Separation and characterization of inner and outer envelope membranes of pea chloroplasts

A novel in-situ-process technique constructs whole circular cpDNA library

BACKGROUND

The chloroplast genome (cp genome) is directly related to the study and analysis of molecular phylogeny and evolution of plants in the phylogenomics era. The cp genome, whereas, is highly plastic and exists as a heterogeneous mixture of sizes and physical conformations. It is advantageous to purify/enrich the circular chloroplast DNA (cpDNA) to reduce sequence complexity in cp genome research. Large-insert, ordered DNA libraries are more practical for genomics research than conventional, unordered ones. From this, a technique of constructing the ordered BAC library with the goal-insert cpDNA fragment is developed in this paper.

RESULTS

This novel in-situ-process technique will efficiently extract circular cpDNA from crops and construct a high-quality cpDNA library. The protocol combines the in-situ chloroplast lysis for the high-purity circular cpDNA with the in-situ substitute/ligation for the high-quality cpDNA library. Individually, a series of original buffers/solutions and optimized procedures for chloroplast lysis in-situ is different than bacterial lysis in-situ; the in-situ substitute/ligation that reacts on the MCE membrane is suitable for constructing the goal-insert, ordered cpDNA library while preventing the large-insert cpDNA fragment breakage. The goal-insert, ordered cpDNA library is arrayed on the microtiter plate by three colonies with the definite cpDNA fragment that are homologous-corresponds to the whole circular cpDNA of the chloroplast.

CONCLUSION

The novel in-situ-process technique amply furtherance of research in genome-wide functional analysis and characterization of chloroplasts, such as genome sequencing, bioinformatics analysis, cloning, physical mapping, molecular phylogeny and evolution.

Purification of Chloroplast Envelope, Thylakoids, and Stroma from Angiosperm Leaves

Plant cell chloroplasts are bounded by a two-membrane envelope. Their photosynthetic function is based on the development of an operational large internal membrane network, called the thylakoids, and on enzymatic processes present in the chloroplast matrix, called the stroma. Thylakoid membranes are distinct from the chloroplast envelope, and their biogenesis is dependent on biosynthetic and transport activities specific of the chloroplast envelope. Starting with the isolation of intact chloroplasts, the method presents the separation by differential centrifugation of the three compartments. A protocol is detailed for leaves of spinach, Arabidopsis or pea.

Isolation of the Inner and Outer Membranes of the Chloroplast Envelope from Angiosperms

The outer and the inner membranes of the chloroplast envelope, also called OEM and IEM, have distinct lipid and protein compositions. They host molecular systems involved in the biogenesis of the organelle, its cellular function, and its communication with other compartments. Here we describe a method for the isolation of these two membranes starting from intact chloroplast preparations, with two alternative procedures based on the starting material. One was developed from spinach leaves, the other from pea leaves. The two procedures differ in the method used to isolate and rupture chloroplasts and separate each membrane.

Mechanisms of temperature acclimatisation in the psychrotolerant green alga Coccomyxa subellipsoidea C-169 (Trebouxiophyceae)

Despite the interest in different temperature acclimatisations of higher plants, few studies have considered the mechanisms that allow psychrotolerant microalgae to live in a cold environment. Although the analysis of the genomes of some algae revealed the presence of specific genes that encode enzymes that can be involved in the response to stress, this area has not been explored deeply. This work aims to clarify the acclimatisation mechanisms that enable the psychrotolerant green alga Coccomyxa subellipsoidea C-169 to grow in a broad temperature spectrum. The contents of various biochemical compounds in cells, the lipid composition of the biological membranes of entire cells, and the thylakoid fraction as well as the electron transport rate and PSII efficiency were investigated. The results demonstrate an acclimatisation mechanism that is specific for C. subellipsoidea and that allows the maintenance of appropriate membrane fluidity, for example, in thylakoid membranes. It is achieved almost exclusively by changes within the unsaturated fatty acid pool, like changes from C18:2 into C18:3 and C16:2 into C16:3 or vice versa. This ensures, for example, an effective transport rate through PSII and in consequence a maximum quantum yield of it in cells growing at different temperatures. Furthermore, reactions characteristic for both psychrotolerant and mesophilic microalgae, involving the accumulation of lipids and soluble sugars in cells at temperatures other than optimal, were observed. These findings add substantially to our understanding of the acclimatisation of psychrotolerant organisms to a wide range of temperatures and prove that this process could be accomplished in a species-specific manner.

Distinctly localized lipid phosphate phosphatases mediate endoplasmic reticulum glycerolipid metabolism in Arabidopsis

Inter-organelle communication is an integral subcellular process in cellular homeostasis. In plants, cellular membrane lipids are synthesized in the plastids and endoplasmic reticulum (ER). However, the crosstalk between these organelles in lipid biosynthesis remains largely unknown. Here, we show that a pair of lipid phosphate phosphatases (LPPs) with differential subcellular localizations is required for ER glycerolipid metabolism in Arabidopsis (Arabidopsis thaliana). LPPα2 and LPPε1, which function as phosphatidic acid phosphatases and thus catalyze the core reaction in glycerolipid metabolism, were differentially localized at ER and chloroplast outer envelopes despite their similar tissue expression pattern. No mutant phenotype was observed in single knockout mutants; however, genetic suppression of these LPPs affected pollen growth and ER phospholipid biosynthesis in mature siliques and seeds with compromised triacylglycerol biosynthesis. Although chloroplast-localized, LPPε1 was localized close to the ER and ER-localized LPPα2. This proximal localization is functionally relevant, because overexpression of chloroplastic LPPε1 enhanced ER phospholipid and triacylglycerol biosynthesis similar to the effect of LPPα2 overexpression in mature siliques and seeds. Thus, ER glycerolipid metabolism requires a chloroplast-localized enzyme in Arabidopsis, representing the importance of inter-organelle communication in membrane lipid homeostasis.

Electron transport in cyanobacterial thylakoid membranes: Are cyanobacteria simple models for photosynthetic organisms?

Cyanobacteria are structurally the simplest oxygenic phototrophs, which makes it difficult to understand the regulation of photosynthesis because the photosynthetic and respiratory processes share the same thylakoid membranes and cytosolic space. This review aimed to summarise the molecular mechanisms and in vivo activities of electron transport in cyanobacterial thylakoid membranes based on the latest progress in photosynthesis research in cyanobacteria. Photosynthetic linear electron transport for CO2 assimilation has the dominant electron flux in the thylakoid membranes. The capacity of O2 photoreduction in cyanobacteria is comparable to the photosynthetic CO2 assimilation, which is mediated by flavodiiron proteins. Additionally, cyanobacterial thylakoid membranes harbour the significant electron flux of respiratory electron transport through a homologue of respiratory complex I, which is also recognized as the part of cyclic electron transport chain if it is coupled with photosystem I in the light. Further, O2-independent alternative electron transports through hydrogenase and nitrate reductase function with reduced ferredoxin as the electron donor. Whereas all these electron transports are recently being understood one by one, the complexity as the whole regulatory system remains to be uncovered in near future.

