Preprotein import into chloroplasts via the Toc and Tic complexes is regulated by redox signals in Pisum sativum

Structural basis of metabolite transport by the chloroplast outer envelope channel OEP21

Triose phosphates (TPs) are the primary products of photosynthetic CO₂ fixation in chloroplasts, which need to be exported into the cytosol across the chloroplast inner envelope (IE) and outer envelope (OE) membranes to sustain plant growth. While transport across the IE is well understood, the mode of action of the transporters in the OE remains unclear. Here we present the high-resolution nuclear magnetic resonance (NMR) structure of the outer envelope protein 21 (OEP21) from garden pea, the main exit pore for TPs in C₃ plants. OEP21 is a cone-shaped β-barrel pore with a highly positively charged interior that enables binding and translocation of negatively charged metabolites in a competitive manner, up to a size of ~1 kDa. ATP stabilizes the channel and keeps it in an open state. Despite the broad substrate selectivity of OEP21, these results suggest that control of metabolite transport across the OE might be possible.

Understanding protein import in diverse non-green plastids

The spectacular diversity of plastids in non-green organs such as flowers, fruits, roots, tubers, and senescing leaves represents a Universe of metabolic processes in higher plants that remain to be completely characterized. The endosymbiosis of the plastid and the subsequent export of the ancestral cyanobacterial genome to the nuclear genome, and adaptation of the plants to all types of environments has resulted in the emergence of diverse and a highly orchestrated metabolism across the plant kingdom that is entirely reliant on a complex protein import and translocation system. The TOC and TIC translocons, critical for importing nuclear-encoded proteins into the plastid stroma, remain poorly resolved, especially in the case of TIC. From the stroma, three core pathways (cpTat, cpSec, and cpSRP) may localize imported proteins to the thylakoid. Non-canonical routes only utilizing TOC also exist for the insertion of many inner and outer membrane proteins, or in the case of some modified proteins, a vesicular import route. Understanding this complex protein import system is further compounded by the highly heterogeneous nature of transit peptides, and the varying transit peptide specificity of plastids depending on species and the developmental and trophic stage of the plant organs. Computational tools provide an increasingly sophisticated means of predicting protein import into highly diverse non-green plastids across higher plants, which need to be validated using proteomics and metabolic approaches. The myriad plastid functions enable higher plants to interact and respond to all kinds of environments. Unraveling the diversity of non-green plastid functions across the higher plants has the potential to provide knowledge that will help in developing climate resilient crops.

Molecular characterization, targeting and expression analysis of chloroplast and mitochondrion protein import components in Nicotiana benthamiana

Improved bioinformatics tools for annotating gene function are becoming increasingly available, but such information must be considered theoretical until further experimental evidence proves it. In the work reported here, the genes for the main components of the translocons of the outer membrane of chloroplasts (Toc) and mitochondria (Tom), including preprotein receptors and protein-conducting channels of N. benthamiana, were identified. Sequence identity searches and phylogenetic relationships with functionally annotated sequences such as those of A. thaliana revealed that N. benthamiana orthologs mainly exist as recently duplicated loci. Only a Toc34 ortholog was found (NbToc34), while Toc159 receptor family was composed of four orthologs but somewhat different from those of A. thaliana. Except for NbToc90, the rest (NbToc120, NbToc159A and NbToc159B) had a molecular weight of about 150 kDa and an acidic domain similar in length. Only two orthologs of the Tom20 receptors, NbTom20-1 and NbTom20-2, were found. The number of the Toc and Tom receptor isoforms in N. benthamiana was comparable to that previously reported in tomato and what we found in BLAST searches in other species in the genera Nicotiana and Solanum. After cloning, the subcellular localization of N. benthamiana orthologs was studied, resulting to be identical to that of A. thaliana receptors. Phenotype analysis after silencing together with relative expression analysis in roots, stems and leaves revealed that, except for the Toc and Tom channel-forming components (NbToc75 and NbTom40) and NbToc34, functional redundancy could be observed either among Toc159 or mitochondrial receptors. Finally, heterodimer formation between NbToc34 and the NbToc159 family receptors was confirmed by two alternative techniques indicating that different Toc complexes could be assembled. Additional work needs to be addressed to know if this results in a functional specialization of each Toc complex.

