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Journal of Bacteriology, June 2005, p. 4207-4213, Vol. 187, No. 12
0021-9193/05/$08.00+0 doi:10.1128/JB.187.12.4207-4213.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Department of Cell and Molecular Biology, Microbiology, Göteborg University, Box 462, 405 30 Göteborg, Sweden,1 Centro Andaluz de Biologia del Desarrollo (CABD), University "Pablo de Olavide," Ctra Utrera km1, ES-41013 Seville, Spain,2 Laboratoire de Chimie Bactérienne, CNRS-UPR9043, 31 Chemin Joseph Aiguier, 13402 Marseille, France3
Received 21 January 2005/ Accepted 8 March 2005
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32, effectively reduced stasis-induced carbonylation. Separate overproduction of the major chaperone systems, DnaK/DnaJ and GroEL/GroES, established that the former of these is more important in counteracting protein carbonylation. Deletion of the heat shock proteases Lon and HslVU enhanced carbonylation whereas a clpP deletion alone had no effect. However, ClpP appears to have a role in reducing protein carbonyls in cells lacking Lon and HslVU. Proteomic immunodetection of carbonylated proteins in the wild-type, lon, and hslVU strains demonstrated that the same spectrum of proteins displayed a higher load of carbonyl groups in the lon and hslVU mutants. These proteins included the ß-subunit of RNA polymerase, elongation factors Tu and G, the E1 subunit of the pyruvate dehydrogenase complex, isocitrate dehydrogenase, 6-phosphogluconate dehydrogenase, and serine hydroxymethyltranferase. |
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The levels of carbonylated proteins increase rapidly as Escherichia coli cells enter stationary phase as a result of carbon/energy (3, 9) or nitrogen (3) starvation. This modification has been associated with the reduced plating efficiency of stationary phase cells (7). Specifically, using an in situ detection of protein carbonyls in single cells and a density gradient centrifugation technique to separate culturable and nonculturable cells of the same chronological age it was demonstrated that proteins of the nonculturable cell population exhibited markedly higher levels of irreversible carbonylation (7). Apart from the fact that the RpoS and OxyR regulons and the superoxide dismutases and catalases are important in mitigating protein carbonylation (10), little is known about the function and identity of cellular defenses against this oxidation. Since heat shock genes have a role in cellular resistance against oxidative stress (14, 24) and are increasingly expressed during oxidant exposure (2, 26), we tested whether the heat shock regulon is involved in attenuating stasis-induced carbonylation. We report that several members of this regulon, both chaperones and proteases, are key factors in the cellular defense against this deleterious oxidative modification.
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32 mouse monoclonal antibodies were from Neoclone. Anti-mouse immunoglobulin G peroxidase conjugates, trypsin, and isopropyl-ß-galactopyranoside (IPTG) were from Sigma. The chemiluminescence blotting substrate (ECL+) was obtained from Amersham Corp., and the Immobilon-P polyvinylidene difluoride (PVDF) membrane was from Millipore. Bicinchoninic protein assay reagents were purchased from Pierce. The precast polyacrylamide gels used for one-dimensional (1-D) electrophoresis were either Criterion 10% Tris-HCl (Bio-Rad) or NuPAGE 12% Bis-Tris gel (Invitrogen). The ampholines (Resolyte 4-8) used for two-dimensional electrophoresis were from BDH (VWR International). The LIVE/DEAD BacLight bacterial viability kit was from Molecular Probe. All chemicals and reagents were used according to instructions provided by the manufacturer.
Bacterial strains, plasmids, and growth conditions
The E. coli K-12 strains and plasmids used in this study are listed in Table 1. Cultures were grown aerobically at 37°C in minimal M9 medium (18) supplemented with thiamine (10 mM), all 20 amino acids, and glucose (0.2%) in Erlenmeyer flasks in a rotary shaker. When appropriate, the medium was supplemented with carbenicillin (100 µg/ml), tetracycline (20 µg/ml), kanamycin (50 µg/ml) or spectinomycin (100 µg/ml). To overproduce Hsps, IPTG was added to the exponentially growing cultures at an optical density at 420 nm (OD420) of 0.05 to induce expression from the plasmid pKV1278 (
32), pBB535 (DnaK/DnaJ), or pBB541 (GroES/GroEL). Early stationary phase samples were collected 30 min after exponential growth (measured as OD420) ceased due to glucose depletion in the medium. For protein stability measurements, cells were grown exponentially at 37°C and at an OD420 of 0.1 and IPTG was added to induce expression from the plasmid pKV1278 (
32). After 3 h, protein synthesis was blocked by addition of spectinomycin (400 µg/ml).