Plant transbilayer lipid asymmetry and the role of lipid flippases

Many biological membranes present an asymmetric lipid distribution between the two leaflets that is known as the transbilayer lipid asymmetry. This asymmetry is essential for cell survival and its loss is related to apoptosis. In mammalian and yeast cells, ATP-dependent transport of lipids to the cytosolic side of the biological membranes, carried out by so-called lipid flippases, contributes to the transbilayer lipid asymmetry. Most of these lipid flippases belong to the P4-ATPase protein family, which is also present in plants. In this review, we summarize the relatively scarce literature concerning the presence of transbilayer lipid asymmetry in different plant cell membranes and revise the potential role of lipid flippases of the P4-ATPase family in generation and/or maintenance of this asymmetry.

Coexpressed subunits of dual genetic origin define a conserved supercomplex mediating essential protein import into chloroplasts

In photosynthetic eukaryotes, thousands of proteins are translated in the cytosol and imported into the chloroplast through the concerted action of two translocons-termed TOC and TIC-located in the outer and inner membranes of the chloroplast envelope, respectively. The degree to which the molecular composition of the TOC and TIC complexes is conserved over phylogenetic distances has remained controversial. Here, we combine transcriptomic, biochemical, and genetic tools in the green alga Chlamydomonas (Chlamydomonas reinhardtii) to demonstrate that, despite a lack of evident sequence conservation for some of its components, the algal TIC complex mirrors the molecular composition of a TIC complex from Arabidopsis thaliana. The Chlamydomonas TIC complex contains three nuclear-encoded subunits, Tic20, Tic56, and Tic100, and one chloroplast-encoded subunit, Tic214, and interacts with the TOC complex, as well as with several uncharacterized proteins to form a stable supercomplex (TIC-TOC), indicating that protein import across both envelope membranes is mechanistically coupled. Expression of the nuclear and chloroplast genes encoding both known and uncharacterized TIC-TOC components is highly coordinated, suggesting that a mechanism for regulating its biogenesis across compartmental boundaries must exist. Conditional repression of Tic214, the only chloroplast-encoded subunit in the TIC-TOC complex, impairs the import of chloroplast proteins with essential roles in chloroplast ribosome biogenesis and protein folding and induces a pleiotropic stress response, including several proteins involved in the chloroplast unfolded protein response. These findings underscore the functional importance of the TIC-TOC supercomplex in maintaining chloroplast proteostasis.

H2O2 Induces Association of RCA with the Thylakoid Membrane to Enhance Resistance of Oryza meyeriana to Xanthomonas oryzae pv. oryzae

Oryza meyeriana is a wild species of rice with high resistance to Xanthomonas oryzae pv. oryzae (Xoo), but the detailed resistance mechanism is unclear. Ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) activase (RCA) is an important enzyme that regulates photosynthesis by activating Rubisco. We have previously reported that Xoo infection induced the relocation of RCA from the chloroplast stroma to the thylakoid membrane in O. meyeriana, but the underlying regulating mechanism and physiological significance of this association remains unknown. In this study, "H₂O₂ burst" with rapid and large increase in the amount of H₂O₂ was found to be induced by Xoo invasion in the leaves of O. meyeriana. 3, 3-diaminobenzidine (DAB) and oxidative 2, 7-Dichlorodi-hydrofluorescein diacetate (H₂DCFDA) staining experiments both showed that H₂O₂ was generated in the chloroplast of O. meyeriana, and that this H₂O₂ generation as well as Xoo resistance of the wild rice were dramatically dependent on light. H₂O₂, methyl viologen with light, and the xanthine-xanthine oxidase system all induced RCA to associate with the thylakoid membrane in vitro, which showed that H₂O₂ could induce the relocation of RCA. In vitro experiments also showed that H₂O₂ induced changes in both the RCA and thylakoid membrane that were required for them to associate and that this association only occurred in O. meyeriana and not in the susceptible cultivated rice. These results suggest that the association of RCA with the thylakoid membrane helps to protect the thylakoid membrane against oxidative damage from H₂O₂. Therefore, in addition to its universal function of activating Rubisco, RCA appears to play a novel role in the resistance of O. meyeriana to Xoo.

Arabidopsis DGD1 SUPPRESSOR1 Is a Subunit of the Mitochondrial Contact Site and Cristae Organizing System and Affects Mitochondrial Biogenesis

Mitochondrial and plastid biogenesis requires the biosynthesis and assembly of proteins, nucleic acids, and lipids. In Arabidopsis (Arabidopsis thaliana), the mitochondrial outer membrane protein DGD1 SUPPRESSOR1 (DGS1) is part of a large multi-subunit protein complex that contains the mitochondrial contact site and cristae organizing system 60-kD subunit, the translocase of outer mitochondrial membrane 40-kD subunit (TOM40), the TOM20s, and the Rieske FeS protein. A point mutation in DGS1, dgs1-1, altered the stability and protease accessibility of this complex. This altered mitochondrial biogenesis, mitochondrial size, lipid content and composition, protein import, and respiratory capacity. Whole plant physiology was affected in the dgs1-1 mutant as evidenced by tolerance to imposed drought stress and altered transcriptional responses of markers of mitochondrial retrograde signaling. Putative orthologs of Arabidopsis DGS1 are conserved in eukaryotes, including the Nuclear Control of ATP Synthase2 (NCA2) protein in yeast (Saccharomyces cerevisiae), but lost in Metazoa. The genes encoding DGS1 and NCA2 are part of a similar coexpression network including genes encoding proteins involved in mitochondrial fission, morphology, and lipid homeostasis. Thus, DGS1 links mitochondrial protein and lipid import with cellular lipid homeostasis and whole plant stress responses.

Isolation of Inner and Outer Membranes of the Chloroplast Envelope from Spinach and Pea

The outer membrane and the inner membrane of the chloroplast envelope, also called OEM and IEM, have distinct functions connected with chloroplast biogenesis and chloroplast communication with the rest of the cell. Here we describe a method for the isolation of these membranes starting from intact chloroplast preparations, with two alternative procedures based on the starting material. One was developed from spinach leaves, the other one from pea leaves. The two procedures differ by the means that are used to isolate and rupture chloroplasts and to liberate each membrane.