Arabidopsis ORANGE protein regulates plastid pre-protein import through interacting with Tic proteins

Chloroplast-targeted proteins are actively imported into chloroplasts via the machinery spanning the double-layered membranes of chloroplasts. While the key translocons at the outer (TOC) and inner (TIC) membranes of chloroplasts are defined, proteins that interact with the core components to facilitate pre-protein import are continuously being discovered. A DnaJ-like chaperone ORANGE (OR) protein is known to regulate carotenoid biosynthesis as well as plastid biogenesis and development. In this study, we found that OR physically interacts with several Tic proteins including Tic20, Tic40, and Tic110 in the classic TIC core complex of the chloroplast import machinery. Knocking out or and its homolog or-like greatly affects the import efficiency of some photosynthetic and non-photosynthetic pre-proteins. Consistent with the direct interactions of OR with Tic proteins, the binding efficiency assay revealed that the effect of OR occurs at translocation at the inner envelope membrane (i.e. at the TIC complex). OR is able to reduce the Tic40 protein turnover rate through its chaperone activity. Moreover, OR was found to interfere with the interaction between Tic40 and Tic110, and reduces the binding of pre-proteins to Tic110 in aiding their release for translocation and processing. Our findings suggest that OR plays a new and regulatory role in stabilizing key translocons and in facilitating the late stage of plastid pre-protein translocation to regulate plastid pre-protein import.

Nanosecond pulsed electric fields modulate the expression of the astaxanthin biosynthesis genes psy, crtR-b and bkt 1 in Haematococcus pluvialis

Nanosecond pulsed electric fields (nsPEFs) have been extensively studied with respect to cellular responses. Whether nsPEFs can regulate gene expression and to modulate the synthesis of valuable compounds, has so far been only tested in the context of apoptosis in cancer cells. We used the unicellular algae Haematococcus pluvialis as system to test, whether nsPEFs could alter gene expression and to promote the biosynthesis of astaxanthin. We find that nsPEFs induce a mild, but significant increase of mortality up to about 20%, accompanied by a moderate increase of astaxanthin accumulation. Steady-state transcript levels of three key genes psy, crtR-b and bkt 1 were seen to increase with a maximum at 3 d after PEF treatment at 50 ns. Pulsing at 25 ns reduce the transcripts of psy, crtR-b from around day 2 after the pulse, while those of bkt 1 remain unchanged. By blocking the membrane-located NADPH oxidase RboH, diphenylene iodonium by itself increased both, the levels of astaxanthin and transcripts of all three biosynthetic genes, and this increase was added up to that produced by nsPEFs. Artificial calcium influx by an ionophore did not induce major changes in the accumulation of astaxanthin, nor in the transcript levels, but amplified the response of crtR-b to nsPEFs at 25 ns, while decreased in 50 ns treatment. When Ca²⁺ influx was inhibited by GdCl₃, the transcript of psy and bkt 1 were decreased for both 25 ns and 50 ns treatments, while crtR-b exhibited an obvious increase for the 25 ns treatment. We interpret these data in a working model, where nsPEFs permeabilise plasma and chloroplast membrane depending on pulse duration leading to a differential release of plastid retrograde signaling to the nucleus.

High Light Acclimation Induces Chloroplast Precursor Phosphorylation and Reduces Import Efficiency

Acclimation is an essential process in plants on many levels, but especially in chloroplasts under changing light conditions. It is partially known how the photosynthetic machinery reacts upon exposure to high light intensities, including rearrangement of numerous protein complexes. Since the majority of proteins residing within chloroplasts needs to be posttranslationally imported into the organelles, we endeavored to study how this important process is regulated upon subjecting plants from pea and Arabidopsis to high light. Our results reveal that acclimation takes place on the one hand in the cytosol by differential phosphorylation of preproteins and resulting from the altered expression of the responsible kinases, and on the other hand at the level of the translocation machineries in the outer (TOC) and inner (TIC) envelope membranes. Intriguingly, while phosphorylation is more pronounced under high light, import itself shows a lower efficiency, along with a reduced accumulation of the Toc receptor proteins Toc34 and Toc159.