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TABLE 1. E. coli strains and plasmids used in this work
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Protein identification.
Proteins separated on two-dimensional gels were blotted onto PVDF membranes and used for immunodetection of carbonylated proteins. The same membranes were stained with Coomassie brilliant blue after exposure in the charge-coupled device camera and were used as references when matching the carbonylated proteins to the spots on the 2-D gels. The matched protein spots were subsequently cut out and used for mass-spectrometric analysis. Samples were analyzed using a matrix-assisted laser desorption ionization-linear reflection mass spectrometer (Micromass, Manchester, UK) in reflection mode. Tryptic digest (0.5 µl) was mixed with 0.5 µl matrix solution (12 mg/ml
-cyano-4-OH-cinnamic acid in acetonitrile/water [1:1]-0.1% trifluoroacetic acid) directly on the matrix-assisted laser desorption ionization probe and allowed to dry at ambient conditions. ACTH (18-39) MH+ 2465.199 was used as an external and tryptic autodigest MH+ 2211.105 as an internal lock mass. Monoisotopic mass values were used for peptide mass mapping using MASCOT (available at www.matrixscience.com) against the nr database at the National Center for Biotechnology Information. All identifications had a score equivalent to a 95% level of confidence. When necessary, samples were purified and concentrated with ZipTips containing C18 resin according to the manufacturer's instructions. The peptides eluted in 3.5 µl of acetonitrile/water (1:1) containing 0.1% formic acid were then analyzed by electrospray ionization mass spectrometry/mass spectrometry (MS/MS) on a quadrupole time-of-flight ultima atmospheric pressure ionization apparatus (Micromass, Manchester, UK). Fragment ion data were acquired by nanoflow electrospray using argon as the collision gas. The MS/MS data were used as input for protein identification by MASCOT (www.matrixscience.com). No restrictions in species, molecular weight, or pI were applied when searching against the nr database of the National Center for Biotechnology Information. Proteins identified all had a score equivalent to a 95% confidence level or higher.
Viability assays. Cells collected after 3 h in stationary phase were serially diluted in M9 medium lacking glucose and amino acids prior to plating onto solid LB medium in the presence and absence of the appropriate antibiotic selection. To induce expression from plasmid pBB535 (ÅF38), IPTG was added at an OD420 of 1.5 before transition into stationary phase. Viability was also determined by microscopic analyses using BacLight LIVE/DEAD methodology, whereby living cells fluoresce in green and dead cells in red when excited at 590 nm (12).
Carbonylation assays. Analyses of carbonylated proteins were performed using the chemical and immunological reagents of an OxyBlot oxidized protein detection kit. The carbonyl groups in the protein side chains were derivatized to 2,4-dinitrophenylhydrazone (DNP-hydrazone) by reaction with 2,4-dinitrophenylhydrazine. The DNP-derivatized proteins were analyzed immunochemically on one- or two-dimensional Western blots or directly dot blotted onto PVDF membranes as described previously (9). In general, 1 µg of protein was loaded into the slot-blot apparatus and 10 µg onto 1- or 2-D gels.
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32 and DnaK/DnaJ prevents stasis-induced oxidation of proteins
Previous studies show that certain proteins are specifically susceptible to stasis-induced oxidation (9), and we determined the specificity of protein carbonylation at the time when this is maximal upon growth arrest (early stationary phase; see Fig. 1). A number of carbonylated proteins, some of which have not previously been shown to be carbonylated, were identified by mass spectrometry (Table 2). The identification suggests that several different processes, including, e.g., information transfer (transcription and translation), central metabolism (glycolysis and TCA cycle), protein folding, and fatty acid biosynthesis, are subjected to stasis-induced deterioration. Some of the identified carbonylated proteins have been demonstrated previously to be targets for the DnaK chaperone, i.e., these proteins are prone to aggregate in the absence of DnaK (19). Based on these results and the fact that aberrant proteins are more susceptible to carbonylation than the native counterparts (3, 8), we entertained the idea that stationary phase carbonylation of proteins may, in part, be a consequence of stasis-induced mistranslation (3) overwhelming the heat shock chaperones. If so, elevated levels of heat shock chaperones could partly counteract the effect of such mistranslation and attenuate stasis-induced carbonylation.