Purification of Chloroplasts and Chloroplast Subfractions: Envelope, Thylakoids, and Stroma-From Spinach, Pea, and Arabidopsis thaliana

Chloroplasts are specific organelles of plant cells dedicated to photosynthesis and delimited by a two-membrane chloroplast envelope. Their photosynthetic function is based on the development of an operational large internal membrane network, called the thylakoids, and on enzymatic processes present in the chloroplast matrix, called the stroma. Thylakoid membranes are clearly different from the chloroplast envelope and their biogenesis is dependent on biosynthetic and transport activities specific of the chloroplast envelope. Starting with the isolation of intact chloroplasts, the method presents the separation by differential centrifugation of the three main compartments of the chloroplast: the stroma, the thylakoids, and the chloroplast envelope. Three different protocols are provided, adapted for starting leaves of spinach, Arabidopsis, and pea.

Arabidopsis TRIGALACTOSYLDIACYLGLYCEROL5 Interacts with TGD1, TGD2, and TGD4 to Facilitate Lipid Transfer from the Endoplasmic Reticulum to Plastids

The biogenesis of photosynthetic membranes in the plastids of higher plants requires an extensive supply of lipid precursors from the endoplasmic reticulum (ER). Four TRIGALACTOSYLDIACYLGLYCEROL (TGD) proteins (TGD1,2,3,4) have thus far been implicated in this lipid transfer process. While TGD1, TGD2, and TGD3 constitute an ATP binding cassette transporter complex residing in the plastid inner envelope, TGD4 is a transmembrane lipid transfer protein present in the outer envelope. These observations raise questions regarding how lipids transit across the aqueous intermembrane space. Here, we describe the isolation and characterization of a novel Arabidopsis thaliana gene, TGD5. Disruption of TGD5 results in similar phenotypic effects as previously described in tgd1,2,3,4 mutants, including deficiency of ER-derived thylakoid lipids, accumulation of oligogalactolipids, and triacylglycerol. Genetic analysis indicates that TGD4 is epistatic to TGD5 in ER-to-plastid lipid trafficking, whereas double mutants of a null tgd5 allele with tgd1-1 or tgd2-1 show a synergistic embryo-lethal phenotype. TGD5 encodes a small glycine-rich protein that is localized in the envelope membranes of chloroplasts. Coimmunoprecipitation assays show that TGD5 physically interacts with TGD1, TGD2, TGD3, and TGD4. Collectively, these results suggest that TGD5 facilitates lipid transfer from the outer to the inner plastid envelope by bridging TGD4 with the TGD1,2,3 transporter complex.

Phosphatidylcholine is transferred from chemically-defined liposomes to chloroplasts through proteins of the chloroplast outer envelope membrane

Chloroplasts maintain their lipid balance through a tight interplay with the endoplasmic reticulum (ER). The outer envelope membrane of chloroplasts contains a large proportion of the phospholipid phosphatidylcholine (PC), which is synthesized in the ER and also a possible precursor for thylakoid galactolipids. The mechanism for PC transport from the ER to chloroplasts is not known. Using isolated chloroplasts and liposomes containing radiolabeled PC we investigated non-vesicular transport of PC in vitro. PC uptake in chloroplasts was time and temperature dependent, but nucleotide independent. Increased radius of liposomes stimulated PC uptake, and protease treatment of the chloroplasts impaired PC uptake. This implies that the chloroplast outer envelopes contains an exposed proteinaceous machinery for the uptake of PC from closely apposed membranes.

Biogenesis of beta-barrel proteins in evolutionary context

The vast majority of outer membrane (OM) proteins in Gram-negative bacteria belongs to the class of membrane-embedded β-barrel proteins. Besides Gram-negative bacteria, the presence of β-barrel proteins is restricted to the OM of the eukaryotic organelles mitochondria and chloroplasts that were derived from prokaryotic ancestors. The assembly of these proteins into the corresponding OM is in each case facilitated by a dedicated protein complex that contains a highly conserved central β-barrel protein termed BamA/YaeT/Omp85 in Gram-negative bacteria and Tob55/Sam50 in mitochondria. However, little is known about the exact mechanism by which these complexes mediate the integration of β-barrel precursors into the lipid bilayer. Interestingly, previous studies showed that during evolution, these complexes retained the ability to functionally assemble β-barrel proteins from different origins. In this review we summarize the current knowledge on the biogenesis pathway of β-barrel proteins in Gram-negative bacteria, mitochondria and chloroplasts and focus on the commonalities and divergences that evolved between the different β-barrel assembly machineries.

Fluorescence recovery after merging a surfactant-covered droplet: a novel technique to measure the diffusion of phospholipid monolayers at fluid/fluid interfaces

We present a novel technique to measure diffusion coefficients of insoluble surfactant monolayers. We merge a surfactant-coated droplet with a fluorescently labeled planar monolayer. During the merging process, a monolayer on a droplet displaces the existing planar monolayer, leaving a dark area when viewed under a fluorescence microscope. We measure fractional intensities as the dyes recover, which allows diffusion coefficients to be computed. We validate this technique with the two most common phospholipid monolayers (DPPC and DOPC) and study the diffusion of their mixtures. The proposed technique has several advantages over the FRAP technique and is potentially capable of measuring the diffusion of any soluble/insoluble surfactant monolayers.

Functional characterization of the chloroplast ferric chelate oxidoreductase enzyme

Iron (Fe) has an essential role in the biosynthesis of chlorophylls and redox cofactors, and thus chloroplast iron uptake is a process of special importance. The chloroplast ferric chelate oxidoreductase (cFRO) has a crucial role in this process but it is poorly characterized. To study the localization and mechanism of action of cFRO, sugar beet (Beta vulgaris cv Orbis) chloroplast envelope fractions were isolated by gradient ultracentrifugation, and their purity was tested by western blotting against different marker proteins. The ferric chelate reductase (FCR) activity of envelope fractions was studied in the presence of NAD(P)H (reductants) and FAD coenzymes. Reduction of Fe(III)-ethylenediaminetetraacetic acid was monitored spectrophotometrically by the Fe(II)-bathophenanthroline disulfonate complex formation. FCR activity, that is production of free Fe(II) for Fe uptake, showed biphasic saturation kinetics, and was clearly associated only to chloroplast inner envelope (cIE) vesicles. The reaction rate was > 2.5 times higher with NADPH than with NADH, which indicates the natural coenzyme preference of cFRO activity and its dependence on photosynthesis. FCR activity of cIE vesicles isolated from Fe-deficient plants also showed clear biphasic kinetics, where the KM of the low affinity component was elevated, and thus this component was down-regulated.

