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First published online January 28, 2005; 10.1104/pp.104.056341 Plant Physiology 137:738-746 (2005) © 2005 American Society of Plant Biologists
A Proposed Mechanism for the Inhibitory Effects of Oxidative Stress on Rubisco Assembly and Its Subunit Expression1Department of Life Sciences, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel
In Chlamydomonas reinhardtii, a light-induced oxidative stress shifts the glutathione pool toward its oxidized form, resulting in a translational arrest of the large subunit (LSU) of Rubisco. We show here that the translational arrest of LSU is tightly coordinated with cessation of Rubisco assembly, and both processes take place after a threshold level of reactive oxygen species is reached. As a result, the small subunit is also eliminated by rapid degradation. We previously showed that the amino terminus of the LSU could bind RNA in a sequence-independent manner, as it shares a structural similarity with the RNA recognition motif. This domain becomes exposed only under oxidizing conditions, thus restricting the RNA-binding activity. Here we show that in vitro, thiol groups of both subunits become oxidized in the presence of oxidized glutathione. The structural changes are mediated by oxidized glutathione, whereas only very high concentrations of H2O2 confer similar results in vitro. Changes in the redox state of the LSU thiol groups are also observed in vivo, in response to a physiological light shock caused by transfer of cells from low light to high light. We propose that during a photooxidative stress, oxidation of thiol groups occurs already in nascent LSU chains, perhaps hindering their association with chaperones. As a result, their RNA recognition motif domain becomes exposed and will bind any RNA in its vicinity, including its own transcript. Due to this binding the ribosome stalls, preventing the assembly of additional ribosomes on the transcript. Polysome analysis using Suc gradients indeed shows that the rbcL RNA is associated with the polysomal fraction at all times but shifts toward fractions that contain smaller polysomes and monosomes during oxidative stress. Thus, translational arrest of the LSU most likely occurs at a postinitiation stage.
Rubisco is responsible for CO2 fixation during photosynthesis. In vascular plants and green algae, it exists as a holoenzyme composed of eight large subunits (LSUs; 55 kD) encoded by the chloroplast rbcL gene and eight small subunits (SSUs; 15 kD) produced by a nuclear family of rbcS genes (Spreitzer, 1993  ). SSU precursors are processed during entry into the chloroplast and are then assembled with the LSUs to yield the holoenzyme. Assembly of the oligomeric protein (approximately 500 kD) is mediated by cpn60 and cpn10 (Gatenby and Ellis, 1990), and a complex of the LSU and cpn60 serves as an intermediate of the assembly process. The chloroplast cpn60 is a homolog of bacterial groEL, and prokaryotic subunits of Rubisco expressed in Escherichia coli can be successfully assembled into a holoenzyme (Goloubinoff et al., 1989). A role for additional chaperone molecules, in accumulation of mature Rubisco complexes, was recently described (Brutnell et al., 1999).
Many protein complexes in the chloroplast are composed of multiple polypeptides, which are expressed in a tightly coordinated manner, as removal of one subunit can have an effect on expression of the other subunits in the complex. The coordinated expression of different subunits of the same complex can be achieved by controlled synthesis, as well as by proteolytic degradation of unassembled subunits. An assembly-dependent regulation was defined as a control by epistasy by Wollman and colleagues (Choquet et al., 1998
Exposure of plant cells to excess illumination generates an oxidative stress due to an imbalance between the antennae size and the inability to dissipate the adsorbed energy (Shapira et al., 1997
The sensitivity of Rubisco holoenzyme to oxidative stress is well established. Exposure of chloroplasts to oxidizing conditions leads to oxidation of Cys residues and to denaturation of Rubisco, as well as to decreased catalytic activity (Marin-Navarro and Moreno, 2003
Oxidizing Conditions Change the Redox State of Thiol Groups on Rubisco Subunits in Vitro and in Vivo
We examined how oxidative stress affects the thiol groups in Rubisco subunits, both in vitro and in vivo, using thiol-reactive agents, such as 4-acetamido-4'-maleimidyl-stilbene-2,2'-disulfonic acid (AMS) and idoacetamide. After oxidation, the Rubisco holoenzyme was precipitated with TCA that protonates free-thiol groups. The protein was then solubilized and the thiol-reactive agent was added to interact irreversibly with the free-thiol groups on Cys residues. This modification increased the size of the target protein and retarded its migration on nonreducing gels. As suspected, if Rubisco holoenzyme was first oxidized in vitro with increasing concentrations of GSSG and then exposed to the thiol-reacting agent, both the LSU and the SSU migrated faster than the nonoxidized and alkylated Rubisco subunits (Fig. 1A, top sections). This indicated that exposure of Rubisco to GSSG resulted in oxidation of the thiol groups on Cys residues. The observed mobility shift was indicative of the redox state of Cys residues in Rubisco subunits, since it was not observed when the GSSG-mediated oxidation of Rubisco was reversed by the addition of