Chloroplast stromal processing peptidase activity is modulated by transit peptide determinants that include inhibitory roles for its N-terminal domain and initial Met

The stromal processing peptidase (SPP) removes transit peptides as precursor proteins enter the chloroplast and different plastid types. SPP is synthesized as a latent, inactive precursor (preSPP) with an atypically long transit peptide. Determinants in the pea (Pisum sativum) SPP transit peptide that regulate mature SPP activation were investigated. Mutational and chemical analyses with protein modifying agents (NEM and APMA) showed a conserved transit peptide Cys-X-Ser/Thr-Cys motif did not inhibit SPP via a "cysteine switch" mechanism through His-X-X-Glu-His site interactions, although cysteines in mature SPP contribute to an active conformation. Significantly, a transit peptide deletion of only the N-terminal 28 amino acids activates SPP located downstream. Short deletions within this region suggest removal of the initial Met plays a pivotal, mechanistic role. Other deletions of ∼30 amino acids along the length of the transit peptide do not individually trigger activity, but larger deletions including Met have an additive effect indicating its removal may be a critical early step during preSPP import. Interestingly, the active preSPP deletion mutants no longer possess predicted Hsp70 binding sites including initial Met, thus Hsp70 interactions may restrict SPP from attaining an active conformation.

RNA-seq highlights parallel and contrasting patterns in the evolution of the nuclear genome of fully mycoheterotrophic plants

BACKGROUND

While photosynthesis is the most notable trait of plants, several lineages of plants (so-called full heterotrophs) have adapted to obtain organic compounds from other sources. The switch to heterotrophy leads to profound changes at the morphological, physiological and genomic levels.

RESULTS

Here, we characterize the transcriptomes of three species representing two lineages of mycoheterotrophic plants: orchids (Epipogium aphyllum and Epipogium roseum) and Ericaceae (Hypopitys monotropa). Comparative analysis is used to highlight the parallelism between distantly related fully heterotrophic plants. In both lineages, we observed genome-wide elimination of nuclear genes that encode proteins related to photosynthesis, while systems associated with protein import to plastids as well as plastid transcription and translation remain active. Genes encoding components of plastid ribosomes that have been lost from the plastid genomes have not been transferred to the nuclear genomes; instead, some of the encoded proteins have been substituted by homologs. The nuclear genes of both Epipogium species accumulated nucleotide substitutions twice as rapidly as their photosynthetic relatives; in contrast, no increase in the substitution rate was observed in H. monotropa.

CONCLUSIONS

Full heterotrophy leads to profound changes in nuclear gene content. The observed increase in the rate of nucleotide substitutions is lineage specific, rather than a universal phenomenon among non-photosynthetic plants.

Import of Soluble Proteins into Chloroplasts and Potential Regulatory Mechanisms

Chloroplasts originated from an endosymbiotic event in which a free-living cyanobacterium was engulfed by an ancestral eukaryotic host. During evolution the majority of the chloroplast genetic information was transferred to the host cell nucleus. As a consequence, proteins formerly encoded by the chloroplast genome are now translated in the cytosol and must be subsequently imported into the chloroplast. This process involves three steps: (i) cytosolic sorting procedures, (ii) binding to the designated receptor-equipped target organelle and (iii) the consecutive translocation process. During import, proteins have to overcome the two barriers of the chloroplast envelope, namely the outer envelope membrane (OEM) and the inner envelope membrane (IEM). In the majority of cases, this is facilitated by two distinct multiprotein complexes, located in the OEM and IEM, respectively, designated TOC and TIC. Plants are constantly exposed to fluctuating environmental conditions such as temperature and light and must therefore regulate protein composition within the chloroplast to ensure optimal functioning of elementary processes such as photosynthesis. In this review we will discuss the recent models of each individual import stage with regard to short-term strategies that plants might use to potentially acclimate to changes in their environmental conditions and preserve the chloroplast protein homeostasis.