Evolution and targeting of Omp85 homologs in the chloroplast outer envelope membrane

Translocon at the outer-envelope-membrane of chloroplasts 75 (Toc75) is the core component of the chloroplast protein import machinery. It belongs to the Omp85 family whose members exist in various Gram-negative bacteria, mitochondria, and chloroplasts of eukaryotes. Chloroplasts of Viridiplantae contain another Omp85 homolog called outer envelope protein 80 (OEP80), whose exact function is unknown. In addition, the Arabidopsis thaliana genome encodes truncated forms of Toc75 and OEP80. Multiple studies have shown a common origin of the Omp85 homologs of cyanobacteria and chloroplasts but their results about evolutionary relationships among cyanobacterial Omp85 (cyanoOmp85), Toc75, and OEP80 are inconsistent. The bipartite targeting sequence-dependent sorting of Toc75 has been demonstrated but the targeting mechanisms of other chloroplast Omp85 homologs remain largely unexplored. This study was aimed to address these unresolved issues in order to further our understanding of chloroplast evolution. Sequence alignments and recently determined structures of bacterial Omp85 homologs were used to predict structures of chloroplast Omp85 homologs. The results enabled us to identify amino acid residues that may indicate functional divergence of Toc75 from cyanoOmp85 and OEP80. Phylogenetic analyses using Omp85 homologs from various cyanobacteria and chloroplasts provided strong support for the grouping of Toc75 and OEP80 sister to cyanoOmp85. However, this support was diminished when the analysis included Omp85 homologs from other bacteria and mitochondria. Finally, results of import assays using isolated chloroplasts support outer membrane localization of OEP80tr and indicate that OEP80 may carry a cleavable targeting sequence.

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.

Comparative proteomics of chloroplasts envelopes from bundle sheath and mesophyll chloroplasts reveals novel membrane proteins with a possible role in c4-related metabolite fluxes and development

As the world population grows, our need for food increases drastically. Limited amounts of arable land lead to a competition between food and fuel crops, while changes in the global climate may impact future crop yields. Thus, a second "green revolution" will need a better understanding of the processes essential for plant growth and development. One approach toward the solution of this problem is to better understand regulatory and transport processes in C4 plants. C4 plants display an up to 10-fold higher apparent CO2 assimilation and higher yields while maintaining high water use efficiency. This requires differential regulation of mesophyll (M) and bundle sheath (BS) chloroplast development as well as higher metabolic fluxes of photosynthetic intermediates between cells and particularly across chloroplast envelopes. While previous analyses of overall chloroplast membranes have yielded significant insight, our comparative proteomics approach using enriched BS and M chloroplast envelopes of Zea mays allowed us to identify 37 proteins of unknown function that have not been seen in these earlier studies. We identified 280 proteins, 84% of which are known/predicted to be present in chloroplasts. Seventy-four percent have a known or predicted membrane association. Twenty-one membrane proteins were 2-15 times more abundant in BS cells, while 36 of the proteins were more abundant in M chloroplast envelopes. These proteins could represent additional candidates of proteins essential for development or metabolite transport processes in C4 plants. RT-PCR confirmed differential expression of 13 candidate genes. Chloroplast association for seven proteins was confirmed using YFP/GFP labeling. Gene expression of four putative transporters was examined throughout the leaf and during the greening of leaves. Genes for a PIC-like protein and an ER-AP-like protein show an early transient increase in gene expression during the transition to light. In addition, PIC gene expression is increased in the immature part of the leaf and was lower in the fully developed parts of the leaf, suggesting a need for/incorporation of the protein during chloroplast development.

AtTPR7 is a chaperone-docking protein of the Sec translocon in Arabidopsis

Chaperone-assisted sorting of post-translationally imported proteins is a general mechanism among all eukaryotic organisms. Interaction of some preproteins with the organellar membranes is mediated by chaperones, which are recognised by membrane-bound tetratricopeptide repeat (TPR) domain containing proteins. We have characterised AtTPR7 as an endoplasmic reticulum protein in plants and propose a potential function for AtTPR7 in post-translational protein import. Our data demonstrate that AtTPR7 interacts with the heat shock proteins HSP90 and HSP70 via a cytosol-exposed TPR domain. We further show by in vitro and in vivo experiments that AtTPR7 is associated with the Arabidopsis Sec63 homologue, AtERdj2. Interestingly, AtTPR7 can functionally complement a Δsec71 yeast mutant that is impaired in post-translational protein transport. These data strongly suggest that AtTPR7 not only has a role in chaperone binding but also in post-translational protein import into the endoplasmic reticulum, pointing to a general mechanism of chaperone-mediated post-translational sorting between the endoplasmic reticulum, mitochondria and chloroplasts in plant cells.

TGD1, -2, and -3 proteins involved in lipid trafficking form ATP-binding cassette (ABC) transporter with multiple substrate-binding proteins

Members of the ATP-binding cassette (ABC) transporter family are essential proteins in species as diverse as archaea and humans. Their domain architecture has remained relatively fixed across these species, with rare exceptions. Here, we show one exception to be the trigalactosyldiacylglycerol 1, 2, and 3 (TGD1, -2, and -3) putative lipid transporter located at the chloroplast inner envelope membrane. TGD2 was previously shown to be in a complex of >500 kDa. We demonstrate that this complex also contains TGD1 and -3 and is very stable because it cannot be broken down by gentle denaturants to form a "core" complex similar in size to standard ABC transporters. The complex was purified from Pisum sativum (pea) chloroplast envelopes by native gel electrophoresis and examined by mass spectrometry. Identified proteins besides TGD1, -2, or -3 included a potassium efflux antiporter and a TIM17/22/23 family protein, but these were shown to be in separate high molecular mass complexes. Quantification of the complex components explained the size of the complex because 8-12 copies of the substrate-binding protein (TGD2) were found per functional transporter.

Inoculation with Xanthomonas oryzae pv. oryzae induces thylakoid membrane association of Rubisco activase in Oryza meyeriana

Oryza meyeriana is a wild species of rice with high resistance to Xanthomonas oryzae pv. oryzae (Xoo), but the resistance mechanism is poorly understood. Protein gel blot analysis and immuno-gold electron microscopy showed that Xoo infection induced an association of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activase (RCA) with the thylakoid membrane in O. meyeriana, which led to considerable decline in the initial activity and the activation state of Rubisco. In susceptible cultivated rice, RCA remained in the chloroplast stroma. RCA may play a role in resistance to Xoo in O. meyeriana that differs from its well-known role in activating Rubisco, perhaps by protecting the thylakoid membrane against damage from Xoo.

