Proteomics of the chloroplast envelope membranes from Arabidopsis thaliana

Purification of intact chloroplasts from marine plant Posidonia oceanica suitable for organelle proteomics

Posidonia oceanica is a marine angiosperm, or seagrass, adapted to grow to the underwater life from shallow waters to 50 m depth. This raises questions of how their photosynthesis adapted to the attenuation of light through the water column and leads to the assumption that biochemistry and metabolism of the chloroplast are the basis of adaptive capacity. In the present study, we described a protocol that was adapted from those optimized for terrestrial plants, to extract chloroplasts from as minimal tissue as possible. We obtained the best balance between tissue amount/intact chloroplasts yield using one leaf from one plant. After isopynic separations, the chloroplasts purity and integrity were evaluated by biochemical assay and using a proteomic approach. Chloroplast proteins were extracted from highly purified organelles and resolved by 1DE SDS-PAGE. Proteins were sequenced by nLC-ESI-IT-MS/MS of 1DE gel bands and identified against NCBInr green plant databases, Dr. Zompo database for seagrasses in a local customized dataset. The curated localization of proteins in sub-plastidial compartments (i.e. envelope, stroma and thylakoids) was retrieved in the AT_CHLORO database. This purification protocol and the validation of compartment markers may serve as basis for sub-cellular proteomics in P. oceanica and other seagrasses.

Identification of Two Conserved Residues Involved in Copper Release from Chloroplast PIB-1-ATPases

Copper is an essential transition metal for living organisms. In the plant model Arabidopsis thaliana, half of the copper content is localized in the chloroplast, and as a cofactor of plastocyanin, copper is essential for photosynthesis. Within the chloroplast, copper delivery to plastocyanin involves two transporters of the PIB-1-ATPases subfamily: HMA6 at the chloroplast envelope and HMA8 in the thylakoid membranes. Both proteins are high affinity copper transporters but share distinct enzymatic properties. In the present work, the comparison of 140 sequences of PIB-1-ATPases revealed a conserved region unusually rich in histidine and cysteine residues in the TMA-L1 region of eukaryotic chloroplast copper ATPases. To evaluate the role of these residues, we mutated them in HMA6 and HMA8. Mutants of interest were selected from phenotypic tests in yeast and produced in Lactococcus lactis for further biochemical characterizations using phosphorylation assays from ATP and Pi Combining functional and structural data, we highlight the importance of the cysteine and the first histidine of the CX3HX2H motif in the process of copper release from HMA6 and HMA8 and propose a copper pathway through the membrane domain of these transporters. Finally, our work suggests a more general role of the histidine residue in the transport of copper by PIB-1-ATPases.

Regulation and structure of the heteromeric acetyl-CoA carboxylase

The enzyme acetyl-CoA carboxylase (ACCase) catalyzes the committed step of the de novo fatty acid biosynthesis (FAS) pathway by converting acetyl-CoA to malonyl-CoA. Two forms of ACCase exist in nature, a homomeric and heteromic form. The heteromeric form of this enzyme requires four different subunits for activity: biotin carboxylase; biotin carboxyl carrier protein; and α- and β-carboxyltransferases. Heteromeric ACCases (htACCase) can be found in prokaryotes and the plastids of most plants. The plant htACCase is regulated by diverse mechanisms reflected by the biochemical and genetic complexity of this multienzyme complex and the plastid stroma where it resides. In this review we summarize the regulation of the plant htACCase and also describe the structural characteristics of this complex from both prokaryotes and plants. This article is part of a Special Issue entitled: Plant Lipid Biology edited by Kent D. Chapman and Ivo Feussner.

Genome-Wide Identification of Jatropha curcas Aquaporin Genes and the Comparative Analysis Provides Insights into the Gene Family Expansion and Evolution in Hevea brasiliensis

Aquaporins (AQPs) are channel-forming integral membrane proteins that transport water and other small solutes across biological membranes. Despite the vital role of AQPs, to date, little is known in physic nut (Jatropha curcas L., Euphorbiaceae), an important non-edible oilseed crop with great potential for the production of biodiesel. In this study, 32 AQP genes were identified from the physic nut genome and the family number is relatively small in comparison to 51 in another Euphorbiaceae plant, rubber tree (Hevea brasiliensis Muell. Arg.). Based on the phylogenetic analysis, the JcAQPs were assigned to five subfamilies, i.e., nine plasma membrane intrinsic proteins (PIPs), nine tonoplast intrinsic proteins (TIPs), eight NOD26-like intrinsic proteins (NIPs), two X intrinsic proteins (XIPs), and four small basic intrinsic proteins (SIPs). Like rubber tree and other plant species, functional prediction based on the aromatic/arginine selectivity filter, Froger's positions, and specificity-determining positions showed a remarkable difference in substrate specificity among subfamilies of JcAQPs. Genome-wide comparative analysis revealed the specific expansion of PIP and TIP subfamilies in rubber tree and the specific gene loss of the XIP subfamily in physic nut. Furthermore, by analyzing deep transcriptome sequencing data, the expression evolution especially the expression divergence of duplicated HbAQP genes was also investigated and discussed. Results obtained from this study not only provide valuable information for future functional analysis and utilization of Jc/HbAQP genes, but also provide a useful reference to survey the gene family expansion and evolution in Euphorbiaceae plants and other plant species.

The chloroplast membrane associated ceQORH putative quinone oxidoreductase reduces long-chain, stress-related oxidized lipids

Under oxidative stress conditions the lipid constituents of cells can undergo oxidation whose frequent consequence is the production of highly reactive α,β-unsaturated carbonyls. These molecules are toxic because they can add to biomolecules (such as proteins and nucleic acids) and several enzyme activities cooperate to eliminate these reactive electrophile species. CeQORH (chloroplast envelope Quinone Oxidoreductase Homolog, At4g13010) is associated with the inner membrane of the chloroplast envelope and imported into the organelle by an alternative import pathway. In the present study, we show that the recombinant ceQORH exhibits the activity of a NADPH-dependent α,β-unsaturated oxoene reductase reducing the double bond of medium-chain (C⩾9) to long-chain (18 carbon atoms) reactive electrophile species deriving from poly-unsaturated fatty acid peroxides. The best substrates of ceQORH are 13-lipoxygenase-derived γ-ketols. γ-Ketols are spontaneously produced in the chloroplast from the unstable allene oxide formed in the biochemical pathway leading to 12-oxo-phytodienoic acid, a precursor of the defense hormone jasmonate. In chloroplasts, ceQORH could detoxify 13-lipoxygenase-derived γ-ketols at their production sites in the membranes. This finding opens new routes toward the understanding of γ-ketols role and detoxification.