Each of the 2 Rubisco SSU chains in C. reinhardtii contains 4 Cys residues, at positions 41, 65, 82, and 96 (Spreitzer, 2003 ). A step-wise decrease in migration of the SSU on a nonreducing gel was observed following incubation with iodoacetamide, in response to a gradual increase in GSSG concentrations. This indicated that the four thiol groups were modified in a sequential manner. It was difficult to observe such a gradual change in the LSU, as its structural switch was rather homogeneous. It is possible that a minor size change in the LSU could not be resolved on our gel due to the size of the protein, and therefore the slower migration of the polypeptide could be observed only after a comprehensive modification of multiple thiol groups. Alternatively, the uniform change in the size of the LSU polypeptide could indicate that this subunit was more sensitive to oxidation. Altogether, in the presence of GSSG, thiol groups on both subunits of Rubisco became oxidized, leading to structural changes in the protein. A photooxidative stress affected the redox state of thiol groups on Rubisco subunits also in vivo. Cells were grown continuously under LL and then shifted to HL for 1.5 and 2.5 h. The LL and HL treated cells were harvested and lysed by TCA to protonate all free-thiol groups. The precipitated proteins were resolubilized and allowed to interact with excess AMS, which alkylated the free-thiol groups of Cys residues. As expected, following the protonation and alkylation procedures, migration of Rubisco LSU on nonreducing gels was faster if it was extracted from HL cells as compared to LL cells. This indicated that upon transfer to HL, thiol groups on cystein residues in Rubisco were oxidized and therefore could not be alkylated by AMS (Fig. 1B). In addition to the change in the redox state, the 37-kD breakdown product was observed at HL, as expected. It was difficult to show the same effect for the SSU of Rubisco in vivo. We assume that under these conditions, SSUs that underwent structural changes were rapidly degraded and therefore could not be detected.
The redox state of thiol groups on the LSU and SSU of Rubisco was modulated by 5 mM GSSG. However, in the presence of H2O2, a change in the redox state of thiol groups could be monitored only at very high concentrations (Fig. 2), suggesting that the mere presence of an oxidizing agent did not have a direct effect on the redox state of thiol groups in Rubisco. Most probably, modulation of protein redox state proceeded in vivo via a mediator molecule such as glutathione (Noctor and Foyer, 1998
Rubisco Assembly during Oxidative Stress Is Tightly Coordinated with Translation of the LSU, andAssembled Particles Remain Stable duringthe First Hour at HL To examine the coordination between the translational arrest of the LSU and cessation of the assembly process, cells were transferred from LL to HL for different time periods (30, 45, and 60 min) and pulse labeled for 3 min with H235S04 at the same light conditions. Immediately after the pulse ended, the cells were divided into two parts. One of them was lysed in SDS-sample buffer and separated over SDS-PAGE denaturing gels to follow the labeling of new LSUs (Fig. 3A). The other was disrupted by sonication, and the soluble fraction was separated over native gels to monitor assembly of new Rubisco particles (Fig. 3B). The inhibitory effect of transferring the cells to HL on translation of the LSU and on assembly of the holoenzyme was nonlinear and could be observed only after a lag period of 30 min at HL. After that period of time (45 and 60 min), inhibition of both processes gradually increased with similar kinetics. It appears that the redox-dependent inhibition occurred only after the reactive oxygen species (ROS) reached a certain threshold and that a tight coordination existed between the synthesis of LSU and its assembly into the holoenzyme.
To test the stability of the holoenzyme particles during the first hour after transfer from LL to HL, a pulse-and-chase experiment was performed. Cells grown at LL were transferred to HL for different periods of time (30, 45, and 60 min) and then pulsed during 3 min with H235S04, as described above. After the pulse ended, an aliquot was removed from each tube (Fig. 4, P lanes) and the radiolabeled cells were further chased with excess cold H2S04 during 60 min at the same light conditions (C lanes). Following the chase, the cells were disrupted by sonication, and the soluble fraction that contained the Rubisco holoenzyme was separated over a native gel. The radiolabeled complexes that were isolated immediately after the pulse labeling (P lanes) remained stable during the first hour (C lanes). The breakdown products of the LSU that usually appear in response to severe oxidative-stress conditions (Desimone et al., 1996 ) could be seen only after longer periods of exposure to HL (data not shown), most probably when ROS levels reach a certain threshold.