Altered regulation of lipid biosynthesis in a mutant of Arabidopsis deficient in chloroplast glycerol-3-phosphate acyltransferase activity

The leaf membrane lipids of many plant species, including Arabidopsis thaliana (L.) Heynh., are synthesized by two complementary pathways that are associated with the chloroplast and the endoplasmic reticulum. By screening directly for alterations in lipid acyl-group composition, we have identified several mutants of Arabidopsis that lack the plastid pathway because of a deficiency in activity of the first enzyme in the plastid pathway of glycerolipid synthesis, acyl-ACP:sn-glycerol-3-phosphate acyltransferase (EC 2.3.1.15) (where ACP is acyl carrier protein). The lesion results in an increased synthesis of lipids by the cytoplasmic pathway that largely compensates for the loss of the plastid pathway and provides nearly normal amounts of all the lipids required for chloroplast biogenesis. However, the fatty acid composition of the leaf membrane lipids of the mutants is altered because the acyltransferases associated with the two pathways normally exhibit different substrate specificities. The remarkable flexibility of the system provides an insight into the nature of the regulatory mechanisms that allocate lipids for membrane biogenesis.

Interaction of actin and the chloroplast protein import apparatus

Actin filaments are major components of the cytoskeleton and play numerous essential roles, including chloroplast positioning and plastid stromule movement, in plant cells. Actin is present in pea chloroplast envelope membrane preparations and is localized at the surface of the chloroplasts, as shown by agglutination of intact isolated chloroplasts by antibodies to actin. To identify chloroplast envelope proteins involved in actin binding, we have carried out actin co-immunoprecipitation and co-sedimentation experiments on detergent-solubilized pea chloroplast envelope membranes. Proteins co-immunoprecipitated with actin were identified by mass spectrometry and by Western blotting and included the Toc159, Toc75, Toc34, and Tic110 components of the TOC-TIC protein import apparatus. A direct interaction of actin with Escherichia coli-expressed Toc159, but not Toc33, was shown by co-sedimentation experiments, suggesting that Toc159 is the component of the TOC complex that interacts with actin on the cytosolic side of the outer envelope membrane. The physiological significance of this interaction is unknown, but it may play a role in the import of nuclear-encoded photosynthesis proteins.

Subcellular localization and enzymatic properties of differentially expressed transketolase genes isolated from the desiccation tolerant resurrection plant Craterostigma plantagineum

The desiccation tolerant resurrection plant Craterostigma plantagineum encodes three classes of transketolase transcripts, which are distinguished by their gene structures and their expression patterns. One class, represented by tkt3, is constitutively expressed and two classes, represented by tkt7 and tkt10, are upregulated upon rehydration of desiccated C. plantagineum plants. The objective of this work was to characterize the differentially expressed transketolase isoforms with respect to subcellular localization and enzymatic activity. Using GFP fusion constructs and enzymatic activity assays, we demonstrate that C. plantagineum has novel forms of transketolase which localize not to the chloroplast, but mainly to the cytoplasm and which are distinct in the enzymatic properties from the transketolase enzymes active in the Calvin cycle or oxidative pentose phosphate pathway. A transketolase preparation from rehydrated leaves was able to synthesize the unusual C8 carbon sugar octulose when glucose-6-phosphate and hydroxy-pyruvate were used as acceptor and donor molecules in in vitro assays. This suggests that a transketolase catalyzed reaction is likely to be involved in the octulose biosynthesis in C. plantagineum.

Purification of intact chloroplasts from Arabidopsis and spinach leaves by isopycnic centrifugation

Chloroplasts are plant-specific organelles. They are the site of photosynthesis but also of many other essential metabolic pathways, such as syntheses of amino acids, vitamins, lipids, and pigments. This unit describes the isolation and purification of chloroplasts from Arabidopsis and spinach leaves. Differential centrifugation is first used to obtain a suspension enriched in chloroplasts (crude chloroplasts extract). In a second step, Percoll density gradient centrifugation is used to recover pure and intact chloroplasts. The Basic Protocol describes the purification of chloroplasts from Arabidopsis leaves. This small flowering plant is now widely used as a model organism in plant biology as it offers important advantages for basic research in genetics and molecular biology. The Alternate Protocol describes the purification of chloroplasts from spinach leaves. Spinach, easily available all through the year, remains a model of choice for the large-scale preparation of pure chloroplasts with a high degree of intactness.

The Omp85-related chloroplast outer envelope protein OEP80 is essential for viability in Arabidopsis

beta-Barrel proteins of the Omp85 (Outer membrane protein, 85 kD) superfamily exist in the outer membranes of Gram-negative bacteria, mitochondria, and chloroplasts. Prominent Omp85 proteins in bacteria and mitochondria mediate biogenesis of other beta-barrel proteins and are indispensable for viability. In Arabidopsis (Arabidopsis thaliana) chloroplasts, there are two distinct types of Omp85-related protein: Toc75 (Translocon at the outer envelope membrane of chloroplasts, 75 kD) and OEP80 (Outer Envelope Protein, 80 kD). Toc75 functions as a preprotein translocation channel during chloroplast import, but the role of OEP80 remains elusive. We characterized three T-DNA mutants of the Arabidopsis OEP80 (AtOEP80) gene. Selectable markers associated with the oep80-1 and oep80-2 insertions segregated abnormally, suggesting embryo lethality of the homozygous genotypes. Indeed, no homozygotes were identified among >100 individuals, and heterozygotes of both mutants produced approximately 25% aborted seeds upon self-pollination. Embryo arrest occurred at a relatively late stage (globular embryo proper) as revealed by analysis using Nomarski optics microscopy. This is substantially later than arrest caused by loss of the principal Toc75 isoform, atToc75-III (two-cell stage), suggesting a more specialized role for AtOEP80. Surprisingly, the oep80-3 T-DNA (located in exon 1 between the first and second ATG codons of the open reading frame) did not cause any detectable developmental defects or affect the size of the AtOEP80 protein in chloroplasts. This indicates that the N-terminal region of AtOEP80 is not essential for the targeting, biogenesis, or functionality of the protein, in contrast with atToc75-III, which requires a bipartite targeting sequence.

