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Mechanism of protonophores-mediated induction of heat-shock response in Escherichia coli.

Jana B, Panja S, Saha S, Basu T - BMC Microbiol. (2009)

Bottom Line: The increase in sigma-32 level was resulted solely from its stabilization, not from its increased synthesis.On the other hand, the protonophores inhibited the translocation of the periplasmic protein alkaline phosphatase (AP), resulting its accumulation in cell cytosol partly in aggregated and partly in dispersed form.As, normally the DnaK-bound sigma-32 was known to be degraded by the heat-shock protease FtsH, our experimental results further suggested that the engagement of DnaK with the non-translocated proteins (like AP) had made the sigma-32 free and stable.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Biochemistry and Biophysics, University of Kalyani, Kalyani - 741 235, West Bengal, India. bimal_edu@rediffmail.com

ABSTRACT

Background: Protonophores are the agents that dissipate the proton-motive-force (PMF) across E. coli plasma membrane. As the PMF is known to be an energy source for the translocation of membrane and periplasmic proteins after their initial syntheses in cell cytoplasm, protonophores therefore inhibit the translocation phenomenon. In addition, protonophores also induce heat-shock-like stress response in E. coli cell. In this study, our motivation was to investigate that how the protonophores-mediated phenomena like inhibition of protein translocation and induction of heat-shock proteins in E. coli were correlated.

Results: Induction of heat-shock-like response in E. coli attained the maximum level after about 20 minutes of cell growth in the presence of a protonophore like carbonyl cyanide m-chloro phenylhydrazone (CCCP) or 2, 4-dinitrophenol (DNP). With induction, cellular level of the heat-shock regulator protein sigma-32 also increased. The increase in sigma-32 level was resulted solely from its stabilization, not from its increased synthesis. On the other hand, the protonophores inhibited the translocation of the periplasmic protein alkaline phosphatase (AP), resulting its accumulation in cell cytosol partly in aggregated and partly in dispersed form. On further cell growth, after withdrawal of the protonophores, the previously accumulated AP could not be translocated out; instead the AP-aggregate had been degraded perhaps by an induced heat-shock protease ClpP. Moreover, the non-translocated AP formed binary complex with the induced heat-shock chaperone DnaK and the excess cellular concentration of DnaK disallowed the induction of heat-shock response by the protonophores.

Conclusion: Our experimental results suggested that the protonophores-mediated accumulation and aggregation of membrane proteins (like AP) in cell cytosol had signaled the induction of heat-shock proteins in E. coli and the non-translocated protein aggregates were possibly degraded by an induced heat-shock protease ClpP. Moreover, the induction of heat-shock response occurred by the stabilization of sigma-32. As, normally the DnaK-bound sigma-32 was known to be degraded by the heat-shock protease FtsH, our experimental results further suggested that the engagement of DnaK with the non-translocated proteins (like AP) had made the sigma-32 free and stable.

