A Homolog of Prokaryotic Thiol Disulfide Transporter CcdA Is Required for the Assembly of the Cytochrome bf Complex in Arabidopsis Chloroplasts

A novel component of the disulfide-reducing pathway required for cytochrome c assembly in plastids

In plastids, the conversion of energy in the form of light to ATP requires key electron shuttles, the c-type cytochromes, which are defined by the covalent attachment of heme to a CXXCH motif. Plastid c-type cytochrome biogenesis occurs in the thylakoid lumen and requires a system for transmembrane transfer of reductants. Previously, CCDA and CCS5/HCF164, found in all plastid-containing organisms, have been proposed as two components of the disulfide-reducing pathway. In this work, we identify a small novel protein, CCS4, as a third component in this pathway. CCS4 was genetically identified in the green alga Chlamydomonas reinhardtii on the basis of the rescue of the ccs4 mutant, which is blocked in the synthesis of holoforms of plastid c-type cytochromes, namely cytochromes f and c(6). Although CCS4 does not display sequence motifs suggestive of redox or heme-binding function, biochemical and genetic complementation experiments suggest a role in the disulfide-reducing pathway required for heme attachment to apoforms of cytochromes c. Exogenous thiols partially rescue the growth phenotype of the ccs4 mutant concomitant with recovery of holocytochrome f accumulation, as does expression of an ectopic copy of the CCDA gene, encoding a trans-thylakoid transporter of reducing equivalents. We suggest that CCS4 might function to stabilize CCDA or regulate its activity.

Regulation of photosynthetic electron transport

The photosynthetic electron transport chain consists of photosystem II, the cytochrome b(6)f complex, photosystem I, and the free electron carriers plastoquinone and plastocyanin. Light-driven charge separation events occur at the level of photosystem II and photosystem I, which are associated at one end of the chain with the oxidation of water followed by electron flow along the electron transport chain and concomitant pumping of protons into the thylakoid lumen, which is used by the ATP synthase to generate ATP. At the other end of the chain reducing power is generated, which together with ATP is used for CO(2) assimilation. A remarkable feature of the photosynthetic apparatus is its ability to adapt to changes in environmental conditions by sensing light quality and quantity, CO(2) levels, temperature, and nutrient availability. These acclimation responses involve a complex signaling network in the chloroplasts comprising the thylakoid protein kinases Stt7/STN7 and Stl1/STN7 and the phosphatase PPH1/TAP38, which play important roles in state transitions and in the regulation of electron flow as well as in thylakoid membrane folding. The activity of some of these enzymes is closely connected to the redox state of the plastoquinone pool, and they appear to be involved both in short-term and long-term acclimation. This article is part of a Special Issue entitled "Regulation of Electron Transport in Chloroplasts".

State transitions at the crossroad of thylakoid signalling pathways

In order to maintain optimal photosynthetic activity under a changing light environment, plants and algae need to balance the absorbed light excitation energy between photosystem I and photosystem II through processes called state transitions. Variable light conditions lead to changes in the redox state of the plastoquinone pool which are sensed by a protein kinase closely associated with the cytochrome b(6)f complex. Preferential excitation of photosystem II leads to the activation of the kinase which phosphorylates the light-harvesting system (LHCII), a process which is subsequently followed by the release of LHCII from photosystem II and its migration to photosystem I. The process is reversible as dephosphorylation of LHCII on preferential excitation of photosystem I is followed by the return of LHCII to photosystem II. State transitions involve a considerable remodelling of the thylakoid membranes, and in the case of Chlamydomonas, they allow the cells to switch between linear and cyclic electron flow. In this alga, a major function of state transitions is to adjust the ATP level to cellular demands. Recent studies have identified the thylakoid protein kinase Stt7/STN7 as a key component of the signalling pathways of state transitions and long-term acclimation of the photosynthetic apparatus. In this article, we present a review on recent developments in the area of state transitions.