Dynamic morphology of plastids and stromules in angiosperm plants

Labelling of plastids with fluorescent proteins has revealed the diversity of their sizes and shapes in different tissues of vascular plants. Stromules, stroma-filled tubules comprising thin extensions of the stroma surrounded by the double envelope membrane, have been observed to emanate from all major types of plastid, though less common on chloroplasts. In some tissue types, stromules are highly dynamic, forming, shrinking, attaching, releasing and fragmenting. Stromule formation is negatively affected by treatment of tissue with cytoskeletal inhibitors. Plastids can be connected by stromules, through which green fluorescent protein (GFP) and fluorescently tagged chloroplast protein complexes have been observed to flow. Within the highly viscous stroma, proteins traffic by diffusion as well as by an active process of directional travel, whose mechanism is unknown. In addition to exchanging materials between plastids, stromules may also serve to increase the surface area of the envelope for import and export, reduce diffusion distance between plastids and other organelles for exchange of materials, and anchor the plastid onto attachment points for proper positioning with the plant cell. Future studies should reveal how these functions may affect plants in adapting to the challenges of a changing environment.

Isolation and characterization of outer and inner envelope membranes of cyanelles from a glaucocystophyte, Cyanophora paradoxa

Envelope membranes were isolated by sucrose density gradient floatation centrifugation from the homogenate of cyanelles prepared from Cyanophora paradoxa. Two yellow bands were separated after 40 h of centrifugation. The buoyant density of one of the two fractions (fraction Y2) coincided with that of inner envelope membranes of spinach or plasma membranes of cyanobacteria. The other yellow fraction (fraction Y1) migrated to top of sucrose-gradient even at 0% sucrose. Pigment analysis revealed that the heavy yellow fraction was rich in zeaxanthin while the light fraction was rich in beta-carotene, and the both fractions contained practically no chlorophylls. Another yellow fraction (fraction Y3) was isolated from the phycobiliprotein fraction, which was the position where the sample was placed for gradient centrifugation. Its buoyant density and absorption spectra were similar to outer membranes of cyanobacteria. We have assigned fractions Y2 and Y3 as inner and outer envelope membrane fractions of cyanelles, respectively. Protein compositions were rather different between the two envelope membranes indicating little cross-contamination among the fractions.

A nitrite transporter associated with nitrite uptake by higher plant chloroplasts

Chloroplasts take up cytosolic nitrite during nitrate assimilation. In this study we identified a nitrite transporter located in the chloroplasts of higher plants. The transporter, CsNitr1-L, a member of the proton-dependent oligopeptide transporter (POT) family, was detected during light-induced chloroplast development in de-etiolating cucumber seedlings. We detected a CsNitr1-L-green fluorescent protein (GFP) fusion protein in the chloroplasts of leaf cells and found that an immunoreactive 51 kDa protein was present in the isolated inner envelope membrane of chloroplasts. CsNitr1-L has an isoform, CsNitr1-S, with an identical 484 amino acid core sequence; however, in CsNitr1-S the 120 amino acid N-terminal extension is missing. Saccharomyces cerevisiae cells expressing CsNitr1-S absorbed nitrite from an acidic medium at a slower rate than mock-transformed control cells, and accumulated nitrite to only one-sixth the concentration of the control cells, suggesting that CsNitr1-S enhances the efflux of nitrite from the cell. Insertion of T-DNA in a single CsNitr1-L homolog (At1g68570) in Arabidopsis resulted in nitrite accumulation in leaves to more than five times the concentration found in the wild type. These results show that it is possible that both CsNitr1-L and CsNitr1-S encode efflux-type nitrite transporters, but with different subcellular localizations. CsNitr1-L may possibly load cytosolic nitrite into chloroplast stroma in the chloroplast envelope during nitrate assimilation. The presence of genes homologous to CsNitr1-L in the genomes of Arabidopsis and rice indicates that facilitated nitrite transport is of general physiological importance in plant nutrition.

The nuclear-encoded factor HCF173 is involved in the initiation of translation of the psbA mRNA in Arabidopsis thaliana

To gain insight into the biogenesis of photosystem II (PSII) and to identify auxiliary factors required for this process, we characterized the mutant hcf173 of Arabidopsis thaliana. The mutant shows a high chlorophyll fluorescence phenotype (hcf) and is severely affected in the accumulation of PSII subunits. In vivo labeling experiments revealed a drastically decreased synthesis of the reaction center protein D1. Polysome association experiments suggest that this is primarily caused by reduced translation initiation of the corresponding psbA mRNA. Comparison of mRNA steady state levels indicated that the psbA mRNA is significantly reduced in hcf173. Furthermore, the determination of the psbA mRNA half-life revealed an impaired RNA stability. The HCF173 gene was identified by map-based cloning, and its identity was confirmed by complementation of the hcf phenotype. HCF173 encodes a protein with weak similarities to the superfamily of the short-chain dehydrogenases/reductases. The protein HCF173 is localized in the chloroplast, where it is mainly associated with the membrane system and is part of a higher molecular weight complex. Affinity chromatography of an HCF173 fusion protein uncovered the psbA mRNA as a component of this complex.

Optical manipulation reveals strong attracting forces at membrane contact sites between endoplasmic reticulum and chloroplasts

Eukaryote cells depend on membrane lipid trafficking from biogenic membranes, like the endoplasmic reticulum (ER), to other membranes in the cell. Two major routes for membrane lipid transport are recognized: vesicular trafficking and lipid transfer at zones of close contact between membranes. Specific ER regions involved in such membrane contact sites (MCSs) have been isolated, and lipid transfer at MCSs as well as protein-protein interactions between the partaking membranes have been demonstrated (reviewed by Holthuis, J. C. M., and Levine, T. P. (2005) Nat. Rev. 6, 209-220). Here we present the first demonstration of the physical association between membranes involved in MCSs: by using optical imaging and manipulation, strong attracting forces between ER and chloroplasts are revealed. We used Arabidopsis thaliana expressing green fluorescent protein in the ER lumen and observed leaf protoplasts by confocal microscopy. The ER network was evident, with ER branch end points apparently localized at chloroplast surfaces. After rupture of a protoplast using a laser scalpel, the cell content was released. ER fragments remained attached to the released chloroplasts and could be stretched out by optical tweezers. The applied force, 400 pN, could not drag a chloroplast free from its attached ER, which could reflect protein-protein interactions at the ER-chloroplast MCSs. As chloroplasts rely on import of ER-synthesized lipids, we propose that lipid transfer occurs at these MCSs. We suggest that lipid transfer at the MCSs also occurs in the opposite direction, for example to channel plastid-synthesized acyl groups to supply substrates for ER-localized synthesis of membrane and storage lipids.

