Molecular aspects of plastid envelope biochemistry

The Chloroplast Envelope of Angiosperms Contains a Peptidoglycan Layer

Plastids in plants are assumed to have evolved from cyanobacteria as they have maintained several bacterial features. Recently, peptidoglycans, as bacterial cell wall components, have been shown to exist in the envelopes of moss chloroplasts. Phylogenomic comparisons of bacterial and plant genomes have raised the question of whether such structures are also part of chloroplasts in angiosperms. To address this question, we visualized canonical amino acids of peptidoglycan around chloroplasts of Arabidopsis and Nicotiana via click chemistry and fluorescence microscopy. Additional detection by different peptidoglycan-binding proteins from bacteria and animals supported this observation. Further Arabidopsis experiments with D-cycloserine and AtMurE knock-out lines, both affecting putative peptidoglycan biosynthesis, revealed a central role of this pathway in plastid genesis and division. Taken together, these results indicate that peptidoglycans are integral parts of plastids in the whole plant lineage. Elucidating their biosynthesis and further roles in the function of these organelles is yet to be achieved.

Identification and Molecular Characterization of the Chloroplast Targeting Domain of Turnip yellow mosaic virus Replication Proteins

Turnip yellow mosaic virus (TYMV) is a positive-strand RNA virus infecting plants. The TYMV 140K replication protein is a key organizer of viral replication complex (VRC) assembly, being responsible for recruitment of the viral polymerase and for targeting the VRCs to the chloroplast envelope where viral replication takes place. However, the structural requirements determining the subcellular localization and membrane association of this essential viral protein have not yet been defined. In this study, we investigated determinants for the in vivo chloroplast targeting of the TYMV 140K replication protein. Subcellular localization studies of deletion mutants identified a 41-residue internal sequence as the chloroplast targeting domain (CTD) of TYMV 140K; this sequence is sufficient to target GFP to the chloroplast envelope. The CTD appears to be located in the C-terminal extension of the methyltransferase domain-a region shared by 140K and its mature cleavage product 98K, which behaves as an integral membrane protein during infection. We predicted the CTD to fold into two amphipathic α-helices-a folding that was confirmed in vitro by circular dichroism spectroscopy analyses of a synthetic peptide. The importance for subcellular localization of the integrity of these amphipathic helices, and the function of 140K/98K, was demonstrated by performing amino acid substitutions that affected chloroplast targeting, membrane association and viral replication. These results establish a short internal α-helical peptide as an unusual signal for targeting proteins to the chloroplast envelope membrane, and provide new insights into membrane targeting of viral replication proteins-a universal feature of positive-strand RNA viruses.

Emerging roles of the chloroplast outer envelope membrane

The chloroplast is essential for the viability of plants. It is enclosed by a double-membrane envelope that originated from the outer and plasma membranes of a cyanobacterial endosymbiont. Chloroplast biogenesis depends on binary fission and import of nuclear-encoded proteins. Our understanding of the mechanisms and evolutionary origins of these processes has been greatly advanced by recent genetic and biochemical studies on envelope-localized multiprotein machines. Furthermore, the latest studies on outer envelope proteins have provided molecular insights into organelle movement and membrane lipid remodeling, activities that are vital for plant survival under diverse environmental conditions. Ongoing and future research on the chloroplast outer envelope should add to our knowledge of organelle biology and the evolution of eukaryotic cells.

The main thylakoid membrane lipid monogalactosyldiacylglycerol (MGDG) promotes the de-epoxidation of violaxanthin associated with the light-harvesting complex of photosystem II (LHCII)