CcdA is a thylakoid membrane protein required for the transfer of reducing equivalents from stroma to thylakoid lumen in the higher plant chloroplast

In order to transfer reducing equivalents into the thylakoid lumen, a specific thylakoid membrane transfer system is suggested that mediates the disulfide bond reduction of proteins in the thylakoid lumen of higher plant chloroplasts. In this system, although stromal thioredoxin can supply the reducing equivalents to a thioredoxin-like protein HCF164 in the thylakoid lumen, a mediator protein for electron transfer in the thylakoid membranes is proposed to be required to link the two suborganellar compartments. CcdA is a candidate protein as a component for this transfer system since CcdA- and HCF164-deficient mutants in Arabidopsis thaliana show the same phenotype. We now show that CcdA is localized in the thylakoid membrane and that its redox state, as well as that of HCF164, is modulated in thylakoids by stromal m-type thioredoxin. Our results strongly suggest that CcdA may act as a mediator in thylakoid membranes by transferring reducing equivalents from the stromal to the lumenal side of the thylakoid membrane in chloroplasts.

Dynamics of reversible protein phosphorylation in thylakoids of flowering plants: the roles of STN7, STN8 and TAP38

Phosphorylation is the most common post-translational modification found in thylakoid membrane proteins of flowering plants, targeting more than two dozen subunits of all multiprotein complexes, including some light-harvesting proteins. Recent progress in mass spectrometry-based technologies has led to the detection of novel low-abundance thylakoid phosphoproteins and localised their phosphorylation sites. Three of the enzymes involved in phosphorylation/dephosphorylation cycles in thylakoids, the protein kinases STN7 and STN8 and the phosphatase TAP38/PPH1, have been characterised in the model species Arabidopsis thaliana. Differential protein phosphorylation is associated with changes in illumination and various other environmental parameters, and has been implicated in several acclimation responses, the molecular mechanisms of which are only partly understood. The phenomenon of State Transitions, which enables rapid adaptation to short-term changes in illumination, has recently been shown to depend on reversible phosphorylation of LHCII by STN7-TAP38/PPH1. STN7 is also necessary for long-term acclimation responses that counteract imbalances in energy distribution between PSII and PSI by changing the rates of accumulation of their reaction-centre and light-harvesting proteins. Another aspect of photosynthetic acclimation, the modulation of thylakoid ultrastructure, depends on phosphorylation of PSII core proteins, mainly executed by STN8. Here we review recent advances in the characterisation of STN7, STN8 and TAP38/PPH1, and discuss their physiological significance within the overall network of thylakoid protein phosphorylation. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.

CCS5, a thioredoxin-like protein involved in the assembly of plastid c-type cytochromes

The c-type cytochromes are metalloproteins with a heme molecule covalently linked to the sulfhydryls of a CXXCH heme-binding site. In plastids, at least six assembly factors are required for heme attachment to the apo-forms of cytochrome f and cytochrome c(6) in the thylakoid lumen. CCS5, controlling plastid cytochrome c assembly, was identified through insertional mutagenesis in the unicellular green alga Chlamydomonas reinhardtii. The complementing gene encodes a protein with similarity to Arabidopsis thaliana HCF164, which is a thylakoid membrane-anchored protein with a lumen-facing thioredoxin-like domain. HCF164 is required for cytochrome b(6)f biogenesis, but its activity and site of action in the assembly process has so far remained undeciphered. We show that CCS5 is a component of a trans-thylakoid redox pathway and operates by reducing the CXXCH heme-binding site of apocytochrome c prior to the heme ligation reaction. The proposal is based on the following findings: 1) the ccs5 mutant is rescued by exogenous thiols; 2) CCS5 interacts with apocytochrome f and c(6) in a yeast two-hybrid assay; and 3) recombinant CCS5 is able to reduce a disulfide in the CXXCH heme-binding site of apocytochrome f.