Light intensity affects chlorophyll synthesis during greening process by metabolite signal from mitochondrial alternative oxidase in Arabidopsis

Although mitochondrial alternative oxidase (AOX) has been proposed to play essential roles in high light stress tolerance, the effects of AOX on chlorophyll synthesis are unclear. Previous studies indicated that during greening, chlorophyll accumulation was largely delayed in plants whose mitochondrial cyanide-resistant respiration was inhibited by knocking out nuclear encoded AOX gene. Here, we showed that this delay of chlorophyll accumulation was more significant under high light condition. Inhibition of cyanide-resistant respiration was also accompanied by the increase of plastid NADPH/NADP(+) ratio, especially under high light treatment which subsequently blocked the import of multiple plastidial proteins, such as some components of the photosynthetic electron transport chain, the Calvin-Benson cycle enzymes and malate/oxaloacetate shuttle components. Overexpression of AOX1a rescued the aox1a mutant phenotype, including the chlorophyll accumulation during greening and plastidial protein import. It thus suggests that light intensity affects chlorophyll synthesis during greening process by a metabolic signal, the AOX-derived plastidial NADPH/NADP(+) ratio change. Further, our results thus revealed a molecular mechanism of chloroplast-mitochondria interactions.

Redox meets protein trafficking

After the engulfment of two prokaryotic organisms, the thus emerged eukaryotic cell needed to establish means of communication and signaling to properly integrate the acquired organelles into its metabolism. Regulatory mechanisms had to evolve to ensure that chloroplasts and mitochondria smoothly function in accordance with all other cellular processes. One essential process is the post-translational import of nuclear encoded organellar proteins, which needs to be adapted according to the requirements of the plant. The demand for protein import is constantly changing depending on varying environmental conditions, as well as external and internal stimuli or different developmental stages. Apart from long-term regulatory mechanisms such as transcriptional/translation control, possibilities for short-term acclimation are mandatory. To this end, protein import is integrated into the cellular redox network, utilizing the recognition of signals from within the organelles and modifying the efficiency of the translocon complexes. Thereby, cellular requirements can be communicated throughout the whole organism. This article is part of a Special Issue entitled: Chloroplast Biogenesis.

The Induction of Recombinant Protein Bodies in Different Subcellular Compartments Reveals a Cryptic Plastid-Targeting Signal in the 27-kDa γ-Zein Sequence

Naturally occurring storage proteins such as zeins are used as fusion partners for recombinant proteins because they induce the formation of ectopic storage organelles known as protein bodies (PBs) where the proteins are stabilized by intermolecular interactions and the formation of disulfide bonds. Endogenous PBs are derived from the endoplasmic reticulum (ER). Here, we have used different targeting sequences to determine whether ectopic PBs composed of the N-terminal portion of mature 27 kDa γ-zein added to a fluorescent protein could be induced to form elsewhere in the cell. The addition of a transit peptide for targeting to plastids causes PB formation in the stroma, whereas in the absence of any added targeting sequence PBs were typically associated with the plastid envelope, revealing the presence of a cryptic plastid-targeting signal within the γ-zein cysteine-rich domain. The subcellular localization of the PBs influences their morphology and the solubility of the stored recombinant fusion protein. Our results indicate that the biogenesis and budding of PBs does not require ER-specific factors and therefore, confirm that γ-zein is a versatile fusion partner for recombinant proteins offering unique opportunities for the accumulation and bioencapsulation of recombinant proteins in different subcellular compartments.

Ectopically expressed sweet pepper ferredoxin PFLP enhances disease resistance to Pectobacterium carotovorum subsp. carotovorum affected by harpin and protease-mediated hypersensitive response in Arabidopsis

Plant ferredoxin-like protein (PFLP) is a photosynthesis-type ferredoxin (Fd) found in sweet pepper. It contains an iron-sulphur cluster that receives and delivers electrons between enzymes involved in many fundamental metabolic processes. It has been demonstrated that transgenic plants overexpressing PFLP show a high resistance to many bacterial pathogens, although the mechanism remains unclear. In this investigation, the PFLP gene was transferred into Arabidopsis and its defective derivatives, such as npr1 (nonexpresser of pathogenesis-related gene 1) and eds1 (enhanced disease susceptibility 1) mutants and NAHG-transgenic plants. These transgenic plants were then infected with the soft-rot bacterial pathogen Pectobacterium carotovorum subsp. carotovorum (Erwinia carotovora ssp. carotovora, ECC) to investigate the mechanism behind PFLP-mediated resistance. The results revealed that, instead of showing soft-rot symptoms, ECC activated hypersensitive response (HR)-associated events, such as the accumulation of hydrogen peroxide (H2 O2 ), electrical conductivity leakage and expression of the HR marker genes (ATHSR2 and ATHSR3) in PFLP-transgenic Arabidopsis. This PFLP-mediated resistance could be abolished by inhibitors, such as diphenylene iodonium (DPI), 1-l-trans-epoxysuccinyl-leucylamido-(4-guanidino)-butane (E64) and benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (z-VAD-fmk), but not by myriocin and fumonisin. The PFLP-transgenic plants were resistant to ECC, but not to its harpin mutant strain ECCAC5082. In the npr1 mutant and NAHG-transgenic Arabidopsis, but not in the eds1 mutant, overexpression of the PFLP gene increased resistance to ECC. Based on these results, we suggest that transgenic Arabidopsis contains high levels of ectopic PFLP; this may lead to the recognition of the harpin and to the activation of the HR and other resistance mechanisms, and is dependent on the protease-mediated pathway.