In higher plants, the major part of the xanthophyll cycle pigment violaxanthin (Vx) is non-covalently bound to the main light-harvesting complex of PSII (LHCII). Under saturating light conditions Vx has to be released from its binding site into the surrounding lipid phase, where it is converted to zeaxanthin (Zx) by the enzyme Vx de-epoxidase (VDE). In the present study we investigated the influence of thylakoid lipids on the de-epoxidation of Vx, which was still associated with the LHCII. We isolated LHCII with different concentrations of native, endogenous lipids and Vx by sucrose gradient centrifugation or successive cation precipitation. Analysis of the different LHCII preparations showed that the concentration of LHCII-associated Vx was correlated with the concentration of the main thylakoid lipid monogalactosyldiacylglycerol (MGDG) associated with the complexes. Decreases in the MGDG content of the LHCII led to a diminished Vx concentration, indicating that a part of the total Vx pool was located in an MGDG phase surrounding the LHCII, whereas another part was bound to the LHCII apoproteins. We further studied the convertibility of LHCII-associated Vx in in-vitro enzyme assays by addition of isolated VDE. We observed an efficient and almost complete Vx conversion in the LHCII fractions containing high amounts of endogenous MGDG. LHCII preparations with low concentrations of MGDG exhibited a strongly reduced Vx de-epoxidation, which could be increased by addition of exogenous, pure MGDG. The de-epoxidation of LHCII-associated Vx was saturated at a much lower concentration of native, endogenous MGDG compared with the concentration of isolated, exogenous MGDG, which is needed for optimal VDE activity in in-vitro assays employing pure isolated Vx.

Arabidopsis ACBP6 is an acyl-CoA-binding protein associated with phospholipid metabolism

In our recent paper in Plant Physiology, we showed that the Arabidopsis thaliana 10-kD acyl-CoA-binding protein, ACBP6, is subcellularly localized to the cytosol and that the overexpression of ACBP6 in transgenic Arabidopsis enhanced freezing tolerance. ACBP6-conferred freezing tolerance was independent of induced cold-regulated (COLD-RESPONSIVE) gene expression, but was correlated to an enhanced expression of phospholipase Ddelta (PLDdelta). Lipid analyses on cold-acclimated freezing-treated ACBP6-overexpressors revealed a decline in phosphatidylcholine (PC) and an elevation of phosphatidic acid (PA) in comparison to wild type. Furthermore, the His-tagged ACBP6 recombinant protein was observed using in vitro filter-binding assays to bind PC, but not PA or lysophosphatidylcholine. Taken together, our results implicate roles for ACBP6 in phospholipid metabolism that is related to gene regulation and PC-binding/transfer. This represents the first report demonstrating the in vitro binding of an ACBP to a phospholipid. The effect of ACBP6 on PLDdelta expression is reminiscent of yeast 10-kD ACBP function in the regulation of genes associated with stress responses, fatty acid synthesis and phospholipid synthesis. However, the yeast ACBP regulates the expression of genes involved in phospholipid synthesis by donation of acyl-CoA esters and its binding to phospholipids remains to be demonstrated.

Photosystem II assembly and repair are differentially localized in Chlamydomonas

Many proteins of the photosynthesis complexes are encoded by the genome of the chloroplast and synthesized by bacterium-like ribosomes within this organelle. To determine where proteins are synthesized for the de novo assembly and repair of photosystem II (PSII) in the chloroplast of Chlamydomonas reinhardtii, we used fluorescence in situ hybridization, immunofluorescence staining, and confocal microscopy. These locations were defined as having colocalized chloroplast mRNAs encoding PSII subunits and proteins of the chloroplast translation machinery specifically under conditions of PSII subunit synthesis. The results revealed that the synthesis of the D1 subunit for the repair of photodamaged PSII complexes occurs in regions of the chloroplast with thylakoids, consistent with the current model. However, for de novo PSII assembly, PSII subunit synthesis was detected in discrete regions near the pyrenoid, termed T zones (for translation zones). In two PSII assembly mutants, unassembled D1 subunits and incompletely assembled PSII complexes localized around the pyrenoid, where we propose that they mark an intermediate compartment of PSII assembly. These results reveal a novel chloroplast compartment that houses de novo PSII biogenesis and the regulated transport of newly assembled PSII complexes to thylakoid membranes throughout the chloroplast.

NMR studies of membranes composed of glycolipids and phospholipids

Lipid membranes composed of monogalactosyldiacylglycerol (MGDG) and dimyristoylphosphatidylcholine (DMPC) were studied by means of NMR spectroscopy. The macroscopic phase behaviour was investigated by (31)P NMR under stationary conditions, whereas microscopic properties such as segmental ordering were probed by two-dimensional (1)H-(13)C separated local field experiments under magic-angle spinning conditions. Our results clearly show that ordering/disordering effects occur for the headgroups as well as for the acyl chains when the sample composition is varied. In particular, the (1)H-(13)C dipolar couplings within the galactose headgroup of MGDG exhibited significant concentration dependence.