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A. Level of active AP in E. coli MPh42 cells grown in the presence of different concentrations of CCCP. Cells were initially grown to log phase (~1.5 × 108 cells/ml) at 30°C in complete MOPS medium and were then transferred to phosphate-less MOPS medium. The re-suspended cells were divided in different parts to treat with the different concentrations of CCCP (0, 10, 30 and 50 μM). The divided cell cultures were then allowed to grow further at 30°C for induction of AP. At different intervals of time, a 1.0 ml cell aliquot was withdrawn from each culture to assay the active AP level. B. Western blot of the different fractions (periplasmic, cytoplasmic and membrane) of E. coli MPh42 cells grown in the presence of CCCP (50 μM). After allowing induction of AP for 30 min, the periplasmic, cytoplasmic and membrane fractions were isolated from equal number of each of the CCCP-treated and the control cells and the western blotting experiment was subsequently performed using anti-AP antibody. Lanes (a, b, c) and (e, f, g) represent the membrane, periplasmic and cytoplasmic fractions of control and CCCP-treated cells respectively; lane d represents purified AP.
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Figure 4: A. Level of active AP in E. coli MPh42 cells grown in the presence of different concentrations of CCCP. Cells were initially grown to log phase (~1.5 × 108 cells/ml) at 30°C in complete MOPS medium and were then transferred to phosphate-less MOPS medium. The re-suspended cells were divided in different parts to treat with the different concentrations of CCCP (0, 10, 30 and 50 μM). The divided cell cultures were then allowed to grow further at 30°C for induction of AP. At different intervals of time, a 1.0 ml cell aliquot was withdrawn from each culture to assay the active AP level. B. Western blot of the different fractions (periplasmic, cytoplasmic and membrane) of E. coli MPh42 cells grown in the presence of CCCP (50 μM). After allowing induction of AP for 30 min, the periplasmic, cytoplasmic and membrane fractions were isolated from equal number of each of the CCCP-treated and the control cells and the western blotting experiment was subsequently performed using anti-AP antibody. Lanes (a, b, c) and (e, f, g) represent the membrane, periplasmic and cytoplasmic fractions of control and CCCP-treated cells respectively; lane d represents purified AP.

Mentions: Besides induction of hsps, protonophores were known to inhibit translocation of the membrane and periplasmic proteins, resulting in their accumulation in cell cytosol as non-functional precursor form [28-30]. In order to find out the detailed molecular correlation between protonophores-mediated induction of heat-shock-like response and inhibition of protein translocation, the inducible periplasmic protein AP of E. coli was selected here as the representative target protein for the translocation experiments. AP was a nonspecific phosphomonoesterase, used to generate inorganic phosphate from a variety of phosphorylated derivatives. The AP gene was known to be inducible as its expression was negatively regulated by the inorganic phosphate – the end product of AP digestion. Thus, the addition of phosphate to the growth medium repressed the induction of AP or in other words, phosphate-less growth medium induced AP in E. coli [31]. When AP was induced in presence of the protonophores, the level of cellular active AP, at any instant of growth, had decreased gradually by the presence of increasing concentrations of CCCP (0 – 50 μM) [fig. 4A] or DNP (0 – 1.5 mM) [not shown] in the growth medium. At 50 μM CCCP concentration, the amount of enzymatically active AP was almost absent. However, the western blot study of the periplasmic, cytoplasmic and membrane fractions of cells using anti-AP antibody (fig. 4B) showed that the lane g, where the cytoplasmic fraction of the CCCP-treated cells was loaded, had contained the induced AP. No considerable AP band was observed in the lanes (f & e), where the periplasmic and membrane fractions of the CCCP-treated cells were loaded respectively. On the other hand, in the case of CCCP-untreated control cells, approximately equal amount of AP was found to be present in both periplasmic (lane b) and cytoplasmic (lane c) fractions; no trace of AP was found in the membrane fraction (lane a). The AP in the cytoplasmic fraction of the control cells (lane c), perhaps, represented the amount of AP that had yet to be translocated to the periplasm. The result of this study revealed that by the presence of CCCP (50 μM) in the growth medium, the induced AP could not be transported out from the cytoplasm to the periplasm. The less intensity of the AP band in lane g compared to the sum of the intensities in lanes b and c implied less induction of AP in cells grown in the presence of CCCP with respect to the control cells; this was consistent with the fact of low growth rate of the CCCP-treated cells (result not shown). The result of this study, therefore, indicated that the presence of CCCP-like protonophore in the growth medium had partially reduced the induction of AP; however, the induced AP could not be transported out from the cytoplasm to the periplasm. AP was known to be synthesized initially in the cytoplasm and then translocated out through the inner membrane to be finally localized as dimeric, active form at the periplasm [32,33]. As the dimerization of AP, through the disulfide bond, could not take place in the reducing milieu of the cytoplasmic environment, the cytosolic pool of the nontranslocated AP in the CCCP-treated cells had shown no activity [34,35].