Thioredoxin targets of the plant chloroplast lumen and their implications for plastid function

The light-dependent regulation of stromal enzymes by thioredoxin (Trx)-catalysed disulphide/dithiol exchange is known as a classical mechanism for control of chloroplast metabolism. Recent proteome studies show that Trx targets are present not only in the stroma but in all chloroplast compartments, from the envelope to the thylakoid lumen. Trx-mediated redox control appears to be a common feature of important pathways, such as the Calvin cycle, starch synthesis and tetrapyrrole biosynthesis. However, the extent of thiol-dependent redox regulation in the thylakoid lumen has not been previously systematically explored. In this study, we addressed Trx-linked redox control in the chloroplast lumen of Arabidopsis thaliana. Using complementary proteomics approaches, we identified 19 Trx target proteins, thus covering more than 40% of the currently known lumenal chloroplast proteome. We show that the redox state of thiols is decisive for degradation of the extrinsic PsbO1 and PsbO2 subunits of photosystem II. Moreover, disulphide reduction inhibits activity of the xanthophyll cycle enzyme violaxanthin de-epoxidase, which participates in thermal dissipation of excess absorbed light. Our results indicate that redox-controlled reactions in the chloroplast lumen play essential roles in the function of photosystem II and the regulation of adaptation to light intensity.

Cytochrome c biogenesis: mechanisms for covalent modifications and trafficking of heme and for heme-iron redox control

Heme is the prosthetic group for cytochromes, which are directly involved in oxidation/reduction reactions inside and outside the cell. Many cytochromes contain heme with covalent additions at one or both vinyl groups. These include farnesylation at one vinyl in hemes o and a and thioether linkages to each vinyl in cytochrome c (at CXXCH of the protein). Here we review the mechanisms for these covalent attachments, with emphasis on the three unique cytochrome c assembly pathways called systems I, II, and III. All proteins in system I (called Ccm proteins) and system II (Ccs proteins) are integral membrane proteins. Recent biochemical analyses suggest mechanisms for heme channeling to the outside, heme-iron redox control, and attachment to the CXXCH. For system II, the CcsB and CcsA proteins form a cytochrome c synthetase complex which specifically channels heme to an external heme binding domain; in this conserved tryptophan-rich "WWD domain" (in CcsA), the heme is maintained in the reduced state by two external histidines and then ligated to the CXXCH motif. In system I, a two-step process is described. Step 1 is the CcmABCD-mediated synthesis and release of oxidized holoCcmE (heme in the Fe(+3) state). We describe how external histidines in CcmC are involved in heme attachment to CcmE, and the chemical mechanism to form oxidized holoCcmE is discussed. Step 2 includes the CcmFH-mediated reduction (to Fe(+2)) of holoCcmE and ligation of the heme to CXXCH. The evolutionary and ecological advantages for each system are discussed with respect to iron limitation and oxidizing environments.

Biogenesis of cytochrome b6 in photosynthetic membranes

In chloroplasts, binding of a c′-heme to cytochrome b₆ on the stromal side of the thylakoid membranes requires a specific mechanism distinct from the one at work for c-heme binding to cytochromes f and c₆ on the lumenal side of membranes. Here, we show that the major protein components of this pathway, the CCBs, are bona fide transmembrane proteins. We demonstrate their association in a series of hetero-oligomeric complexes, some of which interact transiently with cytochrome b₆ in the process of heme delivery to the apoprotein. In addition, we provide preliminary evidence for functional assembly of cytochrome b₆f complexes even in the absence of c′-heme binding to cytochrome b₆. Finally, we present a sequential model for apo- to holo-cytochrome b₆ maturation integrated within the assembly pathway of b₆f complexes in the thylakoid membranes.