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FIG. 1. Protein carbonylation during growth and in stationary phase. Relative protein carbonylation levels (black squares) and the optical density of the wild-type culture (MG1655 lac) (filled circles) are shown during growth and growth arrest caused by glucose depletion. Carbonyl levels were determined by one-dimensional Western blot immunoassays and quantified using Image Gauge software. Carbonyl levels were related to that obtained during exponential growth, which was assigned a value of 1.0. The arrow indicates the time at which samples were obtained for identification of carbonylated proteins (see Table 2). The experiment was repeated at least three times. Representative results are shown, and there was always less than 15% variation between experiments.
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TABLE 2. Proteins subjected to stasis-induced carbonylation at the time when carbonyl content is maximal. Proteins in bold have been demonstrated to be substrates for DnaK (19).
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32 mitigated protein carbonylation in cells entering a growth-arrested state. To confirm that overproduction of
32 resulted in significantly elevated concentrations of Hsps, we checked the levels of DnaK and GroEL (Fig. 2A). As seen in Fig. 2A, overproduction of the heat shock regulon drastically reduced (3.5-fold lower than the control) stasis-induced carbonylation. In fact, carbonylation did not increase upon growth arrest in cells with ectopically elevated Hsp levels.
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FIG. 2. Effects of overproducing the Hsp regulon on protein oxidation. Severalfold change in the levels of DnaK, GroEL, and carbonylated proteins as a consequence of overproducing 32 (ÅF41) (A), DnaK/DnaJ (ÅF38) (B), and GroEL/GroES (ÅF64) (C) in cells entering a growth arrested state (30 min after growth ceased) is shown. "1" on the y axis means no change compared to wild type, while "2" means a twofold increase and "2" means a twofold decrease. The cells were grown with (100 µM) and without IPTG prior to starvation. Protein and carbonyl levels were determined by one-dimensional Western blot immunoassays and quantified using Image Gauge software. All levels were related to that obtained in the control culture without overproduction, which was assigned a value of 1.0.
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32 (Fig. 2B). This was due not to a reduced production of proteins that happen to be carbonylation sensitive but rather to a reduction in such proteins' carbonylation load. For example, the carbonyl level of elongation factor EF-Tu, when normalized to the total levels of EF-Tu present, was substantially reduced by DnaK/DnaJ overproduction (Fig. 3A). As depicted (Fig. 3B), the total amount of carbonylated proteins was generally lower upon DnaK/DnaJ overproduction, indicating a general protective function of this chaperone system.
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FIG. 3. Effects of overproducing DnaK/DnaJ (ÅF38) on EF-Tu carbonylation during glucose starvation (A). Open squares, no IPTG; gray squares; 100 µM IPTG; black squares, 250 µM IPTG. Carbonyl levels are normalized to EF-Tu levels. (B) General pattern and level of carbonylated proteins (equal amounts of total cellular proteins were loaded/well) in growth-arrested (2.5 h after cell division ceased) wild-type (ÅF38) cells harboring the PA1/lacO-1- dnaK dnaJ construct. Cells were grown with (250 µM) and without IPTG prior to starvation. Arrow indicates EF-Tu. Protein and carbonyl levels were determined by one-dimensional Western blot immunoassays and quantified using Image Gauge software. All experiments were repeated at least three times. Representative results are shown, and there was always less than 15% variation between experiments.
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Defense against carbonylation damage requires the Lon and HslVU (ClpQY) proteases.