Mechanism of protonophores-mediated induction of heat-shock response in Escherichia coli.

Jana B, Panja S, Saha S, Basu T - BMC Microbiol. (2009)

A. Level of active AP in E. coli MPh42 cells grown in the presence of different concentrations of CCCP. Cells were initially grown to log phase (~1.5 × 108 cells/ml) at 30°C in complete MOPS medium and were then transferred to phosphate-less MOPS medium. The re-suspended cells were divided in different parts to treat with the different concentrations of CCCP (0, 10, 30 and 50 μM). The divided cell cultures were then allowed to grow further at 30°C for induction of AP. At different intervals of time, a 1.0 ml cell aliquot was withdrawn from each culture to assay the active AP level. B. Western blot of the different fractions (periplasmic, cytoplasmic and membrane) of E. coli MPh42 cells grown in the presence of CCCP (50 μM). After allowing induction of AP for 30 min, the periplasmic, cytoplasmic and membrane fractions were isolated from equal number of each of the CCCP-treated and the control cells and the western blotting experiment was subsequently performed using anti-AP antibody. Lanes (a, b, c) and (e, f, g) represent the membrane, periplasmic and cytoplasmic fractions of control and CCCP-treated cells respectively; lane d represents purified AP.
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Figure 4: A. Level of active AP in E. coli MPh42 cells grown in the presence of different concentrations of CCCP. Cells were initially grown to log phase (~1.5 × 108 cells/ml) at 30°C in complete MOPS medium and were then transferred to phosphate-less MOPS medium. The re-suspended cells were divided in different parts to treat with the different concentrations of CCCP (0, 10, 30 and 50 μM). The divided cell cultures were then allowed to grow further at 30°C for induction of AP. At different intervals of time, a 1.0 ml cell aliquot was withdrawn from each culture to assay the active AP level. B. Western blot of the different fractions (periplasmic, cytoplasmic and membrane) of E. coli MPh42 cells grown in the presence of CCCP (50 μM). After allowing induction of AP for 30 min, the periplasmic, cytoplasmic and membrane fractions were isolated from equal number of each of the CCCP-treated and the control cells and the western blotting experiment was subsequently performed using anti-AP antibody. Lanes (a, b, c) and (e, f, g) represent the membrane, periplasmic and cytoplasmic fractions of control and CCCP-treated cells respectively; lane d represents purified AP.
Mentions: Besides induction of hsps, protonophores were known to inhibit translocation of the membrane and periplasmic proteins, resulting in their accumulation in cell cytosol as non-functional precursor form [28-30]. In order to find out the detailed molecular correlation between protonophores-mediated induction of heat-shock-like response and inhibition of protein translocation, the inducible periplasmic protein AP of E. coli was selected here as the representative target protein for the translocation experiments. AP was a nonspecific phosphomonoesterase, used to generate inorganic phosphate from a variety of phosphorylated derivatives. The AP gene was known to be inducible as its expression was negatively regulated by the inorganic phosphate – the end product of AP digestion. Thus, the addition of phosphate to the growth medium repressed the induction of AP or in other words, phosphate-less growth medium induced AP in E. coli [31]. When AP was induced in presence of the protonophores, the level of cellular active AP, at any instant of growth, had decreased gradually by the presence of increasing concentrations of CCCP (0 – 50 μM) [fig. 4A] or DNP (0 – 1.5 mM) [not shown] in the growth medium. At 50 μM CCCP concentration, the amount of enzymatically active AP was almost absent. However, the western blot study of the periplasmic, cytoplasmic and membrane fractions of cells using anti-AP antibody (fig. 4B) showed that the lane g, where the cytoplasmic fraction of the CCCP-treated cells was loaded, had contained the induced AP. No considerable AP band was observed in the lanes (f & e), where the periplasmic and membrane fractions of the CCCP-treated cells were loaded respectively. On the other hand, in the case of CCCP-untreated control cells, approximately equal amount of AP was found to be present in both periplasmic (lane b) and cytoplasmic (lane c) fractions; no trace of AP was found in the membrane fraction (lane a). The AP in the cytoplasmic fraction of the control cells (lane c), perhaps, represented the amount of AP that had yet to be translocated to the periplasm. The result of this study revealed that by the presence of CCCP (50 μM) in the growth medium, the induced AP could not be transported out from the cytoplasm to the periplasm. The less intensity of the AP band in lane g compared to the sum of the intensities in lanes b and c implied less induction of AP in cells grown in the presence of CCCP with respect to the control cells; this was consistent with the fact of low growth rate of the CCCP-treated cells (result not shown). The result of this study, therefore, indicated that the presence of CCCP-like protonophore in the growth medium had partially reduced the induction of AP; however, the induced AP could not be transported out from the cytoplasm to the periplasm. AP was known to be synthesized initially in the cytoplasm and then translocated out through the inner membrane to be finally localized as dimeric, active form at the periplasm [32,33]. As the dimerization of AP, through the disulfide bond, could not take place in the reducing milieu of the cytoplasmic environment, the cytosolic pool of the nontranslocated AP in the CCCP-treated cells had shown no activity [34,35].