Coordinated Expression of Rubisco Subunits under Oxidative Stress Is Achieved by Rapid Degradation of the SSU Expression of Rubisco subunits should be tightly coordinated, as assembly of the holoenzyme is stoichiometric. Since oxidative stress results in a translational arrest of LSU synthesis and cessation of assembly, we examined how the coordinated expression of the two subunits is achieved. A pulse-and-chase labeling experiment was performed in C. reinhardtii cells that were grown in LL and transferred to HL (Fig. 5). Under LL conditions, both LSU and SSU remained stable for over an hour, with no apparent degradation of either subunit. When the cells were pulsed at after 60 min at HL and then chased at the same conditions, the SSU was rapidly degraded within 15 min postlabeling, while the LSU remained relatively stable during the first hour. When the cells were pulsed after 90 min at HL, incorporation into the LSU was very low and the SSU was not detected at all. Thus, the translational arrest of the LSU during oxidative stress is accompanied by a rapid degradation of the SSU.
The rbcL Transcript Shifts towards Small Polysomes and Monosomes during Oxidative Stress
Changes in the polysomal distribution of the rbcL transcript as a function of illumination were previously demonstrated using Suc gradient analysis (Kim and Mullet, 2003
We show here that inhibition of LSU translation and assembly of the holoenzyme complex are tightly coordinated. The inhibition of both processes is nonlinear, since it can be monitored only after a lag time of 30 min at HL. After this time point, the translational arrest gradually increases along with a cessation of the assembly process (Fig. 3), suggesting that a certain threshold of ROS must be reached before inhibition occurs. Concomitantly, the SSU is removed by rapid degradation of nonassembled molecules. Expression of Rubisco subunits is tightly coordinated. It was previously reported that silencing of SSU expression by introducing antisense RNA (Rodermel et al., 1996 ) or by a gene knockout (Khrebtukova and Spreitzer, 1996) prevented the accumulation of the LSU. Similarly, when translation of Rubisco LSU was inhibited by a nonsense mutation or by chloramphenicol, the SSU was rapidly degraded, indicating that it cannot accumulate in its free and unassembled form (Schmidt and Mishkind, 1983; Spreitzer et al., 1985). Oxidative stress confers structural alterations on both subunits of Rubisco. This is reflected in experiments using AMS and iodoacetamide, thiol-reactive agents that bind reduced-thiol groups on both subunits and retard their migration in nonreducing gels. This binding can be inhibited by the addition of GSSG in experiments carried out in vitro. For the SSU, a gradual oxidation of the four thiol groups is observed in response to an incremental increase in GSSG concentration. Indeed, the Cys residues vary in their surface exposure, possibly affecting their accessibility to the oxidizing agent GSSG. Whereas Cys-65 is exposed to the surface and Cys-41 is found on an external loop, Cys-83 and Cys-96 are buried within the protein. It can be assumed that the exposed Cys residues will be oxidized at lower concentrations of GSSG, as compared to their internal counterparts. A similar analysis of the LSU showed that this polypeptide was more sensitive to oxidation by GSSG, and a maximal effect on its migration was observed already at the lower concentration of GSSG.
It is interesting to note that an oxidizing agent such as H2O2 confers structural changes in vitro only when present at a relatively high concentration (>100 mM), unlike GSSG, which affects the redox of thiol groups on Rubisco already at moderate concentrations (5 mM). Thus, H2O2 by itself most probably does not act directly on Rubisco thiol groups and requires a mediator molecule, such as glutathione. Indeed, we previously showed that elevation of ROS during transfer of C. reinhardtii from LL to HL is accompanied by oxidation of the glutathione pool, and these changes correlate with the temporal arrest of Rubisco LSU synthesis (Irihimovitch and Shapira, 2000 Applying an oxidative stress in vivo by shifting the cells from LL to HL had a similar effect on the redox state of thiol groups in Rubisco. Due to the magnitude of the change observed by western analysis, the oxidizing environment most probably affected the majority of the LSU polypeptide molecules in the cell. We assume that this change precedes the breakdown of LSU, and, moreover, it is possible that the structural changes are not restricted only to the holoenzyme, but they also occur in preassembled de novo-synthesized polypeptides.