Analysis of the chloroplast protein kinase Stt7 during state transitions

State transitions allow for the balancing of the light excitation energy between photosystem I and photosystem II and for optimal photosynthetic activity when photosynthetic organisms are subjected to changing light conditions. This process is regulated by the redox state of the plastoquinone pool through the Stt7/STN7 protein kinase required for phosphorylation of the light-harvesting complex LHCII and for the reversible displacement of the mobile LHCII between the photosystems. We show that Stt7 is associated with photosynthetic complexes including LHCII, photosystem I, and the cytochrome b₆f complex. Our data reveal that Stt7 acts in catalytic amounts. We also provide evidence that Stt7 contains a transmembrane region that separates its catalytic kinase domain on the stromal side from its N-terminal end in the thylakoid lumen with two conserved Cys that are critical for its activity and state transitions. On the basis of these data, we propose that the activity of Stt7 is regulated through its transmembrane domain and that a disulfide bond between the two lumen Cys is essential for its activity. The high-light–induced reduction of this bond may occur through a transthylakoid thiol–reducing pathway driven by the ferredoxin-thioredoxin system which is also required for cytochrome b₆f assembly and heme biogenesis.

To grow optimally, photosynthetic organisms need to constantly adjust to changing light conditions. One of these adjustments, called state transitions, allows light energy to be redistributed between the two photosynthetic reaction center complexes in a cell's chloroplasts. These complexes act in concert with other components of the photosynthetic machinery to turn light energy into cellular energy. A key component in the regulation of state transitions is the chloroplast protein Stt7 (also known as STN7), which can modify other proteins by adding a phosphate group. When light levels change, the oxidation level of a pool of another chloroplast component, plastoquinone, changes, which in turn activates Stt7, inducing it to phosphorylate specific proteins of the light-harvesting complex of one reaction center. As a result, a portion of this light-harvesting complex is transferred from one photosynthetic reaction center to the other, thereby optimizing photosynthetic efficiency. Here, we have addressed the configuration of Stt7 within the thylakoid membrane of the chloroplast and the molecular mechanisms underlying its activation. Our data reveal that the level of Stt7 protein changes drastically under specific environmental conditions, that the protein does not need to be present in a one-to-one ratio with its targets for activity, and that it associates directly with a number of components of the photosynthetic machinery. The protein-modifying domain of Stt7 is exposed to the outer side of the thylakoid membrane, whereas the domain critical for regulation of its activity lies on the inner side of the thylakoid membrane. These results shed light on the molecular mechanisms that allow photosynthetic organisms to adjust to fluctuations in light levels.

The Stt7/STN7 chloroplast protein is involved in the phosphorylation and remodeling of the light-harvesting apparatus of photosynthetic organisms and plays a key role in the acclimation of the photosynthetic machinery following changes in light levels.

A novel pathway of cytochrome c biogenesis is involved in the assembly of the cytochrome b6f complex in arabidopsis chloroplasts

We recently characterized a novel heme biogenesis pathway required for heme c(i)' covalent binding to cytochrome b6 in Chlamydomonas named system IV or CCB (cofactor assembly, complex C (b6f), subunit B (PetB)). To find out whether this CCB pathway also operates in higher plants and extend the knowledge of the c-type cytochrome biogenesis, we studied Arabidopsis insertion mutants in the orthologs of the CCB genes. The ccb1, ccb2, and ccb4 mutants show a phenotype characterized by a deficiency in the accumulation of the subunits of the cytochrome b6f complex and lack covalent heme binding to cytochrome b6. These mutants were functionally complemented with the corresponding wild type cDNAs. Using fluorescent protein reporters, we demonstrated that the CCB1, CCB2, CCB3, and CCB4 proteins are targeted to the chloroplast compartment of Arabidopsis. We have extended our study to the YGGT family, to which CCB3 belongs, by studying insertion mutants of two additional members of this family for which no mutants were previously characterized, and we showed that they are not functionally involved in the CCB system. Thus, we demonstrate the ubiquity of the CCB proteins in chloroplast heme c(i)' binding.