Oxidized aconitase has been shown to be recognized and degraded by mitochondrial Lon in mammalian cells (4). Since the bacterial Lon protease is a member of the heat shock regulon, we investigated whether attenuated carbonylation caused by Hsp overproduction resulted, in part, from increased degradation of carbonylated proteins. To approach this, we blocked protein synthesis with spectinomycin in cells with and without overproduced levels of
32 and subsequently analyzed protein carbonyl levels at time intervals. As depicted (Fig. 4A), protein carbonyl levels decreased when
32 had been overproduced, in stark contrast to the vector control results. This provided the impetus to analyze carbonylation levels in mutants lacking Lon and other Hsp proteases. Deletion of Lon increased protein carbonylation upon entry of cells into stationary phase, and deletion of the HslVU (ClpQY) protease had an even more pronounced effect (Fig. 5). However, a clpP deletion alone did not affect total protein carbonyl levels (Fig. 5). It is possible that increased levels of
S in the clpP mutant (22) may mask a possible role for ClpP in degrading carbonylated proteins, since
S-dependent genes have been shown to counteract protein oxidation (10). However, the clpP mutations had no significant effect on total protein carbonyl levels even in a background lacking rpoS (not shown). In contrast, the lack of ClpP further elevated carbonylation in cells lacking Lon, suggesting that ClpP has a role in the defense against protein carbonylation but that Lon can fully compensate for the absence of ClpP (Fig. 5). In addition, HslVU appears to partly compensate for the lack of Lon and ClpP since carbonylation was further increased by introducing an hslVU deletion (Fig. 5).
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FIG. 4. Relative stability of carbonylated proteins determined by one-dimensional Western blot immunoassays in cells with different levels of 32. Protein synthesis was inhibited with spectinomycin during entry to stationary phase (30 min after growth ceased) in cells containing either the empty vector control (ÅF42; open squares) or the Ptrc- rpoH construct (ÅF41) grown in the presence (black squares) or absence (gray squares) of 100 µM IPTG.
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FIG. 5. Severalfold change in the levels of carbonylated proteins in wild-type (MG1655 lac) cells and in cells lacking the proteases Lon (ÅF66), HslVU (ÅF49), and ClpPX (PhB1907), ClpPX and Lon (PhB1465), and ClpPX, Lon, and HslVU (PhB1466). Protein extracts were from exponentially growing cells (open bars) and cells from an overnight stationary phase sample (grey bars). Carbonyl levels were determined by a slot-blot immunoassay and quantified using Image Gauge software. All levels were related to that obtained for the wild type during exponential growth, which was assigned a value of 1.0.
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FIG. 6. Proteomic immunodetection of carbonylated proteins in wild type (MG1655 lac), lon (ÅF66), and hslVU (ÅF49) cells entering a growth-arrested state due to glucose starvation. The major target proteins of carbonylation were identified by mass spectrometry and included the ß-subunit of RNA polymerase (RpoB), elongation factors Tu and G (EF-Tu, EF-G), the E1 component (AceE) of the pyruvate dehydrogenase complex, isocitrate dehydrogenase (Icd), 6-phosphogluconate dehydrogenase (Gnd), and Serine hydroxymethyltranferase (GlyA). The boxed proteins are GroEL and DnaK. The experiment was repeated three times, and the patterns of carbonylated proteins were the same in each experiment. Representative results are shown.
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32 is not informative with respect to stasis survival since it drastically reduces expression, by sigma factor competition (15), of
S- and
E-dependent genes, which are essential for stasis survival. Thus, while it is clear that mutant cells lacking specific members of the heat shock regulon (e.g., DnaK, ClpP, HslVU, and Lon) (16, 23, 27) die at an accelerated rate in stationary phase, it is uncertain whether the level of heat shock proteins constitutes a bottleneck in stationary phase survival of wild-type E. coli cells.
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FIG. 7. Stasis-survival of cells overproducing DnaK/DnaJ (ÅF38). Open squares, no IPTG; black squares, 250 µM IPTG. Plating efficiency (A) and membrane integrity (B) was monitored in cells growth arrested due to glucose starvation. Membrane integrity of cells was analyzed using the BacLight LIVE/DEAD methodology (12). Intact cells appear fluorescent green (C; 1 h starvation), whereas cells with a debilitated and leaky membrane appear red (D; 7 days of starvation). Representative results are shown, and there was always less than 15% variation between experiments.
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This work was sponsored by grants from the Swedish Natural Research Council and the Foundation for Strategic Research in Sweden and an award from the Göran Gustafsson Foundation for Scientific Research in Molecular Biology.
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