Proteomic analysis of plasma membrane and tonoplast from the leaves of mangrove plant Avicennia officinalis

In order to understand the salt tolerance and secretion in mangrove plant species, gel electrophoresis coupled with LC-MS-based proteomics was used to identify key transport proteins in the plasma membrane (PM) and tonoplast fractions of Avicennia officinalis leaves. PM and tonoplast proteins were purified using two-aqueous-phase partitioning and density gradient centrifugation, respectively. Forty of the 254 PM proteins and 31 of the 165 tonoplast proteins identified were predicted to have transmembrane domains. About 95% of the identified proteins could be classified based on their functions. The major classes of proteins were predicted to be involved in transport, metabolic processes, defense/stress response, and signal transduction, while a few of the proteins were predicted to be involved in other functions such as membrane trafficking. The main classes of transporter proteins identified included H(+) -ATPases, ATP-binding cassette transporters, and aquaporins, all of which could play a role in salt secretion. These data will serve as the baseline membrane proteomic dataset for Avicennia species. Further, this information can contribute to future studies on understanding the mechanism of salt tolerance in halophytes in addition to salt secretion in mangroves. All MS data have been deposited in the ProteomeXchange with identifier PXD000837 (http://proteomecentral.proteomexchange.org/dataset/PXD000837).

Aquaporins in Plants

Aquaporins are membrane channels that facilitate the transport of water and small neutral molecules across biological membranes of most living organisms. In plants, aquaporins occur as multiple isoforms reflecting a high diversity of cellular localizations, transport selectivity, and regulation properties. Plant aquaporins are localized in the plasma membrane, endoplasmic reticulum, vacuoles, plastids and, in some species, in membrane compartments interacting with symbiotic organisms. Plant aquaporins can transport various physiological substrates in addition to water. Of particular relevance for plants is the transport of dissolved gases such as carbon dioxide and ammonia or metalloids such as boron and silicon. Structure-function studies are developed to address the molecular and cellular mechanisms of plant aquaporin gating and subcellular trafficking. Phosphorylation plays a central role in these two processes. These mechanisms allow aquaporin regulation in response to signaling intermediates such as cytosolic pH and calcium, and reactive oxygen species. Combined genetic and physiological approaches are now integrating this knowledge, showing that aquaporins play key roles in hydraulic regulation in roots and leaves, during drought but also in response to stimuli as diverse as flooding, nutrient availability, temperature, or light. A general hydraulic control of plant tissue expansion by aquaporins is emerging, and their role in key developmental processes (seed germination, emergence of lateral roots) has been established. Plants with genetically altered aquaporin functions are now tested for their ability to improve plant tolerance to stresses. In conclusion, research on aquaporins delineates ever expanding fields in plant integrative biology thereby establishing their crucial role in plants.

Proteomic insights into synthesis of isoflavonoids in soybean seeds

Soybean seeds are the major human dietary source of isoflavonoids, a class of plant natural products almost entirely exclusive to legumes. Isoflavonoids reduce the risk of a number of chronic human illnesses. Biosynthesis and accumulation of this class of compounds is a multigenic and complex trait, with a great deal of variability among soybean cultivars and with respect to the environment. There is a wealth of genomic, transcriptomic, and metabolomics data regarding isoflavonoid biosynthesis, but the connection between multigene families and their cognate proteins is a missing link that could provide us with a great deal of functional information. The changing proteome of the developing seed can shed light on the correlative increase in isoflavonoids, while the maternal seed coat proteome can provide the link with inherited metabolic and signaling machinery. In this effort, 'seed-filling' proteomics has revealed key secondary metabolite enzymes that quantitatively vary throughout seed development. Seed coat proteomics has revealed the existence of metabolic apparatus specific to isoflavonoid biosynthesis (isoflavonoid reductase) that could potentially influence the chemical content of this organ. The future of proteomic analysis of isoflavonoid biosynthesis should be centered on the development of quantitative, tissue-specific proteomes that emphasize low-abundance metabolic proteins to extract the whole suite of factors involved.

Calcium-dependent regulation of photosynthesis

The understanding of calcium as a second messenger in plants has been growing intensively over the last decades. Recently, attention has been drawn to the organelles, especially the chloroplast but focused on the stromal Ca2+ transients in response to environmental stresses. Herein we will expand this view and discuss the role of Ca2+ in photosynthesis. Moreover we address of how Ca2+ is delivered to chloroplast stroma and thylakoids. Thereby, new light is shed on the regulation of photosynthetic electron flow and light-dependent metabolism by the interplay of Ca2+, thylakoid acidification and redox status. This article is part of a Special Issue entitled: Chloroplast biogenesis.

Hsp90 is involved in the regulation of cytosolic precursor protein abundance in tomato

Cytosolic chaperones are involved in the regulation of cellular protein homeostasis in general. Members of the families of heat stress proteins 70 (Hsp70) and 90 (Hsp90) assist the transport of preproteins to organelles such as chloroplasts or mitochondria. In addition, Hsp70 was described to be involved in the degradation of chloroplast preproteins that accumulate in the cytosol. Because a similar function has not been established for Hsp90, we analyzed the influences of Hsp90 and Hsp70 on the protein abundance in the cellular context using an in vivo system based on mesophyll protoplasts. We observed a differential behavior of preproteins with respect to the cytosolic chaperone-dependent regulation. Some preproteins such as pOE33 show a high dependence on Hsp90, whereas the abundance of preproteins such as pSSU is more strongly dependent on Hsp70. The E3 ligase, C-terminus of Hsp70-interacting protein (Chip), appears to have a more general role in the control of cytosolic protein abundance. We discuss why the different reaction modes are comparable with the cytosolic unfolded protein response.

Posttranslational modifications of FERREDOXIN-NADP+ OXIDOREDUCTASE in Arabidopsis chloroplasts

Rapid responses of chloroplast metabolism and adjustments to photosynthetic machinery are of utmost importance for plants' survival in a fluctuating environment. These changes may be achieved through posttranslational modifications of proteins, which are known to affect the activity, interactions, and localization of proteins. Recent studies have accumulated evidence about the crucial role of a multitude of modifications, including acetylation, methylation, and glycosylation, in the regulation of chloroplast proteins. Both of the Arabidopsis (Arabidopsis thaliana) leaf-type FERREDOXIN-NADP(+) OXIDOREDUCTASE (FNR) isoforms, the key enzymes linking the light reactions of photosynthesis to carbon assimilation, exist as two distinct forms with different isoelectric points. We show that both AtFNR isoforms contain multiple alternative amino termini and undergo light-responsive addition of an acetyl group to the α-amino group of the amino-terminal amino acid of proteins, which causes the change in isoelectric point. Both isoforms were also found to contain acetylation of a conserved lysine residue near the active site, while no evidence for in vivo phosphorylation or glycosylation was detected. The dynamic, multilayer regulation of AtFNR exemplifies the complex regulatory network systems controlling chloroplast proteins by a range of posttranslational modifications, which continues to emerge as a novel area within photosynthesis research.

Ions channels/transporters and chloroplast regulation

Ions play fundamental roles in all living cells and their gradients are often essential to fuel transports, to regulate enzyme activities and to transduce energy within and between cells. Their homeostasis is therefore an essential component of the cell metabolism. Ions must be imported from the extracellular matrix to their final subcellular compartments. Among them, the chloroplast is a particularly interesting example because there, ions not only modulate enzyme activities, but also mediate ATP synthesis and actively participate in the building of the photosynthetic structures by promoting membrane-membrane interaction. In this review, we first provide a comprehensive view of the different machineries involved in ion trafficking and homeostasis in the chloroplast, and then discuss peculiar functions exerted by ions in the frame of photochemical conversion of absorbed light energy.