Photosynthetic acclimation: state transitions and adjustment of photosystem stoichiometry--functional relationships between short-term and long-term light quality acclimation in plants

In dense plant populations, individuals shade each other resulting in a low-light habitat that is enriched in far-red light. This light quality gradient decreases the efficiency of the photosynthetic light reaction as a result of imbalanced excitation of the two photosystems. Plants counteract such conditions by performing acclimation reactions. Two major mechanisms are known to assure efficient photosynthesis: state transitions, which act on a short-term timescale; and a long-term response, which enables the plant to re-adjust photosystem stoichiometry in favour of the rate-limiting photosystem. Both processes start with the perception of the imbalanced photosystem excitation via reduction/oxidation (redox) signals from the photosynthetic electron transport chain. Recent data in Arabidopsis indicate that initialization of the molecular processes in both cases involve the activity of the thylakoid membrane-associated kinase, STN7. Thus, redox-controlled phosphorylation events may not only adjust photosystem antenna structure but may also affect plastid, as well as nuclear, gene expression. Both state transitions and the long-term response have been described mainly in molecular terms, while the physiological relevance concerning plant survival and reproduction has been poorly investigated. Recent studies have shed more light on this topic. Here, we give an overview on the long-term response, its physiological effects, possible mechanisms and its relationship to state transitions as well as to nonphotochemical quenching, another important short-term mechanism that mediates high-light acclimation. Special emphasis is given to the functional roles and potential interactions between the different light acclimation strategies. A working model displays the various responses as an integrated molecular system that helps plants to acclimate to the changing light environment.

Feedback regulation of photosynthetic electron transport by NADP(H) redox poise

When plants experience an imbalance between the absorption of light energy and the use of that energy to drive metabolism, they are liable to suffer from oxidative stress. Such imbalances arise due to environmental conditions (e.g. heat, chilling or drought), and can result in the production of reactive oxygen species (ROS). Here, we present evidence for a novel protective process - feedback redox regulation via the redox poise of the NADP(H) pool. Photosynthetic electron transport was studied in two transgenic tobacco (Nicotiana tabacum) lines - one having reduced levels of ferredoxin NADP+-reductase (FNR), the enzyme responsible for reducing NADP+, and the other reduced levels of glyceraldehyde 3-phosphate dehydrogenase (GAPDH), the principal consumer of NADPH. Both had a similar degree of inhibition of carbon fixation and impaired electron transport. However, whilst FNR antisense plants were obviously stressed, with extensive bleaching of leaves, GAPDH antisense plants showed no visible signs of stress, beyond having a slowed growth rate. Examination of electron transport in these plants indicated that this difference is due to feedback regulation occurring in the GAPDH but not the FNR antisense plants. We propose that this reflects the occurrence of a previously undescribed regulatory pathway responding to the redox poise of the NADP(H) pool.

Role of thylakoid protein kinases in photosynthetic acclimation

Photosynthetic organisms are able to adjust to changes in light quality through state transition, a process which leads to a balancing of the light excitation energy between the antennae systems of photosystem II and photosystem I. A genetic approach has been used in Chlamydomonas with the aim of elucidating the signaling chain involved in state transitions. This has led to the identification of a small family of Ser-Thr protein kinases associated with the thylakoid membrane and conserved in algae and land plants. These kinases appear to be involved both in short and long term adaptations to changes in the light environment.

Knock-out of the plastid-encoded PetL subunit results in reduced stability and accelerated leaf age-dependent loss of the cytochrome b6f complex