Exploring mechanisms linked to differentiation and function of dimorphic chloroplasts in the single cell C4 species Bienertia sinuspersici

BACKGROUND

In the model single-cell C4 plant Bienertia sinuspersici, chloroplast- and nuclear-encoded photosynthetic enzymes, characteristically confined to either bundle sheath or mesophyll cells in Kranz-type C4 leaves, all occur together within individual leaf chlorenchyma cells. Intracellular separation of dimorphic chloroplasts and key enzymes within central and peripheral compartments allow for C4 carbon fixation analogous to NAD-malic enzyme (NAD-ME) Kranz type species. Several methods were used to investigate dimorphic chloroplast differentiation in B. sinuspersici.

RESULTS

Confocal analysis revealed that Rubisco-containing chloroplasts in the central compartment chloroplasts (CCC) contained more photosystem II proteins than the peripheral compartment chloroplasts (PCC) which contain pyruvate,Pi dikinase (PPDK), a pattern analogous to the cell type-specific chloroplasts of many Kranz type NAD-ME species. Transient expression analysis using GFP fusion constructs containing various lengths of a B. sinuspersici Rubisco small subunit (RbcS) gene and the transit peptide of PPDK revealed that their import was not specific to either chloroplast type. Immunolocalization showed the rbcL-specific mRNA binding protein RLSB to be selectively localized to the CCC in B. sinuspersici, and to Rubisco-containing BS chloroplasts in the closely related Kranz species Suaeda taxifolia. Comparative fluorescence analyses were made using redox-sensitive and insensitive GFP forms, as well comparative staining using the peroxidase indicator 3,3-diaminobenzidine (DAB), which demonstrated differences in stromal redox potential, with the CCC having a more negative potential than the PCC.

CONCLUSIONS

Both CCC RLSB localization and the differential chloroplast redox state are suggested to have a role in post-transcriptional rbcL expression.

Proteome analysis of plastids from developing seeds of Jatropha curcas L

In this study, we performed a proteomic analysis of plastids isolated from the endosperm of developing Jatropha curcas seeds that were in the initial stage of deposition of protein and lipid reserves. Proteins extracted from the plastids were digested with trypsin, and the peptides were applied to an EASY-nano LC system coupled inline to an ESI-LTQ-Orbitrap Velos mass spectrometer, and this led to the identification of 1103 proteins representing 804 protein groups, of which 923 proteins were considered as true identifications, and this considerably expands the repertoire of J. curcas proteins identified so far. Of the identified proteins, only five are encoded in the plastid genome, and none of them are involved in photosynthesis, evidentiating the nonphotosynthetic nature of the isolated plastids. Homologues for 824 out of 923 identified proteins were present in PPDB, SUBA, or PlProt databases while homologues for 13 proteins were not found in any of the three plastid proteins databases but were marked as plastidial by at least one of the three prediction programs used. Functional classification showed that proteins belonging to amino acids metabolism comprise the main functional class, followed by carbohydrate, energy, and lipid metabolisms. The small and large subunits of Rubisco were identified, and their presence in the plastids is considered to be an adaptive feature counterbalancing for the loss of one-third of the carbon as CO2 as a result of the conversion of carbohydrate to oil through glycolysis. While several enzymes involved in the biosynthesis of several precursors of diterpenoids were identified, we were unable to identify any terpene synthase/cyclase, which suggests that the plastids isolated from the endosperm of developing seeds do not synthesize phorbol esters. In conclusion, our study provides insights into the major biosynthetic pathways and certain unique features of the plastids from the endosperm of developing seeds at the whole proteome level.