It was previously shown that a severe oxidative stress leads to the proteolytic breakdown of Rubisco in intact chloroplasts (Desimone et al., 1996
We recently showed that the amino-terminal domain of the LSU has a structural resemblance to the RRM, typical of many RNA-binding proteins, such as the U1A splicing factor (Yosef et al., 2004
Further support of this mechanism is obtained from analyzing the profile of polysomes that are associated with the rbcL transcript, using Suc gradients. Indeed, the rbcL transcript remains associated with polysomes even after translation of LSU stops, although upon transfer of the cells from LL to HL, the rbcL RNA shifts toward smaller polysomes and monosomes. The continuous association of the rbcL transcript with the polysomal fraction suggests that the arrest in translation occurs at a postinitiation stage. If exposure of the RRM motif in a newly synthesized chain and its interaction with the rbcL transcript causes the ribosome to pause at an early stage of elongation, then new ribosomes cannot be added, causing the observed shift toward smaller polysomes and monosomes. A similar mechanism that controls ribosome loading was reported in other systems for hydroxyperoxide glutathione peroxidase in eukaryotes (Fletcher et al., 2001 Whether the regulatory mechanism in C. reinhardtii is also conserved in land plants is yet to be shown. However, preliminary experiments (data not shown) using Rubisco holoenzyme that was purified from tobacco (Nicotiana tabacum) and spinach (Spinacia oleracea) show RNA-binding activity in vitro in the presence of GSSG.
Here we examine the inhibitory effect of oxidative stress on Rubisco assembly and on expression of its subunits. Oxidative stress affects thiol groups of Rubisco subunits, as observed in vivo during light-induced stress and in vitro in the presence of GSSG. We propose that the redox of the thiol groups causes structural changes of newly synthesized polypeptide chains, resulting in the exposure of an RRM domain found in the amino terminus of Rubisco LSU and in binding of the rbcL transcript while still on the polysomes. Thus, translation of the LSU is inhibited almost completely during oxidative stress, most probably by an autoregulatory loop. Assembly of the holoenzyme therefore halts, and the unassembled SSU polypeptides are rapidly degraded. Upon recovery from oxidative stress, synthesis of the LSU and assembly of both subunits resume.
Strains and Growth Conditions
Chlamydomonas reinhardtii wild-type strain CC-125, obtained from the Chlamydomonas Genetics Center, Duke University at Durham, NC, was used in all experiments. Cultures (300 mL) in high-salt-reduced sulfate (HSRS; Schmidt et al., 1985
The effect of GSSG or H2O2 on the oxidation state of Rubisco subunits was examined by the use of thiol-reacting agents. The free-thiol groups on Cys residues were modified by iodoacetamide for the SSU and by AMS for the LSU. Briefly, purified Rubisco (100 µL; 0.2 mg/mL) was subjected to various concentrations of GSSG and H2O2 for 1 h at room temperature. The protein was then precipitated by the addition of TCA to a final concentration of 20% for 30 min to protonate all free thiols. Protein precipitates were collected by centrifugation at 13,000 rpm for 10 min, washed with acetone, and dissolved in freshly prepared solution containing 1% SDS, 100 mM Tris-HCl, pH 8, 1 mM EDTA, and a cocktail of protease inhibitors. The thiol-reacting agent was added to a final concentration of 20 mM for AMS or 75 mM for iodoacetamide, and the mixture was incubated for 1 h at room temperature. Protein samples were then separated over nonreducing gels, 15% (for SSU) or 10% (for LSU) SDS-PAGE (30:0.4 acrylamide:bisacrylamide), lacking any reducing agent, and subjected to western analysis using anti-Rubisco antibodies (1:4,000). In control samples, oxidation by GSSG was reversed prior to the incubation with iaodoacetamide or AMS, by the addition of
The in vivo redox state of Rubisco subunits was determined as described (Vestweber and Schatz, 1988
In vivo pulse-and-chase labeling of plastid and nuclear proteins with H235SO4 was preformed essentially as described (Schmidt and Mishkind, 1983
Wild-type C. reinhardtii cells (CC-125) were grown in a Tris-acetate phosphate medium (3 L) and harvested to yield 8 g fresh weight. The cell pellet was resuspended in 30 mL buffer containing 50 mM Tris-HCl, pH 8, 10 mM Mg2Cl, 10 mM NaHCO3, 10 mM dithiothreitol (DTT), 1 mM EDTA, and a cocktail of protease inhibitors (Sigma-Aldrich). The cells were disrupted by a triple passage through a French Pressure Cell Press (Spectronic Instruments, Rochester, NY) at 4,000 psi and centrifuged for 10 min at 20,000g (Sorvall SS-34 rotor, 10,000 rpm; Sorvall Products, Newtown, CT). The supernatant was collected and further centrifuged at 200,000g for 1 h at 4°C (Beckman TI50 rotor, 50,000 rpm; Beckman Instruments, Fullerton, CA). The protein was precipitated by a 25% to 50% ammonium sulfate cut of the 200,000g supernatant, and the pellet was resuspended and dialyzed against a buffer containing 50 mM Tris-HCl, pH 8, 10 mM Mg2Cl, 10 mM NaHCO3, 10 mM DTT, and 1 mM EDTA. The dialyzed solution was loaded on a linear Suc gradient (10%–30%) prepared in dialysis buffer. The gradient was centrifuged for 16 h at 4°C at 164,000g (Beckman SW40, at 40,000 rpm) and fractions (1 mL) were collected. Migration of Rubisco holoenzyme was monitored by western analysis of samples removed from each fraction.