Targeted proteomic quantitation of the absolute expression and turnover of cystic fibrosis transmembrane conductance regulator in the apical plasma membrane

Deficient chloride transport through cystic fibrosis (CF) transmembrane conductance regulator (CFTR) causes lethal complications in CF patients. CF is the most common autosomal recessive genetic disease, which is caused by mutations in the CFTR gene; thus, CFTR mutants can serve as primary targets for drugs to modulate and rescue the ion channel’s function. The first step of drug modulation is to increase the expression of CFTR in the apical plasma membrane (PM); thus, accurate measurement of CFTR in the PM is desired. This work reports a tandem enrichment strategy to prepare PM CFTR and uses a stable isotope labeled CFTR sample as the quantitation reference to measure the absolute amount of apical PM expression of CFTR in CFBE 41o- cells. It was found that CFBE 41o- cells expressing wild-type CFTR (wtCFTR), when cultured on plates, had 2.9 ng of the protein in the apical PM per million cells; this represented 10% of the total CFTR found in the cells. When these cells were polarized on filters, the apical PM expression of CFTR increased to 14%. Turnover of CFTR in the apical PM of baby hamster kidney cells overexpressing wtCFTR (BHK-wtCFTR) was also quantified by targeted proteomics based on multiple reaction monitoring mass spectrometry; wtCFTR had a half-life of 29.0 ± 2.5 h in the apical PM. This represents the first direct measurement of CFTR turnover using stable isotopes. The absolute quantitation and turnover measurements of CFTR in the apical PM can significantly facilitate understanding the disease mechanism of CF and thus the development of new disease-modifying drugs. Absolute CFTR quantitation allows for direct result comparisons among analyses, analysts, and laboratories and will greatly amplify the overall outcome of CF research and therapy.

Border control: selectivity of chloroplast protein import and regulation at the TOC-complex

Plants have evolved complex and sophisticated molecular mechanisms to regulate their development and adapt to their surrounding environment. Particularly the development of their specific organelles, chloroplasts and other plastid-types, is finely tuned in accordance with the metabolic needs of the cell. The normal development and functioning of plastids require import of particular subsets of nuclear encoded proteins. Most preproteins contain a cleavable sequence at their N terminal (transit peptide) serving as a signal for targeting to the organelle and recognition by the translocation machinery TOC-TIC (translocon of outer membrane complex-translocon of inner membrane complex) spanning the dual membrane envelope. The plastid proteome needs constant remodeling in response to developmental and environmental factors. Therefore selective regulation of preprotein import plays a crucial role in plant development. In this review we describe the diversity of transit peptides and TOC receptor complexes, and summarize the current knowledge and potential directions for future research concerning regulation of the different Toc isoforms.

Unknown components of the plastidial permeome

Beyond their role in photosynthesis plastids provide a plethora of additional metabolic functions to plant cells. For example, they harbor complete biosynthetic pathways for the de novo synthesis of carotenoids, fatty acids, and amino acids. Furthermore plastids contribute important reactions to multi-compartmentalized pathways, such as photorespiration or plant hormone syntheses, and they depend on the import of essential molecules that they cannot synthesize themselves, such as ascorbic acid. This causes a high traffic of metabolites across the plastid envelope. Although it was recently shown that non-polar substrates could be exchanged between the plastid and the ER without involving transporters, various essential transport processes are mediated by highly selective but still unknown metabolite transporters. This review focuses on selected components of the plastidial permeome that are predicted to exist but that have not yet been identified as molecular entities, such as the transporters for isopentenyl diphosphate (IPP) or ascorbic acid.

Ultraweak photon emission and proteomics analyses in soybean under abiotic stress

Biophotons are ultraweak photon emissions that are closely related to various biological activities and processes. In mammals, biophoton emissions originate from oxidative bursts in immunocytes during immunological responses. Biophotons emitted from plant organs provide novel information about the physiological state of plant under in vivo condition. In this review, the principles and recent advances in the measurement of biophoton emissions in plants are described. Furthermore, examples of biophoton emission and proteomics in soybean under abiotic stress are reviewed and discussed. Finally, this review suggests that the application of proteomics should provide a better interpretation of plant response to biophoton emission and allow the identification of genes that will allow the screening of crops able to produce maximal yields, even in stressful environments.

Nitrite transport activity of a novel HPP family protein conserved in cyanobacteria and chloroplasts

Some cyanobacterial genomes encode an integral membrane protein of the HPP family, which exhibited nitrite transport activity when expressed in the nitrite transport-less NA4 mutant of the cyanobacterium Synechococcus elongatus strain PCC 7942. AT5G62720 and AT3G47980 were found to encode Arabidopsis homologs of the cyanobacterial protein. The product of AT5G62720 was localized to the chloroplast envelope membrane and was shown to confer nitrite uptake activity on the NA4 mutant when expressed with an N-terminally truncated transit peptide or as a fusion with the N-terminal region of the cyanobacterial HPP family protein. Kinetic analyses showed that the Arabidopsis protein has much higher affinity for nitrite (K(m) = 13 µM) than the cyanobacterial protein (K(m) = 150 µM). Illuminated chloroplasts isolated from the mutant lines of AT5G62720 showed much lower activity of nitrite uptake than the chloroplasts isolated from the wild-type Col-0 plants, while the chloroplasts of the mutants of AT1G68570 (AtNPF3.1), the gene previously reported to encode a plastid nitrite transporter AtNitr1, showed wild-type levels of nitrite uptake activity. AT3G47980 was expressed in roots but not in shoots. It has a putative transit peptide similar to that of AT5G62720 and its fusion with the N-terminal region of the cyanobacterial HPP protein showed low but significant activity of nitrite transport in the cyanobacterial cell. Transcription of AT5G62720 (AtNITR2;1) and AT3G47980 (AtNITR2;2) was stimulated by nitrate under the control of the NIN-like proteins, suggesting that the HPP proteins represent nitrate-inducible components of the nitrite transport system of plastids.

The distinct functional roles of the inner and outer chloroplast envelope of Pea (Pisum sativum) as revealed by proteomic approaches

Protein profiles of inner (IE) and outer (OE) chloroplast envelope membrane preparations from pea were studied using shotgun nLC-MS/MS and two-dimensional electrophoresis, and 589 protein species (NCBI entries) were identified. The relative enrichment of each protein in the IE/OE pair of membranes was used to provide an integrated picture of the chloroplast envelope. From the 546 proteins identified with shotgun, 321 showed a significant differential distribution, with 180 being enriched in IE and 141 in OE. To avoid redundancy and facilitate in silico localization, Arabidopsis homologues were used to obtain a nonredundant list of 409 envelope proteins, with many showing significant OE or IE enrichment. Functional classification reveals that IE is a selective barrier for transport of many metabolites and plays a major role in controlling protein homeostasis, whereas proteins in OE are more heterogeneous and participate in a wide range of processes. Data support that metabolic processes previously described to occur in the envelope such as chlorophyll and tocopherol biosynthesis can be ascribed to the IE, whereas others such as carotenoid or lipid biosynthesis occur in both membranes. Furthermore, results allow empirical assignation to the IE and/or OE of many proteins previously assigned to the bulk chloroplast envelope proteome.