The Plasmodium translocon of exported proteins (PTEX) component thioredoxin-2 is important for maintaining normal blood-stage growth

Plasmodium parasites remodel their vertebrate host cells by translocating hundreds of proteins across an encasing membrane into the host cell cytosol via a putative export machinery termed PTEX. Previously PTEX150, HSP101 and EXP2 have been shown to be bona fide members of PTEX. Here we validate that PTEX88 and TRX2 are also genuine members of PTEX and provide evidence that expression of PTEX components are also expressed in early gametocytes, mosquito and liver stages, consistent with observations that protein export is not restricted to asexual stages. Although amenable to genetic tagging, HSP101, PTEX150, EXP2 and PTEX88 could not be genetically deleted in Plasmodium berghei, in keeping with the obligatory role this complex is postulated to have in maintaining normal blood-stage growth. In contrast, the putative thioredoxin-like protein TRX2 could be deleted, with knockout parasites displaying reduced grow-rates, both in vivo and in vitro, and reduced capacity to cause severe disease in a cerebral malaria model. Thus, while not essential for parasite survival, TRX2 may help to optimize PTEX activity. Importantly, the generation of TRX2 knockout parasites that display altered phenotypes provides a much-needed tool to dissect PTEX function.

Widespread dual targeting of proteins in land plants: when, where, how and why

Since the discovery of the first dual targeted protein in plants in 1995 the number of dual targeted proteins in plants has grown to over 250 proteins. Much work and investigations have focused on identifying how or what makes a protein dual targeted. Recently, more research has focused on the evolution and conservation of dual targeting of proteins in plants. This new work has demonstrated that dual targeting arose early within the evolution of plants and because it is rarely lost, once gained, it must be under some positive selection pressure. The possible reasons as why proteins are dual targeted and why it was conserved during the evolution of plants are discussed.

An intermolecular disulfide-based light switch for chloroplast psbD gene expression in Chlamydomonas reinhardtii

Expression of the chloroplast psbD gene encoding the D2 protein of the photosystem II reaction center is regulated by light. In the green alga Chlamydomonas reinhardtii, D2 synthesis requires a high-molecular-weight complex containing the RNA stabilization factor Nac2 and the translational activator RBP40. Based on size exclusion chromatography analyses, we provide evidence that light control of D2 synthesis depends on dynamic formation of the Nac2/RBP40 complex. Furthermore, 2D redox SDS-PAGE assays suggest an intermolecular disulfide bridge between Nac2 and Cys11 of RBP40 as the putative molecular basis for attachment of RBP40 to the complex in light-grown cells. This covalent link is reduced in the dark, most likely via NADPH-dependent thioredoxin reductase C, supporting the idea of a direct relationship between chloroplast gene expression and chloroplast carbon metabolism during dark adaption of algal cells.

Thioredoxin and glutaredoxin systems in plants: molecular mechanisms, crosstalks, and functional significance

Thioredoxins (Trx) and glutaredoxins (Grx) constitute families of thiol oxidoreductases. Our knowledge of Trx and Grx in plants has dramatically increased during the last decade. The release of the Arabidopsis genome sequence revealed an unexpectedly high number of Trx and Grx genes. The availability of several genomes of vascular and nonvascular plants allowed the establishment of a clear classification of the genes and the chronology of their appearance during plant evolution. Proteomic approaches have been developed that identified the putative Trx and Grx target proteins which are implicated in all aspects of plant growth, including basal metabolism, iron/sulfur cluster formation, development, adaptation to the environment, and stress responses. Analyses of the biochemical characteristics of specific Trx and Grx point to a strong specificity toward some target enzymes, particularly within plastidial Trx and Grx. In apparent contradiction with this specificity, genetic approaches show an absence of phenotype for most available Trx and Grx mutants, suggesting that redundancies also exist between Trx and Grx members. Despite this, the isolation of mutants inactivated in multiple genes and several genetic screens allowed the demonstration of the involvement of Trx and Grx in pathogen response, phytohormone pathways, and at several control points of plant development. Cytosolic Trxs are reduced by NADPH-thioredoxin reductase (NTR), while the reduction of Grx depends on reduced glutathione (GSH). Interestingly, recent development integrating biochemical analysis, proteomic data, and genetics have revealed an extensive crosstalk between the cytosolic NTR/Trx and GSH/Grx systems. This crosstalk, which occurs at multiple levels, reveals the high plasticity of the redox systems in plants.