Bottom Line: The increase in sigma-32 level was resulted solely from its stabilization, not from its increased synthesis.On the other hand, the protonophores inhibited the translocation of the periplasmic protein alkaline phosphatase (AP), resulting its accumulation in cell cytosol partly in aggregated and partly in dispersed form.As, normally the DnaK-bound sigma-32 was known to be degraded by the heat-shock protease FtsH, our experimental results further suggested that the engagement of DnaK with the non-translocated proteins (like AP) had made the sigma-32 free and stable.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Biochemistry and Biophysics, University of Kalyani, Kalyani - 741 235, West Bengal, India. bimal_edu@rediffmail.com

ABSTRACT

Background: Protonophores are the agents that dissipate the proton-motive-force (PMF) across E. coli plasma membrane. As the PMF is known to be an energy source for the translocation of membrane and periplasmic proteins after their initial syntheses in cell cytoplasm, protonophores therefore inhibit the translocation phenomenon. In addition, protonophores also induce heat-shock-like stress response in E. coli cell. In this study, our motivation was to investigate that how the protonophores-mediated phenomena like inhibition of protein translocation and induction of heat-shock proteins in E. coli were correlated.

Results: Induction of heat-shock-like response in E. coli attained the maximum level after about 20 minutes of cell growth in the presence of a protonophore like carbonyl cyanide m-chloro phenylhydrazone (CCCP) or 2, 4-dinitrophenol (DNP). With induction, cellular level of the heat-shock regulator protein sigma-32 also increased. The increase in sigma-32 level was resulted solely from its stabilization, not from its increased synthesis. On the other hand, the protonophores inhibited the translocation of the periplasmic protein alkaline phosphatase (AP), resulting its accumulation in cell cytosol partly in aggregated and partly in dispersed form. On further cell growth, after withdrawal of the protonophores, the previously accumulated AP could not be translocated out; instead the AP-aggregate had been degraded perhaps by an induced heat-shock protease ClpP. Moreover, the non-translocated AP formed binary complex with the induced heat-shock chaperone DnaK and the excess cellular concentration of DnaK disallowed the induction of heat-shock response by the protonophores.

Conclusion: Our experimental results suggested that the protonophores-mediated accumulation and aggregation of membrane proteins (like AP) in cell cytosol had signaled the induction of heat-shock proteins in E. coli and the non-translocated protein aggregates were possibly degraded by an induced heat-shock protease ClpP. Moreover, the induction of heat-shock response occurred by the stabilization of sigma-32. As, normally the DnaK-bound sigma-32 was known to be degraded by the heat-shock protease FtsH, our experimental results further suggested that the engagement of DnaK with the non-translocated proteins (like AP) had made the sigma-32 free and stable.

Show MeSH
Related in: MedlinePlus