Alternative transcription initiation and the AUG context configuration control dual-organellar targeting and functional competence of Arabidopsis Lon1 protease

Cellular homeostasis relies on components of protein quality control including chaperones and proteases. In bacteria and eukaryotic organelles, Lon proteases play a critical role in removing irreparably damaged proteins and thereby preventing the accumulation of deleterious degradation-resistant aggregates. Gene expression, live-cell imaging, immunobiochemical, and functional complementation approaches provide conclusive evidence for Lon1 dual-targeting to chloroplasts and mitochondria. Dual-organellar deposition of Lon1 isoforms depends on both transcriptional regulation and alternative translation initiation via leaky ribosome scanning from the first AUG sequence context that deviates extensively from the optimum Kozak consensus. Organelle-specific Lon1 targeting results in partial complementation of Arabidopsis lon1-1 mutants, whereas full complementation is solely accomplished by dual-organellar targeting. Both the optimal and non-optimal AUG sequence contexts are functional in yeast and facilitate leaky ribosome scanning complementing the pim1 phenotype when the mitochondrial presequence is used. Bioinformatic search identified a limited number of Arabidopsis genes with Lon1-type dual-targeting sequence organization. Lon4, the paralog of Lon1, has an ambiguous presequence likely evolved from the twin presequences of an ancestral Lon1-like gene, generating a single dual-targeted protein isoform. We postulate that Lon1 and its subfunctional paralog Lon4 evolved complementary subsets of transcriptional and posttranscriptional regulatory components responsive to environmental cues for dual-organellar targeting.

Deciphering thylakoid sub-compartments using a mass spectrometry-based approach

Photosynthesis has shaped atmospheric and ocean chemistries and probably changed the climate as well, as oxygen is released from water as part of the photosynthetic process. In photosynthetic eukaryotes, this process occurs in the chloroplast, an organelle containing the most abundant biological membrane, the thylakoids. The thylakoids of plants and some green algae are structurally inhomogeneous, consisting of two main domains: the grana, which are piles of membranes gathered by stacking forces, and the stroma-lamellae, which are unstacked thylakoids connecting the grana. The major photosynthetic complexes are unevenly distributed within these compartments because of steric and electrostatic constraints. Although proteomic analysis of thylakoids has been instrumental to define its protein components, no extensive proteomic study of subthylakoid localization of proteins in the BBY (grana) and the stroma-lamellae fractions has been achieved so far. To fill this gap, we performed a complete survey of the protein composition of these thylakoid subcompartments using thylakoid membrane fractionations. We employed semiquantitative proteomics coupled with a data analysis pipeline and manual annotation to differentiate genuine BBY and stroma-lamellae proteins from possible contaminants. About 300 thylakoid (or potentially thylakoid) proteins were shown to be enriched in either the BBY or the stroma-lamellae fractions. Overall, present findings corroborate previous observations obtained for photosynthetic proteins that used nonproteomic approaches. The originality of the present proteomic relies in the identification of photosynthetic proteins whose differential distribution in the thylakoid subcompartments might explain already observed phenomenon such as LHCII docking. Besides, from the present localization results we can suggest new molecular actors for photosynthesis-linked activities. For instance, most PsbP-like subunits being differently localized in stroma-lamellae, these proteins could be linked to the PSI-NDH complex in the context of cyclic electron flow around PSI. In addition, we could identify about a hundred new likely minor thylakoid (or chloroplast) proteins, some of them being potential regulators of the chloroplast physiology.

New functions of the chloroplast Preprotein and Amino acid Transporter (PRAT) family members in protein import

Plant cells contain distinct compartments such as the nucleus, the endomembrane system comprising the endoplasmic reticulum and Golgi apparatus, peroxisomes, vacuoles, as well as mitochondria and chloroplasts. All of these compartments are surrounded by 1 or 2 limiting membranes and need to import proteins from the cytosol. Previous work led to the conclusion that mitochondria and chloroplasts use structurally different protein import machineries in their outer and inner membranes for the uptake of cytosolic precursor proteins. Our most recent data show that there is some unexpected overlap. Three members of the family of preprotein and amino acid transporters, PRAT, were identified in chloroplasts that mediate the uptake of transit sequence-less proteins into the inner plastid envelope membrane. By analogy, mitochondria contain with TIM22 a related PRAT protein that is involved in the import of transit sequence-less proteins into the inner mitochondrial membrane. Both mitochondria and chloroplasts thus make use of similar import mechanisms to deliver some of their proteins to their final place. Because single homologs of HP20- and HP30-like proteins are present in algae such as Chlamydomonas, Ostreococcus, and Volvox, which diverged from land plants approximately 1 billion years ago, it is likely that the discovered PRAT-mediated mechanism of protein translocation evolved concomitantly with the secondary endosymbiotic event that gave rise to green plants.

Robust feature generation for protein subchloroplast location prediction with a weighted GO transfer model

Chloroplasts are crucial organelles of green plants and eukaryotic algae since they conduct photosynthesis. Predicting the subchloroplast location of a protein can provide important insights for understanding its biological functions. The performance of subchloroplast location prediction algorithms often depends on deriving predictive and succinct features from genomic and proteomic data. In this work, a novel weighted Gene Ontology (GO) transfer model is proposed to generate discriminating features from sequence data and GO Categories. This model contains two components. First, we transfer the GO terms of the homologous protein, and then assign the bit-score as weights to GO features. Second, we employ term-selection methods to determine weights for GO terms. This model is capable of improving prediction accuracy due to the tolerance of the noise derived from homolog knowledge transfer. The proposed weighted GO transfer method based on bit-score and a logarithmic transformation of CHI-square (WS-LCHI) performs better than the baseline models, and also outperforms the four off-the-shelf subchloroplast prediction methods.

Understanding protein trafficking in plant cells through proteomics

The functions of approximately one-third of the proteins encoded by the Arabidopsis thaliana genome are completely unknown. Moreover, many annotations of the remainder of the genome supply tentative functions, at best. Knowing the ultimate localization of these proteins, as well as the pathways used for getting there, may provide clues as to their functions. The putative localization of most proteins currently relies on in silico-based bioinformatics approaches, which, unfortunately, often result in erroneous predictions. Emerging proteomics techniques coupled with other systems biology approaches now provide researchers with a plethora of methods for elucidating the final location of these proteins on a large scale, as well as the ability to dissect protein-sorting pathways in plants.