The chloroplast protein import system: from algae to trees

Chloroplasts are essential organelles in the cells of plants and algae. The functions of these specialized plastids are largely dependent on the ~3000 proteins residing in the organelle. Although chloroplasts are capable of a limited amount of semiautonomous protein synthesis - their genomes encode ~100 proteins - they must import more than 95% of their proteins after synthesis in the cytosol. Imported proteins generally possess an N-terminal extension termed a transit peptide. The importing translocons are made up of two complexes in the outer and inner envelope membranes, the so-called Toc and Tic machineries, respectively. The Toc complex contains two precursor receptors, Toc159 and Toc34, a protein channel, Toc75, and a peripheral component, Toc64/OEP64. The Tic complex consists of as many as eight components, namely Tic22, Tic110, Tic40, Tic20, Tic21 Tic62, Tic55 and Tic32. This general Toc/Tic import pathway, worked out largely in pea chloroplasts, appears to operate in chloroplasts in all green plants, albeit with significant modifications. Sub-complexes of the Toc and Tic machineries are proposed to exist to satisfy different substrate-, tissue-, cell- and developmental requirements. In this review, we summarize our understanding of the functions of Toc and Tic components, comparing these components of the import machinery in green algae through trees. We emphasize recent findings that point to growing complexities of chloroplast protein import process, and use the evolutionary relationships between proteins of different species in an attempt to define the essential core translocon components and those more likely to be responsible for regulation. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.

A systematic screen to discover and analyze apicoplast proteins identifies a conserved and essential protein import factor

Parasites of the phylum Apicomplexa cause diseases that impact global health and economy. These unicellular eukaryotes possess a relict plastid, the apicoplast, which is an essential organelle and a validated drug target. However, much of its biology remains poorly understood, in particular its elaborate compartmentalization: four membranes defining four different spaces. Only a small number of organellar proteins have been identified in particular few proteins are known for non-luminal apicoplast compartments. We hypothesized that enlarging the catalogue of apicoplast proteins will contribute toward identifying new organellar functions and expand the realm of targets beyond a limited set of characterized pathways. We developed a bioinformatic screen based on mRNA abundance over the cell cycle and on phyletic distribution. We experimentally assessed 57 genes, and of 30 successful epitope tagged candidates eleven novel apicoplast proteins were identified. Of those, seven appear to target to the lumen of the organelle, and four localize to peripheral compartments. To address their function we then developed a robust system for the construction of conditional mutants via a promoter replacement strategy. We confirm the feasibility of this system by establishing conditional mutants for two selected genes – a luminal and a peripheral apicoplast protein. The latter is particularly intriguing as it encodes a hypothetical protein that is conserved in and unique to Apicomplexan parasites and other related organisms that maintain a red algal endosymbiont. Our studies suggest that this peripheral plastid protein, PPP1, is likely localized to the periplastid compartment. Conditional disruption of PPP1 demonstrated that it is essential for parasite survival. Phenotypic analysis of this mutant is consistent with a role of the PPP1 protein in apicoplast biogenesis, specifically in import of nuclear-encoded proteins into the organelle.

Apicomplexa are a group of parasites that cause important diseases, including malaria and several AIDS associated opportunistic infections. The parasites depend on an algal endosymbiont, the apicoplast, and this provides an Achilles' heel for drug development. We use Toxoplasma gondii as a model to characterize the biology and function of the apicoplast. In this study we apply a strategy to identify new apicoplast proteins and to prioritize them as potential targets through the analysis of genetic mutants. To aid this goal we develop a new parasite line and a protocol enabling the streamlined construction of conditional mutants. Using this new approach we discover numerous new apicoplast proteins, many of them have no assigned function yet. We demonstrate that function can be deduced using our genetic approach by establishing the essential role in apicoplast protein import for a new factor with intriguing localization and evolutionary history.

How do single cell C4 species form dimorphic chloroplasts?

