs) utilizeextracellular cystine, glutathione (GSH), andN-acetylcysteine (NAC) to synthesizecellular GSH. Extracellular cystine was effective from 5 µM, whereasGSH and NAC were required at 100 µM for comparable effects. Theefficacy of extracellular GSH was dependent on de novo GSH synthesis,indicating a dependence on cellular -glutamyltransferase (glutamyltranspeptidase). Coculture of syngenetic HUVSMCs and corresponding human umbilical vein endothelial cells (HUVECs) on poroussupports restricted cystine- or GSH-stimulated synthesis of HUVSMC GSHwhen supplied on the "luminal" endothelial side. Thus HUVSMC GSHrapidly attained a steady-state level below that achieved in theabsence of interposed HUVECs. HUVSMCs also readily utilizeboth reduced ascorbate (AA) and oxidized dehydroascorbate (DHAA) overthe range 50-500 µM. Phloretin effectively blocked both AA- andDHAA-stimulated assimilation of intracellular AA, indicating a role fora glucose transporter in their transport. Uptake of extracellular AAwas also sensitive to extracellular, but not intracellular, thioldepletion. When AA was applied to the endothelial side of the coculturemodel, assimilation of intracellular AA in HUVSMCs was restricted to asteady-state level below that achieved by free access. 相似文献
18.
19.
The effect of exogenous application of 28-Homobrassinolide (HBR) on radish (Raphanus sativus L.) seedlings under zinc (Zn2+) stress on glutathione (GSH) production, consumption and changes in redox status was investigated. Zinc toxicity resulted in oxidative burst as evidenced by increased accumulation of hydrogen peroxide (H2O2) and malondialdehyde (MDA) content. These stress indices were significantly decreased by HBR supplementation. Under Zn2+ stress, GSH pool was decreased, while the contribution of oxidized glutathione (GSSG) to total GSH increased (GSSH/GSH ratio), this translated into significant reduction of GSH redox homeostasis. In addition, an increase of phytochelatins (PCs) was observed. In radish seedlings under Zn2+ stress, the activities of gamma-glutamylcysteine synthetase (γ-ECS), glutathione synthetase (GS), glutathione peroxidase (GPX), glutathione-S-transferase (GST) and cysteine (Cys) levels increased but the activity of glutathione reductase (GR) decreased. However, application of HBR increased the GSH pool and maintained their redox ratio by increasing the enzyme activities of GSH biosynthesis (γ-ECS and GS) and GSH metabolism (GR, GPX and GST). The results of present study are novel in being the first to demonstrate that exogenous application of HBR modulates the GSH synthesis, metabolism and redox homeostasis to confer resistance against Zn2+ induced oxidative stress. 相似文献
20.
Transmembrane helices (TMHs) in membrane proteins can be orientated with their N-terminus towards the cytoplasm (Nin), or facing the non-cytoplasmic side (Nout). Most membrane proteins are inserted co-translationally into membranes, aided by Sec-type translocons. Since the final orientation of Nin- and Nout-orientated TMHs differs, they could also interact differently with the translocon and the surrounding membrane during insertion. We measured pulling forces exerted on Nin-orientated TMHs during co-translational insertion into the inner membrane of Escherichia coli. Our results demonstrate that Nin-orientated TMHs experience a weaker pulling force but retain the overall biphasic force profile seen previously for Nout-orientated TMHs (Ismail et al., 2012 [1]). 相似文献