A new member of the psToc159 family contributes to distinct protein targeting pathways in pea chloroplasts

Protein import into chloroplasts relies on specific targeting of preproteins from the cytosol to the organelles and coordinated translocation processes across the double envelope membrane. Here, two complex machineries constitute the so called general import pathway, which consists of the TOC and TIC complexes (translocon at the outer envelope of chloroplasts and translocon at the inner envelope of chloroplasts, respectively). The majority of canonical preproteins feature an N-terminal cleavable transit peptide, which is necessary for targeting and recognition at the chloroplast surface by receptors of TOC, where Toc159 acts as the primary contact site. We identified a non-canonical preprotein without the classical transit peptide, the superoxide dismutase (FSD1), which was then used in chemical crosslinking approaches to find new interaction partners at the outer envelope from pea chloroplasts. In this way we could link FSD1 to members of the Toc159 family in pea, namely psToc132 and psToc120. Using deletion mutants as well as a peptide scanning approach we defined regions of the preprotein, which are involved in receptor binding. These are distributed across the entire sequence; however the extreme N-terminus as well as a C-proximal domain turned out to be essential for targeting and import. En route into the plastid FSD1 engages components of the general import pathway, implying that in spite of the non-canonical targeting information and recognition by a specific receptor this preprotein follows a similar way across the envelope as the majority of plastid preproteins.

Photosynthesis of Quercus suber is affected by atmospheric NH3 generated by multifunctional agrosystems

Montados are evergreen oak woodlands dominated by Quercus species, which are considered to be key to biodiversity conservation and ecosystem services. This ecosystem is often used for cattle breeding in most regions of the Iberian Peninsula, which causes plants to receive extra nitrogen as ammonia (NH(3)) through the atmosphere. The effect of this atmospheric NH(3) (NH(3atm)) on ecosystems is still under discussion. This study aimed to evaluate the effects of an NH(3atm) concentration gradient downwind of a cattle barn in a Montado area. Leaves from the selected Quercus suber L. trees along the gradient showed a clear influence of the NH(3) on δ(13)C, as a consequence of a strong limitation on the photosynthetic machinery by a reduction of both stomatal and mesophyll conductance. A detailed study of the impact of NH(3atm) on the photosynthetic performance of Q. suber trees is presented, and new mechanisms by which NH(3) affects photosynthesis at the leaf level are suggested.

Three proteins mediate import of transit sequence-less precursors into the inner envelope of chloroplasts in Arabidopsis thaliana

A family of 17 putative preprotein and amino acid transporters designated PRAT has been identified in Arabidopsis thaliana, comprising PRAT proteins in mitochondria and chloroplasts. Although some PRAT proteins, such as the translocon of the mitochondrial inner membrane (TIM) proteins TIM22 and TIM23, play decisive roles for the translocation and import of mitochondrial inner membrane proteins, little is known about the role of the different PRAT members in chloroplasts. Here we report the identification of three distinct PRAT proteins as part of a unique protein import site. One of the identified PRAT proteins is identical with a previously characterized hypothetical protein (HP) of 20 kDa designated HP20 of the outer plastid envelope membrane. The second PRAT component is represented by HP30, and the third is identical to HP30-2, a close relative of HP30. Both HP30 and HP30-2 are inner plastid envelope membrane proteins of chloroplasts. Using biochemical, cell biological, and genetic approaches we demonstrate that all three PRAT proteins cooperate during import of transit sequence-less proteins, such as the quinone oxidoreductase homolog ceQORH used as model, into the inner chloroplast envelope membrane. Our data are reminiscent of findings reported for the TIM22 translocase, which is involved in the import of carrier proteins and other, hydrophobic membrane proteins lacking cleavable transit sequences into the inner mitochondrial membrane. Together our results establish the PRAT family as a widely used system of protein translocases in different membranes of endosymbiotic origin.

Analysis of essential Arabidopsis nuclear genes encoding plastid-targeted proteins

The Chloroplast 2010 Project (http://www.plastid.msu.edu/) identified and phenotypically characterized homozygous mutants in over three thousand genes, the majority of which encode plastid-targeted proteins. Despite extensive screening by the community, no homozygous mutant alleles were available for several hundred genes, suggesting that these might be enriched for genes of essential function. Attempts were made to generate homozygotes in ~1200 of these lines and 521 of the homozygous viable lines obtained were deposited in the Arabidopsis Biological Resource Center (http://abrc.osu.edu/). Lines that did not yield a homozygote in soil were tested as potentially homozygous lethal due to defects either in seed or seedling development. Mutants were characterized at four stages of development: developing seed, mature seed, at germination, and developing seedlings. To distinguish seed development or seed pigment-defective mutants from seedling development mutants, development of seeds was assayed in siliques from heterozygous plants. Segregating seeds from heterozygous parents were sown on supplemented media in an attempt to rescue homozygous seedlings that could not germinate or survive in soil. Growth of segregating seeds in air and air enriched to 0.3% carbon dioxide was compared to discover mutants potentially impaired in photorespiration or otherwise responsive to CO2 supplementation. Chlorophyll fluorescence measurements identified CO2-responsive mutants with altered photosynthetic parameters. Examples of genes with a viable mutant allele and one or more putative homozygous-lethal alleles were documented. RT-PCR of homozygotes for potentially weak alleles revealed that essential genes may remain undiscovered because of the lack of a true null mutant allele. This work revealed 33 genes with two or more lethal alleles and 73 genes whose essentiality was not confirmed with an independent lethal mutation, although in some cases second leaky alleles were identified.

Proteome changes in wild and modern wheat leaves upon drought stress by two-dimensional electrophoresis and nanoLC-ESI-MS/MS

To elucidate differentially expressed proteins and to further understand post-translational modifications of transcripts, full leaf proteome profiles of two wild emmer (Triticum turgidum ssp. dicoccoides TR39477 and TTD22) and one modern durum wheat (Triticum turgidum ssp. durum cv. Kızıltan) genotypes were compared upon 9-day drought stress using two-dimensional gel electrophoresis and nano-scale liquid chromatographic electrospray ionization tandem mass spectrometry methods. The three genotypes compared exhibit distinctive physiological responses to drought as previously shown by our group. Results demonstrated that many of the proteins were common in both wild emmer and modern wheat proteomes; of which, 75 were detected as differentially expressed proteins. Several proteins identified in all proteomes exhibited drought regulated patterns of expression. A number of proteins were observed with higher expression levels in response to drought in wild genotypes compared to their modern relative. Eleven protein spots with low peptide matches were identified as candidate unique drought responsive proteins. Of the differentially expressed proteins, four were selected and further analyzed by quantitative real-time PCR at the transcriptome level to compare with the proteomic data. The present study provides protein level differences in response to drought in modern and wild genotypes of wheat that may account for the differences of the overall responses of these genotypes to drought. Such comparative proteomics analyses may aid in the better understanding of complex drought response and may suggest candidate genes for molecular breeding studies to improve tolerance against drought stress and, thus, to enhance yields.