Characterization of plant aquaporins

Plants have been reported to contain a large set of aquaporins (38 for Arabidopsis), which has been divided into four subfamilies on the basis of similarities in their amino acid sequences. They belong to the large superfamily of major intrinsic proteins (MIP), which was the basis for the nomenclature PIP, TIP, and NIP, also indicating the subcellular localization plasma membrane, tonoplast, and nodule of the respective founding member. The fourth subfamily of small and basic intrinsic proteins is not well characterized so far. The increasing number of reports dealing with various aspects of plant aquaporins is starting to advance our understanding of aquaporin biology in plants. Fundamental questions include: what is the basic function of the different plant aquaporins, what is their primary substrate, and what is the consequence of function/malfunction of a particular aquaporin for the overall function of the plant? Biochemical and biophysical techniques can be employed to get information on the basic functional characteristics of plant aquaporins. An impressive set of techniques has been used to study aquaporin function on molecular, subcellular, and cellular levels in plants, as well as in heterologous expression systems. The physiological role of aquaporins in plants is much less well understood, but reports unraveling the physiological role of aquaporins, mainly employing genetic techniques and functional measurement on the whole plant level, are emerging. The goal of this chapter is to give an overview on the applied methods, together with some exemplary findings.

A role for falcilysin in transit peptide degradation in thePlasmodium falciparumapicoplast

Falcilysin (FLN) is a zinc metalloprotease thought to degrade globin peptides in the acidic vacuole of the human malaria parasite Plasmodium falciparum. The enzyme has been found to have acidic or neutral pH optima on different peptides and to have additional distribution outside the food vacuole. These data suggested that FLN has an additional function in the parasite. To further probe the functions of FLN, we created a transgenic parasite clone expressing a chromosomally encoded FLN-GFP fusion. Unexpectedly, FLN was found in the apicoplast, an essential chloroplast-like organelle. Nuclear encoded apicoplast proteins are targeted to the organelle by a bipartite N-terminal sequence comprised of a signal sequence followed by a positively charged transit peptide domain. Free transit peptides are thought to be toxic to the plastid and need to be rapidly degraded after proteolytic release from proproteins. We hypothesized that FLN may participate in transit peptide degradation in the apicoplast based on its preference for basic residues at neutral pH and on phylogenetic comparison with other M16 family metalloproteases. In vitro cleavage by FLN of the transit peptide from the apicoplast-resident acyl carrier protein supports this idea. The importance of FLN for parasite development is suggested by our inability to truncate the chromosomal FLN open reading frame. Our work indicates that FLN is an attractive target for antimalarial development.

Reconstitution of protein targeting to the inner envelope membrane of chloroplasts

The chloroplast envelope plays critical roles in the synthesis and regulated transport of key metabolites, including intermediates in photosynthesis and lipid metabolism. Despite this importance, the biogenesis of the envelope membranes has not been investigated in detail. To identify the determinants of protein targeting to the inner envelope membrane (IM), we investigated the targeting of the nucleus-encoded integral IM protein, atTic40. We found that pre-atTic40 is imported into chloroplasts and processed to an intermediate size (int-atTic40) before insertion into the IM. Int-atTic40 is soluble and inserts into the IM from the internal stromal compartment. We also show that atTic40 and a second IM protein, atTic110, can target and insert into isolated IM vesicles in vitro. Collectively, our experiments are consistent with a "postimport" mechanism in which the IM proteins are first imported from the cytoplasm and subsequently inserted into the IM from the stroma.

Proteomics of chloroplast envelope membranes

Proteomics is a very powerful approach to link the information contained in sequenced genomes, like Arabidopsis, to the functional knowledge provided by studies of plant cell compartments, such as chloroplast envelope membranes. This review summarizes the present state of proteomic analyses of highly purified spinach and Arabidopsis envelope membranes. Methods targeted towards the hydrophobic core of the envelope allow identifying new proteins, and especially new transport systems. Common features were identified among the known and newly identified putative envelope inner membrane transporters and were used to mine the complete Arabidopsis genome to establish a virtual plastid envelope integral protein database. Arabidopsis envelope membrane proteins were extracted using different methods, that is, chloroform/methanol extraction, alkaline or saline treatments, in order to retrieve as many proteins as possible, from the most to the less hydrophobic ones. Mass spectrometry analyses lead to the identification of more than 100 proteins. More than 50% of the identified proteins have functions known or very likely to be associated with the chloroplast envelope. These proteins are (a) involved in ion and metabolite transport, (b) components of the protein import machinery and (c) involved in chloroplast lipid metabolism. Some soluble proteins, like proteases, proteins involved in carbon metabolism or in responses to oxidative stress, were associated with envelope membranes. Almost one third of the newly identified proteins have no known function. The present stage of the work demonstrates that a combination of different proteomics approaches together with bioinformatics and the use of different biological models indeed provide a better understanding of chloroplast envelope biochemical machinery at the molecular level.

The predicted candidates of Arabidopsis plastid inner envelope membrane proteins and their expression profiles

Plastid envelope proteins from the Arabidopsis nuclear genome were predicted using computational methods. Selection criteria were: first, to find proteins with NH(2)-terminal plastid-targeting peptides from all annotated open reading frames from Arabidopsis; second, to search for proteins with membrane-spanning domains among the predicted plastidial-targeted proteins; and third, to subtract known thylakoid membrane proteins. Five hundred forty-one proteins were selected as potential candidates of the Arabidopsis plastid inner envelope membrane proteins (AtPEM candidates). Only 34% (183) of the AtPEM candidates could be assigned to putative functions based on sequence similarity to proteins of known function (compared with the 69% function assignment of the total predicted proteins in the genome). Of the 183 candidates with assigned functions, 40% were classified in the category of "transport facilitation," indicating that this collection is highly enriched in membrane transporters. Information on the predicted proteins, tissue expression data from expressed sequence tags and microarrays, and publicly available T-DNA insertion lines were collected. The data set complements proteomic-based efforts in the increased detection of integral membrane proteins, low-abundance proteins, or those not expressed in tissues selected for proteomic analysis. Digital northern analysis of expressed sequence tags suggested that the transcript levels of most AtPEM candidates were relatively constant among different tissues in contrast to stroma and the thylakoid proteins. However, both digital northern and microarray analyses identified a number of AtPEM candidates with tissue-specific expression patterns.