The cytochrome-b6f complex, a key component of the photosynthetic electron transport chain, contains a number of very small protein subunits whose functions are not well defined. Here we have investigated the function of the 31-amino acid PetL subunit encoded in the chloroplast genome in all higher plants. Chloroplast-transformed petL knock-out tobacco plants display no obvious phenotype, suggesting that PetL is not essential for cytochrome b6f complex biogenesis and function (Fiebig, A., Stegemann, S., and Bock, R. (2004) Nucleic Acids Res. 32, 3615-3622). We show here that, whereas young mutant leaves accumulate comparable amounts of cytochrome b6f complex and have an identical assimilation capacity as wild type leaves, both cytochrome b6f complex contents and assimilation capacities of mature and old leaves are strongly reduced in the mutant, indicating that the cytochrome b6f complex is less stable than in the wild type. Reduced complex stability was also confirmed by in vitro treatments of isolated thylakoids with chaotropic reagents. Adaptive responses observed in the knockout mutants, such as delayed down-regulation of plastocyanin contents, indicate that plants can sense the restricted electron flux to photosystem I yet cannot compensate the reduced stability of the cytochrome b6f complex by adaptive up-regulation of complex synthesis. We propose that efficient cytochrome b6f complex biogenesis occurs only in young leaves and that the capacity for de novo synthesis of the complex is very low in mature and aging leaves. Gene expression analysis indicates that the ontogenetic down-regulation of cytochrome b6f complex biogenesis occurs at the post-transcriptional level.

HCF164 receives reducing equivalents from stromal thioredoxin across the thylakoid membrane and mediates reduction of target proteins in the thylakoid lumen

HCF164 is a membrane-anchored thioredoxin-like protein known to be indispensable for assembly of cytochrome b6 f in the thylakoid membranes. In this study, we report the finding that chloroplast stroma m-type thioredoxin is the source of reducing equivalents for reduction of HCF164 in the thylakoid lumen, providing strong evidence that higher plant chloroplasts possess a trans-membrane reducing equivalent transfer system similar to that found in bacteria. To probe the function of HCF164 in the lumen, a screen to identify the reducing equivalent acceptor proteins of HCF164 was carried out by using a resin-immobilized HCF164 single cysteine mutant, leading to the isolation of putative target thylakoid proteins. Among the newly identified target proteins, the reduction of the PSI-N subunit of photosystem I by HCF164 was confirmed both in vitro and in isolated thylakoids. Two components of the cytochrome b6 f complex, the cytochrome f and Rieske FeS proteins, were also identified as novel potential target proteins. The data presented here suggest that HCF164 serves as an important transducer of reducing equivalents to proteins in the thylakoid lumen.

Role of membrane-bound thiol-disulfide oxidoreductases in endospore-forming bacteria

Thiol-disulfide oxidoreductases catalyze formation, disruption, or isomerization of disulfide bonds between cysteine residues in proteins. Much is known about the functional roles and properties of this class of redox enzymes in vegetative bacterial cells but their involvement in sporulation has remained unknown until recently. Two membrane-embedded thiol-disulfide oxidoreductases, CcdA and StoA/SpoIVH, conditionally required for efficient production of Bacillus subtilis heat-resistant endospores, have now been identified. Properties of mutant cells lacking the two enzymes indicate new aspects in the molecular details of endospore envelope development. This mini-review presents an overview of membrane-bound thiol-disulfide oxidoreductases in the Gram-positive bacterium B. subtilis and endospore synthesis. Accumulated experimental findings on CcdA and StoA/SpoIVH are reviewed. A model for the role of these proteins in endospore cortex biogenesis in presented.

HCF153, a novel nuclear-encoded factor necessary during a post-translational step in biogenesis of the cytochrome bf complex

We have isolated the nuclear photosynthetic mutant hcf153 which shows reduced accumulation of the cytochrome b(6)f complex. The levels and processing patterns of the RNAs encoding the cytochrome b(6)f subunits are unaltered in the mutant. In vivo protein labeling experiments and analysis of polysome association revealed normal synthesis of the large chloroplast-encoded cytochrome b(6)f subunits. The mutation resulted from a T-DNA insertion and the affected nuclear gene was cloned. HCF153 encodes a 15 kDa protein containing a chloroplast transit peptide. Sequence similarity searches revealed that the protein is restricted to higher plants. A HCF153-Protein A fusion construct introduced into hcf153 mutant plants was able to substitute the function of the wild-type protein. Fractionation of intact chloroplasts from these transgenic plants suggests that most or all of the fusion protein is tightly associated with the thylakoid membrane. Our data show that the identified factor is a novel protein that could be involved in a post-translational step during biogenesis of the cytochrome b(6)f complex. It is also possible that HCF153 is necessary for translation of one of the very small subunits of the cytochrome b(6)f complex.