Identification of CP12 as a Novel Calcium-Binding Protein in Chloroplasts

Calcium plays an important role in the regulation of several chloroplast processes. However, very little is still understood about the calcium fluxes or calcium-binding proteins present in plastids. Indeed, classical EF-hand containing calcium-binding proteins appears to be mostly absent from plastids. In the present study we analyzed the stroma fraction of Arabidopsis chloroplasts for the presence of novel calcium-binding proteins using 2D-PAGE separation followed by calcium overlay assay. A small acidic protein was identified by mass spectrometry analyses as the chloroplast protein CP12 and the ability of CP12 to bind calcium was confirmed with recombinant proteins. CP12 plays an important role in the regulation of the Calvin-Benson-Bassham Cycle participating in the assembly of a supramolecular complex between phosphoribulokinase and glyceraldehyde 3-phosphate dehydrogenase, indicating that calcium signaling could play a role in regulating carbon fixation.

Evolution of chloroplast J proteins

Hsp70 chaperones are involved in multiple biological processes and are recruited to specific processes by designated J domain-containing cochaperones, or J proteins. To understand the evolution and functions of chloroplast Hsp70s and J proteins, we identified the Arabidopsis chloroplast J protein constituency using a combination of genomic and proteomic database searches and individual protein import assays. We show that Arabidopsis chloroplasts have at least 19 J proteins, the highest number of confirmed J proteins for any organelle. These 19 J proteins are classified into 11 clades, for which cyanobacteria and glaucophytes only have homologs for one clade, green algae have an additional three clades, and all the other 7 clades are specific to land plants. Each clade also possesses a clade-specific novel motif that is likely used to interact with different client proteins. Gene expression analyses indicate that most land plant-specific J proteins show highly variable expression in different tissues and are down regulated by low temperatures. These results show that duplication of chloroplast Hsp70 in land plants is accompanied by more than doubling of the number of its J protein cochaperones through adding new J proteins with novel motifs, not through duplications within existing families. These new J proteins likely recruit chloroplast Hsp70 to perform tissue specific functions related to biosynthesis rather than to stress resistance.

The 'protein complex proteome' of chloroplasts in Arabidopsis thaliana

Here, a first GelMap of the chloroplast "protein complex proteome" of Arabidopsis thaliana is presented. The GelMap software tool allows assigning multiple proteins to gel spots, thereby taking advantage of the high sensitivity of state-of-the-art mass spectrometry systems. Furthermore, the software allows functional annotation of all identified proteins. If applied to a 2D blue native (BN)/SDS gel, GelMap can selectively display protein complexes of low abundance. For the chloroplast GelMap, highly purified organelles were separated by 2D BN/SDS PAGE and spots were automatically detected using Delta 2D software. Within 287 spots, a total of 1841 proteins were identified (on average 6.4 proteins per spot), representing a set of 436 non redundant proteins. Most of these proteins form part of protein complexes. The quality of the map is reflected by its inclusion of a more or less complete set of protein complexes described for chloroplasts in the literature. The GelMap is publically available at www.gelmap.de/arabidopsis-chloro and may be used as a resource for identifying novel protein complexes within any of its functional categories.

BIOLOGICAL SIGNIFICANCE

The chloroplast GelMap represents a data resource for the definition of protein complexes in the model plant A. thaliana. It should be useful for in depth understanding of chloroplast biochemistry, as illustrated by the discovery of so far unknown protein complexes. The GelMap is publically available at www.gelmap.de/arabidopsis-chloro.

Functional Redundancy and Divergence within the Arabidopsis RETICULATA-RELATED Gene Family

A number of Arabidopsis (Arabidopsis thaliana) mutants exhibit leaf reticulation, having green veins that stand out against paler interveinal tissues, fewer cells in the interveinal mesophyll, and normal perivascular bundle sheath cells. Here, to examine the basis of leaf reticulation, we analyzed the Arabidopsis RETICULATA-RELATED (RER) gene family, several members of which cause leaf reticulation when mutated. Although transcripts of RE, RER1, and RER3 were mainly detected in the bundle sheath cells of expanded leaves, functional RER3:GREEN FLUORESCENT PROTEIN was visualized in the chloroplast membranes of all photosynthetic cells. Leaf reticulation in the re and rer3 loss-of-function mutants occurred, along with accumulation of reactive oxygen species, in a photoperiod-dependent manner. A comparison of re and rer3 leaf messenger RNA expression profiles showed more than 200 genes were similarly misexpressed in both mutants. In addition, metabolic profiles of mature leaves revealed that several biosynthetic pathways downstream of pyruvate are altered in re and rer3. Double mutant analysis showed that only re rer1 and rer5 rer6 exhibited synergistic phenotypes, indicating functional redundancy. The redundancy between RE and its closest paralog, RER1, was confirmed by overexpressing RER1 in re mutants, which partially suppressed leaf reticulation. Our results show that RER family members can be divided into four functional modules with divergent functions. Moreover, these results provide insights into the origin of the reticulated phenotype, suggesting that the RER proteins functionally interconnect photoperiodic growth, amino acid homeostasis, and reactive oxygen species metabolism during Arabidopsis leaf growth.

Assessment of the requirement for aquaporins in the thylakoid membrane of plant chloroplasts to sustain photosynthetic water oxidation

Oxygenic photosynthetic organisms use sunlight energy to oxidize water to molecular oxygen. This process is mediated by the photosystem II complex at the lumenal side of the thylakoid membrane. Most research efforts have been dedicated to understanding the mechanism behind the unique water oxidation reactions, whereas the delivery pathways for water molecules into the thylakoid lumen have not yet been studied. The most common mechanisms for water transport are simple diffusion and diffusion facilitated by specialized channel proteins named aquaporins. Calculations using published data for plant chloroplasts indicate that aquaporins are not necessary to sustain water supply into the thylakoid lumen at steady state photosynthetic rates. Yet, arguments for their presence in the plant thylakoid membrane and beneficial action are presented.

The wheat chloroplastic proteome

With the availability of plant genome sequencing, analysis of plant proteins with mass spectrometry has become promising and admired. Determining the proteome of a cell is still a challenging assignment, which is convoluted by proteome dynamics and convolution. Chloroplast is fastidious curiosity for plant biologists due to their intricate biochemical pathways for indispensable metabolite functions. In this review, an overview on proteomic studies conducted in wheat with a special focus on subcellular proteomics of chloroplast, salt and water stress. In recent years, we and other groups have attempted to understand the photosynthesis in wheat and abiotic stress under salt imposed and water deficit during vegetative stage. Those studies provide interesting results leading to better understanding of the photosynthesis and identifying the stress-responsive proteins. Indeed, recent studies aimed at resolving the photosynthesis pathway in wheat. Proteomic analysis combining two complementary approaches such as 2-DE and shotgun methods couple to high through put mass spectrometry (LTQ-FTICR and MALDI-TOF/TOF) in order to better understand the responsible proteins in photosynthesis and abiotic stress (salt and water) in wheat chloroplast will be focused.