The multisubunit acetyl-CoA carboxylase is strongly associated with the chloroplast envelope through non-ionic interactions to the carboxyltransferase subunits

The committed step for de novo fatty acid biosynthesis is the carboxylation of acetyl-CoA catalyzed by acetyl-CoA carboxylase (ACCase). Plastidial ACCase from most plants is a multisubunit complex composed of multiple copies of four different polypeptides, biotin carboxyl carrier protein (BCCP), biotin carboxylase (BC), and carboxyltransferase (alpha-CT and beta-CT). Immunoblot analyses revealed these four proteins were mostly (69% of total) associated with a 17,000 g insoluble fraction from lysed pea chloroplasts. Under the same conditions only 8% of ribulose-1,5-bisphosphate carboxylase was associated with this insoluble fraction. BCCP and biotin carboxylase BC subunits freely dissociated from 17 kg insoluble fractions under high ionic strength conditions, whereas alpha-CT and beta-CT subunits remained tightly associated. Both CT subunits were highly enriched in envelope versus stroma and thylakoid preparations whereas BC and BCCP subunits were predominantly stromal-localized due to partial dissociation. Rapid solubilization of intact chloroplasts with Triton X-100 followed by centrifugation at 30 kg resulted in a pellet that was up to 8-fold enriched in ACCase activity and 21-fold enriched in BC activity. Triton-insoluble 30 kg pellets were reduced in lipid and chlorophyll content but enriched in chloroplast DNA due to the isolation of nucleoid particles. However, ACCase was not directly associated with nucleoids since enzymatic digestion of DNA or RNA had no effect on the association with Triton-insoluble matter. The amount of Triton-insoluble ACCase was similar in chloroplasts isolated from dark- or light-adapted leaves suggesting transitory starch granules were also not involved in this association. It is proposed that ACCase is associated with envelope membranes through interactions with an unidentified integral membrane protein.

HCF164 encodes a thioredoxin-like protein involved in the biogenesis of the cytochrome b(6)f complex in Arabidopsis

To understand the biogenesis of the plastid cytochrome b(6)f complex and to identify the underlying auxiliary factors, we have characterized the nuclear mutant hcf164 of Arabidopsis and isolated the affected gene. The mutant shows a high chlorophyll fluorescence phenotype and is severely deficient in the accumulation of the cytochrome b(6)f complex subunits. In vivo protein labeling experiments indicated that the mutation acts post-translationally by interfering with the assembly of the complex. Because of its T-DNA tag, the corresponding gene was cloned and its identity confirmed by complementation of homozygous mutant plants. HCF164 encodes a thioredoxin-like protein that possesses disulfide reductase activity. The protein was found in the chloroplast, where it is anchored to the thylakoid membrane at its lumenal side. HCF164 is closely related to the thioredoxin-like protein TxlA of Synechocystis sp PCC6803, most probably reflecting its evolutionary origin. The protein also shows a limited similarity to the eubacterial CcsX and CcmG proteins, which are required for the maturation of periplasmic c-type cytochromes. The putative roles of HCF164 for the assembly of the cytochrome b(6)f complex are discussed.

HCF164 encodes a thioredoxin-like protein involved in the biogenesis of the cytochrome b(6)f complex in Arabidopsis

To understand the biogenesis of the plastid cytochrome b(6)f complex and to identify the underlying auxiliary factors, we have characterized the nuclear mutant hcf164 of Arabidopsis and isolated the affected gene. The mutant shows a high chlorophyll fluorescence phenotype and is severely deficient in the accumulation of the cytochrome b(6)f complex subunits. In vivo protein labeling experiments indicated that the mutation acts post-translationally by interfering with the assembly of the complex. Because of its T-DNA tag, the corresponding gene was cloned and its identity confirmed by complementation of homozygous mutant plants. HCF164 encodes a thioredoxin-like protein that possesses disulfide reductase activity. The protein was found in the chloroplast, where it is anchored to the thylakoid membrane at its lumenal side. HCF164 is closely related to the thioredoxin-like protein TxlA of Synechocystis sp PCC6803, most probably reflecting its evolutionary origin. The protein also shows a limited similarity to the eubacterial CcsX and CcmG proteins, which are required for the maturation of periplasmic c-type cytochromes. The putative roles of HCF164 for the assembly of the cytochrome b(6)f complex are discussed.

HCF164 encodes a thioredoxin-like protein involved in the biogenesis of the cytochrome b(6)f complex in Arabidopsis

To understand the biogenesis of the plastid cytochrome b(6)f complex and to identify the underlying auxiliary factors, we have characterized the nuclear mutant hcf164 of Arabidopsis and isolated the affected gene. The mutant shows a high chlorophyll fluorescence phenotype and is severely deficient in the accumulation of the cytochrome b(6)f complex subunits. In vivo protein labeling experiments indicated that the mutation acts post-translationally by interfering with the assembly of the complex. Because of its T-DNA tag, the corresponding gene was cloned and its identity confirmed by complementation of homozygous mutant plants. HCF164 encodes a thioredoxin-like protein that possesses disulfide reductase activity. The protein was found in the chloroplast, where it is anchored to the thylakoid membrane at its lumenal side. HCF164 is closely related to the thioredoxin-like protein TxlA of Synechocystis sp PCC6803, most probably reflecting its evolutionary origin. The protein also shows a limited similarity to the eubacterial CcsX and CcmG proteins, which are required for the maturation of periplasmic c-type cytochromes. The putative roles of HCF164 for the assembly of the cytochrome b(6)f complex are discussed.

Acyl-CoA dependent acylation of phospholipids in the chloroplast envelope

Acyl-CoAs are substrates for acyl lipid synthesis in the endoplasmic reticulum. In addition, they may also be substrates for lipid acylation in other membranes. In order to assess whether lipid acylation may have a role in plastid lipid metabolism, we have studied the incorporation of radiolabelled fatty acids from acyl-CoAs into lipids in isolated, intact pea chloroplasts. The labelled lipids were phosphatidylcholine (PC), phosphatidylglycerol (PG), phosphatidylinositol and free fatty acids. With oleoyl-CoA, the fatty acid was incorporated preferably into the sn-2 position of PC and the acylation activity mainly occurred in fractions enriched in inner chloroplast envelope. Added lysoPC stimulated the activity. With palmitoyl-CoA, the fatty acid was incorporated primarily into the sn-1 position of PG and the reaction occurred at the surface of the chloroplasts. As chloroplast-synthesized PG generally contains 16C fatty acids in the sn-2 position, we propose that the acylation of PG studied represents activities present in a domain of the endoplasmic reticulum or an endoplasmic reticulum-derived fraction that is associated with chloroplasts and maintains this association during isolation. This domain or fraction contains a discreet population of lipid metabolizing activities, different from that of bulk endoplasmic reticulum, as shown by that with isolated endoplasmic reticulum, acyl-CoAs strongly labelled phosphatidic acid and phosphatidylethanolamine, lipids that were never labelled in the isolated chloroplasts.