Bienertia sinuspersici is one of only three higher land plant species known to perform C 4 photosynthesis without Kranz anatomy through partitioning of photosynthetic functions between dimorphic chloroplasts in a single photosynthetic cell. We recently reported the successful separation of the two chloroplast types, and biochemical and functional analyses revealed differences in protein composition and specialization of photosynthetic functions. In Kranz type C 4 species, spatial (or cell-specific) control of transcription of nuclear genes contributes to development of dimorphic chloroplasts, but obviously this cannot be involved in formation of dimorphic chloroplasts within individual photosynthetic cells. Therefore, we address here the question of how nuclear encoded proteins could be selectively targeted to plastids within a cell to form two types of chloroplasts. We discuss current knowledge of chloroplast differentiation in single cell C 4 species and present three hypothetical mechanisms for how this could occur.

Ferredoxin:NADPH oxidoreductase is recruited to thylakoids by binding to a polyproline type II helix in a pH-dependent manner

Ferredoxin:NADPH oxidoreductase (FNR) is a key enzyme of photosynthetic electron transport required for generation of reduction equivalents. Recently, two proteins were found to be involved in membrane-anchoring of FNR by specific interaction via a conserved Ser/Pro-rich motif: Tic62 and Trol. Our crystallographic study reveals that the FNR-binding motif, which forms a polyproline type II helix, induces self-assembly of two FNR monomers into a back-to-back dimer. Because binding occurs opposite to the FNR active sites, its activity is not affected by the interaction. Surface plasmon resonance analyses disclose a high affinity of FNR to the binding motif, which is strongly increased under acidic conditions. The pH of the chloroplast stroma changes dependent on the light conditions from neutral to slightly acidic in complete darkness or to alkaline at saturating light conditions. Recruiting of FNR to the thylakoids could therefore represent a regulatory mechanism to adapt FNR availability/activity to photosynthetic electron flow.

A new concept for ferredoxin-NADP(H) oxidoreductase binding to plant thylakoids

During the evolution of photosynthesis, regulatory circuits were established that allow the precise coupling of light-driven electron transfer chains with downstream processes such as carbon fixation. The ferredoxin (Fd):ferredoxin-NADP(+) oxidoreductase (FNR) couple is an important mediator for these processes because it provides the transition from exclusively membrane-bound light reactions to the mostly stromal metabolic pathways. Recent progress has allowed us to revisit how FNR is bound to thylakoids and to revaluate the current view that only membrane-bound FNR is active in photosynthetic reactions. We argue that the vast majority of thylakoid-bound FNR of higher plants is not necessary for photosynthesis. We furthermore propose that the correct distribution of FNR between stroma and thylakoids is used to efficiently regulate Fd-dependent electron partitioning in the chloroplast.

Protein import into chloroplasts--how chaperones feature into the game

Chloroplasts originated from an endosymbiotic event, in which an ancestral photosynthetic cyanobacterium was engulfed by a mitochondriate eukaryotic host cell. During evolution, the endosymbiont lost its autonomy by means of a massive transfer of genetic information from the prokaryotic genome to the host nucleus. Consequently, the development of protein import machineries became necessary for the relocation of proteins that are now nuclear-encoded and synthesized in the cytosol but destined for the chloroplast. Organelle biogenesis and maintenance requires a tight coordination of transcription, translation and protein import between the host cell and the organelle. This review focuses on the translocation complexes in the outer and inner envelope membrane with a special emphasis on the role of molecular chaperones. This article is part of a Special Issue entitled Protein translocation across or insertion into membranes.

Protein import into chloroplasts: the Tic complex and its regulation

Chloroplasts like mitochondria were derived from an endosymbiontic event. Due to the massive gene transfer to the nucleus during endosymbiosis, only a limited number of chloroplastic proteins are still encoded for in the plastid genome. Most of the nuclear-encoded plastidic proteins are post-translationally translocated back to the chloroplast via the general import pathway through distinct outer and inner envelope membrane protein complexes, the Toc and Tic translocons (Translocon at the outer/inner envelope membrane of chloroplasts). Eight Tic subunits have been described so far, including two potential channel proteins (Tic110 and Tic20), the "motor complex" (Tic40 associated with the stromal chaperone Hsp93) and the "redox regulon" (Tic62, Tic55, and Tic32) involved in regulation of protein import via the metabolic redox status of the chloroplast. Regulation can additionally occur via thioredoxins (Tic110 and Tic55) or via the calcium/calmodulin network (Tic110 and Tic32). In this review we present the current knowledge about the Tic complex focusing on its regulation and addressing some still open questions.