BIOLOGICAL SIGNIFICANCE

In this review we discussed the identification of the most abundant protein in wheat chloroplast and stress-responsive under salt and water stress in chloroplast of wheat seedlings, thus providing the proteomic view of the events during the development of this seedling under stress conditions. Chloroplast is fastidious curiosity for plant biologists due to their intricate biochemical pathways for indispensable metabolite functions. An overview on proteomic studies conducted in wheat with a special focus on subcellular proteomics of chloroplast, salt and water stress. We have attempted to understand the photosynthesis in wheat and abiotic stress under salt imposed and water deficit during seedling stage. Those studies provide interesting results leading to a better understanding of the photosynthesis and identifying the stress-responsive proteins. In reality, our studies aspired at resolving the photosynthesis pathway in wheat. Proteomic analysis united two complementary approaches such as Tricine SDS-PAGE and 2-DE methods couple to high through put mass spectrometry (LTQ-FTICR and MALDI-TOF/TOF) in order to better understand the responsible proteins in photosynthesis and abiotic stress (salt and water) in wheat chloroplast will be highlighted. This article is part of a Special Issue entitled: Translational Plant Proteomics.

Complementary biochemical approaches applied to the identification of plastidial calmodulin-binding proteins

Ca(2+)/Calmodulin (CaM)-dependent signaling pathways play a major role in the modulation of cell responses in eukaryotes. In the chloroplast, few proteins such as the NAD(+) kinase 2 have been previously shown to interact with CaM, but a general picture of the role of Ca(2+)/CaM signaling in this organelle is still lacking. Using CaM-affinity chromatography and mass spectrometry, we identified 210 candidate CaM-binding proteins from different Arabidopsis and spinach chloroplast sub-fractions. A subset of these proteins was validated by an optimized in vitro CaM-binding assay. In addition, we designed two fluorescence anisotropy assays to quantitatively characterize the binding parameters and applied those assays to NAD(+) kinase 2 and selected candidate proteins. On the basis of our results, there might be many more plastidial CaM-binding proteins than previously estimated. In addition, we showed that an array of complementary biochemical techniques is necessary in order to characterize the mode of interaction of candidate proteins with CaM.

Identification of a dual-targeted protein belonging to the mitochondrial carrier family that is required for early leaf development in rice

A dual-targeted protein belonging to the mitochondrial carrier family was characterized in rice (Oryza sativa) and designated 3'-Phosphoadenosine 5'-Phosphosulfate Transporter1 (PAPST1). The papst1 mutant plants showed a defect in thylakoid development, resulting in leaf chlorosis at an early leaf developmental stage, while normal leaf development was restored 4 to 6 d after leaf emergence. OsPAPST1 is highly expressed in young leaves and roots, while the expression is reduced in mature leaves, in line with the recovery of chloroplast development seen in the older leaves of papst1 mutant plants. OsPAPST1 is located on the outer mitochondrial membrane and chloroplast envelope. Whole-genome transcriptomic analysis reveals reduced expression of genes encoding photosynthetic components (light reactions) in papst1 mutant plants. In addition, sulfur metabolism is also perturbed in papst1 plants, and it was seen that PAPST1 can act as a nucleotide transporter when expressed in Escherichia coli that can be inhibited significantly by 3'-phosphoadenosine 5'-phosphosulfate. Given these findings, together with the altered phenotype seen only when leaves are first exposed to light, it is proposed that PAPST1 may act as a 3'-phosphoadenosine 5'-phosphosulfate carrier that has been shown to act as a retrograde signal between chloroplasts and the nucleus.

CHAPERONIN 20 mediates iron superoxide dismutase (FeSOD) activity independent of its co-chaperonin role in Arabidopsis chloroplasts

Iron superoxide dismutases (FeSODs; FSDs) are primary antioxidant enzymes in Arabidopsis thaliana chloroplasts. The stromal FSD1 conferred the only detectable FeSOD activity, whereas the thylakoid membrane- and nucleoid-co-localized FSD2 and FSD3 double mutant showed arrested chloroplast development. FeSOD requires cofactor Fe for its activity, but its mechanism of activation is unclear. We used reversed-phase high-performance liquid chromatography (HPLC), gel filtration chromatography, LC-MS/MS, protoplast transient expression and virus-induced gene silencing (VIGS) analyses to identify and characterize a factor involved in FeSOD activation. We identified the chloroplast-localized co-chaperonin CHAPERONIN 20 (CPN20) as a mediator of FeSOD activation by direct interaction. The relationship between CPN20 and FeSOD was confirmed by in vitro experiments showing that CPN20 alone could enhance FSD1, FSD2 and FSD3 activity. The in vivo results showed that CPN20-overexpressing mutants and mutants with defective co-chaperonin activity increased FSD1 activity, without changing the chaperonin CPN60 protein level, and VIGS-induced downregulation of CPN20 also led to decreased FeSOD activity. Our findings reveal that CPN20 can mediate FeSOD activation in chloroplasts, a role independent of its known function in the chaperonin system.

Modulation of the light-harvesting chlorophyll antenna size in Chlamydomonas reinhardtii by TLA1 gene over-expression and RNA interference

Truncated light-harvesting antenna 1 (TLA1) is a nuclear gene proposed to regulate the chlorophyll (Chl) antenna size in Chlamydomonas reinhardtii. The Chl antenna size of the photosystems and the chloroplast ultrastructure were manipulated upon TLA1 gene over-expression and RNAi downregulation. The TLA1 over-expressing lines possessed a larger chlorophyll antenna size for both photosystems and contained greater levels of Chl b per cell relative to the wild type. Conversely, TLA1 RNAi transformants had a smaller Chl antenna size for both photosystems and lower levels of Chl b per cell. Western blot analyses of the TLA1 over-expressing and RNAi transformants showed that modulation of TLA1 gene expression was paralleled by modulation in the expression of light-harvesting protein, reaction centre D1 and D2, and VIPP1 genes. Transmission electron microscopy showed that modulation of TLA1 gene expression impacts the organization of thylakoid membranes in the chloroplast. Over-expressing lines showed well-defined grana, whereas RNAi transformants possessed loosely held together and more stroma-exposed thylakoids. Cell fractionation suggested localization of the TLA1 protein in the inner chloroplast envelope and potentially in association with nascent thylakoid membranes, indicating a role in Chl antenna assembly and thylakoid membrane biogenesis. The results provide a mechanistic understanding of the Chl antenna size regulation by the TLA1 gene.

The role of calcium in chloroplasts--an intriguing and unresolved puzzle

More than 70 years of studies have indicated that chloroplasts contain a significant amount of calcium, are a potential storage compartment for this ion, and might themselves be prone to calcium regulation. Many of these studies have been performed on the photosynthetic light reaction as well as CO(2) fixation via the Calvin-Benson-Bassham cycle, and they showed that calcium is required in several steps of these processes. Further studies have indicated that calcium is involved in other chloroplast functions that are not directly related to photosynthesis and that there is a calcium-dependent regulation similar to cytoplasmic calcium signal transduction. Nevertheless, the precise role that calcium has as a functional and regulatory component of chloroplast processes remains enigmatic. Calcium concentrations in different chloroplast subcompartments have been measured, but the extent and direction of intra-plastidal calcium fluxes or calcium transport into and from the cytosol are not yet very well understood. In this review we want to give an overview over the current knowledge on the relationship between chloroplasts and calcium and discuss questions that need to be addressed in future research.