Towards a Functional Dissection of Thioredoxin Networks in Plant Cells

Thioredoxins are a ubiquitous family of redox equivalent mediators, long considered to possess a limited number of target enzymes. Recent progress in proteomic research has allowed the identification of a wide variety of candidate proteins with which this small protein may interact in vivo. Moreover, the activity of thioredoxin itself has been recently found to be subject to regulation by posttranslational modifications, adding an additional level of complexity to the function of this intriguing enzyme family. The current review charts the technical progress made in the continuing discovery of the numerous and diverse roles played by these proteins in the regulation of redox networks in plant cells.

Cyc2p, a Membrane-bound Flavoprotein Involved in the Maturation of Mitochondrial c-Type Cytochromes

Mitochondrial apocytochrome c and c1 are converted to their holoforms in the intermembrane space by attachment of heme to the cysteines of the CXXCH motif through the activity of assembly factors cytochrome c heme lyase and cytochrome c1 heme lyase (CCHL and CC1HL). The maintenance of apocytochrome sulfhydryls and heme substrates in a reduced state is critical for the ligation of heme. Factors that control the redox chemistry of the heme attachment reaction to apocytochrome c are known in bacteria and plastids but not in mitochondria. We have explored the function of Cyc2p, a candidate redox cytochrome c assembly component in yeast mitochondria. We show that Cyc2p is required for the activity of CCHL toward apocytochrome c and c1 and becomes essential for the heme attachment to apocytochrome c1 carrying a CAPCH instead of CAACH heme binding site. A redox function for Cyc2p in the heme lyase reaction is suggested from 1) the presence of a noncovalently bound FAD molecule in the C-terminal domain of Cyc2p, 2) the localization of Cyc2p in the inner membrane with the FAD binding domain exposed to the intermembrane space, and 3) the ability of recombinant Cyc2p to carry the NADPH-dependent reduction of ferricyanide. We postulate that, in vivo, Cyc2p interacts with CCHL and is involved in the reduction of heme prior to its ligation to apocytochrome c by CCHL.

Thioredoxins in Arabidopsis and other plants

Regulation of disulfide dithiol exchange has become increasingly important in our knowledge of plant life. Initially discovered as regulators of light-dependent malate biosynthesis in the chloroplast, plant thioredoxins are now implicated in a large panel of reactions related to metabolism, defense and development. In this review we describe the numerous thioredoxin types encoded by the Arabidopsis genome, and provide evidence that they are present in all higher plants. Some results suggest cross-talk between thioredoxins and glutaredoxins, the second family of disulfide dithiol reductase. The development of proteomics in plants revealed an unexpectedly large number of putative target proteins for thioredoxins and glutaredoxins. Nevertheless, we are far from a clear understanding of the actual function of each thioredoxin in planta. Although hampered by functional redundancies between genes, genetic approaches are probably unavoidable to define which thioredoxin interacts with which target protein and evaluate the physiological consequences.

The novel cytochrome c6 of chloroplasts: a case of evolutionary bricolage?

Cytochrome c6 has long been known as a redox carrier of the thylakoid lumen of cyanobacteria and some eukaryotic algae that can substitute for plastocyanin in electron transfer. Until recently, it was widely accepted that land plants lack a cytochrome c6. However, a homologue of the protein has now been identified in several plant species together with an additional isoform in the green alga Chlamydomonas reinhardtii. This form of the protein, designated cytochrome c6A, differs from the 'conventional' cytochrome c6 in possessing a conserved insertion of 12 amino acids that includes two absolutely conserved cysteine residues. There are conflicting reports of whether cytochrome c6A can substitute for plastocyanin in photosynthetic electron transfer. The evidence for and against this is reviewed and the likely evolutionary history of cytochrome c6A is discussed. It is suggested that it has been converted from a primary role in electron transfer to one in regulation within the chloroplast, and is an example of evolutionary 'bricolage'.