Polyclonal rabbit antisera used in this study were directed against the Rubisco holoenzyme from tobacco (Nicotiana tabacum).
Cells were grown at LL conditions and transferred to HL for time periods of 30, 45, and 60 min. Following this treatment, the cells were pulse labeled for 3 min and chased, as described above. Samples (2 mL) were removed at time 0 and after a chase of 60 min, centrifuged, washed once with HSRS, and resuspended in 500 µL of sonication buffer (2 mM DTT, 10 mM MgCl2, 2 mM NaHCO3, and 50 mM Tris-HCl, pH 8.0). Samples were immediately sonicated twice using a microtip of XL 2020 MISONIX sonicator at 0°C. Sonication was performed by applying pulses of 20 s at intervals of 20 s. The disrupted cells were then centrifuged for 10 min at 4°C (20,000g), and supernatants containing the soluble proteins were collected. Incorporation of the radiolabel was measured by TCA precipitation. The assembled Rubisco holoenzyme was analyzed using nondenaturing 5% to 15% gradient polyacrylamide gels separated at 4°C in the standard Tris-Gly buffer lacking SDS, at 100 mV for 5 h. For western blotting, the gels were first soaked in transfer buffer containing 10% SDS for 10 min, and blotting time was extended (x2).
A 300-mL culture was grown at LL until log phase (OD750 0.2–0.3). To produce an oxidative stress, MeV was added at a concentration of 10–5 M for 1 h at LL. Cycloheximide and chloramphenicol were added to concentrations of 10 µg/mL and 100 µg/mL, respectively, for 5 min. The cells were harvested and resuspended in a buffer containing 25 mM Tris-HCl, pH 8, 25 mM MgCl, 25 mM KCl, 5 mM DTT, 10 µg/mL cycloheximide, 100 µg/mL chloramphenicol, a proteinase inhibitor cocktail (Sigma-Aldrich, according to the manufacturer), and 1 mg/mL Heparin. The cells were disrupted three times in a French Pressure Cell Press (at 5,000 psi). The cell debris and insoluble membranes were removed by centrifugation at 18,000g for 40 min at 4°C. The supernatant (5 mL) was layered over a 10%-to-40% Suc gradient prepared in the resuspention buffer, and the gradients were centrifuged at 120,000g for 4.5 h at 4°C in a SW28.1 rotor (Beckman). RNA was isolated from the fractions using phenol/chloroform extraction followed by ethanol precipitation. The RNA was separated using 1% agarose gels containing formaldehyde, blotted, and hybridized with a 1.5-kb radioactive fragment containing the entire rbcL gene, amplified by PCR from p266 (provided by the Chlamydomonas Genetics Center, Duke University, Durham, NC).
We thank T.J. Andrews from the Australian National University, Canberra, Australia for antibodies against Rubisco, and T. Trebitsh from the Ben-Gurion University for helpful discussions. Received November 11, 2004; returned for revision December 2, 2004; accepted December 8, 2004.
1 This work was supported by the Israel Science Foundation (grant no. 515/02). 
2 Present address: Boyce Thompson Institute for Plant Research, Cornell University, Tower Road, Ithaca, NY 14853. Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.056341. * Corresponding author; e-mail shapiram{at}bgu.ac.il; fax 972–8–6479185 or 972–8–6461710.
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ABSTRACT
RESULTS
-mercaptoethanol prior to alkylation with the thiol-reacting agent and separation of the modified proteins on reducing gels (
) and SSU (
) polypeptides were determined by Phosphorimager analysis, and the results are presented in the accompanying graphs.