The involvement of Arabidopsis glutathione peroxidase 8 in the suppression of oxidative damage in the nucleus and cytosol

A family of eight genes with homology to mammalian glutathione peroxidase (GPX) isoenzymes, designated AtGPX1-AtGPX8, has been identified in Arabidopsis thaliana. In this study we demonstrated the functional analysis of Arabidopsis AtGPX8 with peroxidase activity toward H(2)O(2) and lipid hydroperoxides using thioredoxin as an electron donor. The transcript and protein levels of AtGPX8 in Arabidopsis were up-regulated coordinately in response to oxidative damage caused by high-light (HL) stress or treatment with paraquat (PQ). Furthermore, the knockout Arabidopsis mutants of AtGPX8 (KO-gpx8) exhibited increased sensitivity to oxidative damage caused by PQ treatment in root elongation compared with the wild-type plants. In contrast, transgenic lines overexpressing AtGPX8 (Ox-AtGPX8) were less sensitive to oxidative damage than the wild-type plants. The levels of oxidized proteins in the KO-gpx8 and Ox-AtGPX8 lines were enhanced and suppressed, respectively, compared with the wild-type plants under HL stress or PQ treatment. The fusion protein of AtGPX8 tagged with green fluorescent protein was localized in the cytosol and nucleus of onion epidermal cells. In addition, the AtGPX8 protein was detected in the cytosolic and nuclear fractions prepared from leaves of Arabidopsis plants using the AtGPX8 antibody. Oxidative DNA damage under treatment with PQ increased in the wild-type and KO-gpx8 plants, while it decreased in the OX-AtGPX8 plants. These results suggest that AtGPX8 plays an important role in the protection of cellular components including nuclear DNA against oxidative stress.

Components of mitochondrial oxidative phosphorylation vary in abundance following exposure to cold and chemical stresses

Plant mitochondria are highly responsive organelles that vary their metabolism in response to a wide range of chemical and environmental conditions. Quantitative proteomics studies have begun to allow the analysis of these large-scale protein changes in mitochondria. However studies of the integral membrane proteome of plant mitochondria, arguably the site responsible for the most fundamental mitochondrial processes of oxidative phosphorylation, protein import and metabolite transport, remain a technical challenge. Here we have investigated the changes in protein abundance in response to a number of chemical stresses and cold. In addition to refining the subcellular localization of 66 proteins, we have been able to characterize 596 protein × treatment combinations following a range of stresses. To date it has been assumed that the main mitochondrial response to stress involved the induction of alternative respiratory proteins such as AOX, UCPs, and alternative NAD(P)H dehydrogenases; we now provide evidence for a number of very specific protein abundance changes that have not been highlighted previously by transcript studies. This includes both previously characterized stress responsive proteins as well as major components of oxidative phosphorylation, protein import/export, and metabolite transport.

TGD4 involved in endoplasmic reticulum-to-chloroplast lipid trafficking is a phosphatidic acid binding protein

The synthesis of galactoglycerolipids, which are prevalent in photosynthetic membranes, involves enzymes at the endoplasmic reticulum (ER) and the chloroplast envelope membranes. Genetic analysis of trigalactosyldiacylglycerol (TGD) proteins in Arabidopsis has demonstrated their role in polar lipid transfer from the ER to the chloroplast. The TGD1, 2, and 3 proteins resemble components of a bacterial-type ATP-binding cassette (ABC) transporter, with TGD1 representing the permease, TGD2 the substrate binding protein, and TGD3 the ATPase. However, the function of the TGD4 protein in this process is less clear and its location in plant cells remains to be firmly determined. The predicted C-terminal β-barrel structure of TGD4 is weakly similar to proteins of the outer cell membrane of Gram-negative bacteria. Here, we show that, like TGD2, the TGD4 protein when fused to DsRED specifically binds phosphatidic acid (PtdOH). As previously shown for tgd1 mutants, tgd4 mutants have elevated PtdOH content, probably in extraplastidic membranes. Using highly purified and specific antibodies to probe different cell fractions, we demonstrated that the TGD4 protein was present in the outer envelope membrane of chloroplasts, where it appeared to be deeply buried within the membrane except for the N-terminus, which was found to be exposed to the cytosol. It is proposed that TGD4 is either directly involved in the transfer of polar lipids, possibly PtdOH, from the ER to the outer chloroplast envelope membrane or in the transfer of PtdOH through the outer envelope membrane.

FtsH proteases located in the plant chloroplast

FtsHs are a well-characterized family of membrane bound proteases containing an AAA (ATPase associated with various cellular activities) and a Zn(2+) metalloprotease domain. FtsH proteases are found in eubacteria, animals and plants and are known to have a crucial role in housekeeping proteolysis of membrane proteins. In Arabidopsis thaliana, 12 FtsH family members are present (FtsH 1-12) and their subcellular localization is restricted to mitochondria and chloroplasts. In addition, five genes coding for proteins homologous to FtsH (FtsHi 1-5) have been detected in the genome, lacking the conserved zinc-binding motif HEXXH, which presumably renders them inactive for proteolysis. These inactive FtsHs as well as nine of the active FtsHs are thought to be localized in the chloroplast. In this article, we shortly summarize the recent findings on plastidic FtsH proteases in text and figures. We will mainly focus on FtsH 1, 2, 5 and 8 localized in the thylakoid membrane and known for their importance in photosynthesis.

Membrane topology and predicted RNA-binding function of the 'early responsive to dehydration (ERD4)' plant protein

Functional annotation of uncharacterized genes is the main focus of computational methods in the post genomic era. These tools search for similarity between proteins on the premise that those sharing sequence or structural motifs usually perform related functions, and are thus particularly useful for membrane proteins. Early responsive to dehydration (ERD) genes are rapidly induced in response to dehydration stress in a variety of plant species. In the present work we characterized function of Brassica juncea ERD4 gene using computational approaches. The ERD4 protein of unknown function possesses ubiquitous DUF221 domain (residues 312-634) and is conserved in all plant species. We suggest that the protein is localized in chloroplast membrane with at least nine transmembrane helices. We detected a globular domain of 165 amino acid residues (183-347) in plant ERD4 proteins and expect this to be posited inside the chloroplast. The structural-functional annotation of the globular domain was arrived at using fold recognition methods, which suggested in its sequence presence of two tandem RNA-recognition motif (RRM) domains each folded into βαββαβ topology. The structure based sequence alignment with the known RNA-binding proteins revealed conservation of two non-canonical ribonucleoprotein sub-motifs in both the putative RNA-recognition domains of the ERD4 protein. The function of highly conserved ERD4 protein may thus be associated with its RNA-binding ability during the stress response. This is the first functional annotation of ERD4 family of proteins that can be useful in designing experiments to unravel crucial aspects of stress tolerance mechanism.