Redox regulation in the chloroplast thylakoid lumen: a new frontier in photosynthesis research

Initially linked to photosynthesis, regulation by change in the redox state of thiol groups (S-S<-- -->2SH) is now known to occur throughout biology. Thus, in addition to serving important structural and catalytic functions, it is recognized that, in many cases, disulphide bonds can be broken and reformed for regulation. Several systems, each linking a hydrogen donor to an intermediary disulphide protein, act to effect changes that alter the activity of target proteins by change in the thiol redox state. Pertinent to the present discussion is the chloroplast ferredoxin/thioredoxin system, comprised of photoreduced ferredoxin, a thioredoxin, and the enzyme ferredoxin-thioredoxin reductase, that occur in the stroma. In this system, thioredoxin links the activity of enzymes to light: those enzymes functional in biosynthesis are reductively activated by light via thioredoxin (S-S-->2SH), whereas counterparts acting in degradation are deactivated under illumination conditions and are oxidatively activated in the dark (2SH-->S-S). Recent research has uncovered a new paradigm in which an immunophilin, FKBP13, and potentially other enzymes of the chloroplast thylakoid lumen are oxidatively activated in the light (2SH-->S-S). The present review provides a perspective on this recent work.

Redox regulation: a broadening horizon

Initially discovered in the context of photosynthesis, regulation by change in the redox state of thiol groups (S-S <--> 2SH) is now known to occur throughout biology. Several systems, each linking a hydrogen donor to an intermediary disulfide protein, act to effect changes that alter the activity of target proteins: the ferredoxin/thioredoxin system, comprised of reduced ferredoxin, a thioredoxin, and the enzyme, ferredoxin-thioredoxin reductase; the NADP/thioredoxin system, including NADPH, a thioredoxin, and NADP-thioredoxin reductase; and the glutathione/glutaredoxin system, composed of reduced glutathione and a glutaredoxin. A related disulfide protein, protein disulfide isomerase (PDI) acts in protein assembly. Regulation linked to plastoquinone and signaling induced by reactive oxygen species (ROS) and other agents are also being actively investigated. Progress made on these systems has linked redox to the regulation of an increasing number of processes not only in plants, but in other types of organisms as well. Research in areas currently under exploration promises to provide a fuller understanding of the role redox plays in cellular processes, and to further the application of this knowledge to technology and medicine.

Structural analysis uncovers a role for redox in regulating FKBP13, an immunophilin of the chloroplast thylakoid lumen

Change in redox status has long been known to link light to the posttranslational regulation of chloroplast enzymes. So far, studies have been conducted primarily with thioredoxin-linked members of the stroma that function in a broad array of biosynthetic and degradatory processes. Consequently, little is known about the role of redox in regulating the growing number of enzymes found to occur in the lumen, the site of oxygen evolution in thylakoid membranes. To help fill this gap, we have studied AtFKBP13, an FKBP-type immunophilin earlier shown to interact with a redox-active protein of the lumen, and found the enzyme to contain a pair of disulfide bonds in x-ray structural studies. These disulfides, which in protein mutagenesis experiments were shown to be essential for the associated peptidyl-prolyl isomerase activity, are unique to chloroplast FKBPs and are absent in animal and yeast counterparts. Both disulfide bonds were redox-active and were reduced by thioredoxin from either chloroplast or bacterial sources in a reaction that led to loss of enzyme activity. The results suggest a previously unrecognized paradigm for redox regulation in chloroplasts in which activation by light is achieved in concert with oxygen evolution by the oxidation of sulfhydryl groups (conversion of SH to S-S). Such a mechanism, occurring in the thylakoid lumen, is in direct contrast to regulation of enzymes in the stroma, where reduction of disulfides targeted by thioredoxin (S-S converted to SH) leads to an increase in activity in the light.