Folding kinetics and structure of OEP16

The chloroplast outer membrane contains different, specialized pores that are involved in highly specific traffic processes from the cytosol into the chloroplast and vice versa. One representative member of these channels is the outer envelope protein 16 (OEP16), a cation-selective high conductance channel with high selectivity for amino acids. Here we study the mechanism and kinetics of the folding of this membrane protein by fluorescence and circular dichroism spectroscopy, using deletion mutants of the two single tryptophanes Trp-77-->Phe-77 and Trp-100-->Phe-100. In addition, the wild-type spectra were deconvoluted, depicting the individual contributions from each of the two tryptophan residues. The results show that both tryptophan residues are located in a completely different environment. The Trp-77 is deeply buried in the hydrophobic part of the protein, whereas the Trp-100 is partially solvent exposed. These results were further confirmed by studies of fluorescence quenching with I(-). The kinetics of the protein folding are studied by stopped flow fluorescence and circular dichroism measurements. The folding process depends highly on the detergent concentration and can be divided into an ultrafast phase (k > 1000 s(-1)), a fast phase (200-800 s(-1)), and a slow phase (25-70 s(-1)). The slow phase is absent in the W100F mutant. Secondary structure analysis and comparision with closely related proteins led to a new model of the structure of OEP16, suggesting that the protein is, in contrast to most other outer membrane proteins studied so far, purely alpha-helical, consisting of four transmembrane helices. Trp-77 is located in helix II, whereas the Trp-100 is located in the loop between helices II and III, close to the interface to helix III. We suggest that the first, very fast process corresponds to the formation of the helices, whereas the insertion of the helices into the detergent micelle and the correct folding of the II-III loop may be the later, rate-limiting steps of the folding process.

Preprotein recognition by the Toc complex

The Toc core complex consists of the pore-forming Toc75 and the GTPases Toc159 and Toc34. We confirm that the receptor form of Toc159 is integrated into the membrane. The association of Toc34 to Toc75/Toc159 is GTP dependent and enhanced by preprotein interaction. The N-terminal half of the pSSU transit peptide interacts with high affinity with Toc159, whereas the C-terminal part stimulates its GTP hydrolysis. The phosphorylated C-terminal peptide of pSSU interacts strongly with Toc34 and therefore inhibits binding and translocation of pSSU into Toc proteoliposomes. In contrast, Toc159 recognises only the dephosphorylated forms. The N-terminal part of the pSSU presequence does not influence binding to the Toc complex, but is able to block import into proteoliposomes through its interaction with Toc159. We developed a model of differential presequence recognition by Toc34 and Toc159.

Analysis of the plastidic phosphate translocator gene family in Arabidopsis and identification of new phosphate translocator-homologous transporters, classified by their putative substrate-binding site

Analysis of the Arabidopsis genome revealed the complete set of plastidic phosphate translocator (pPT) genes. The Arabidopsis genome contains 16 pPT genes: single copies of genes coding for the triose phosphate/phosphate translocator and the xylulose phosphate/phosphate translocator, and two genes coding for each the phosphoenolpyruvate/phosphate translocator and the glucose-6-phosphate/phosphate translocator. A relatively high number of truncated phosphoenolpyruvate/phosphate translocator genes (six) and glucose-6-phosphate/phosphate translocator genes (four) could be detected with almost conserved intron/exon structures as compared with the functional genes. In addition, a variety of PT-homologous (PTh) genes could be identified in Arabidopsis and other organisms. They all belong to the drug/metabolite transporter superfamily showing significant similarities to nucleotide sugar transporters (NSTs). The pPT, PTh, and NST proteins all possess six to eight transmembrane helices. According to the analysis of conserved motifs in these proteins, the PTh proteins can be divided into (a) the lysine (Lys)/arginine group comprising only non-plant proteins, (b) the Lys-valine/alanine/glycine group of Arabidopsis proteins, (c) the Lys/asparagine group of Arabidopsis proteins, and (d) the Lys/threonine group of plant and non-plant proteins. None of these proteins have been characterized so far. The analysis of the putative substrate-binding sites of the pPT, PTh, and NST proteins led to the suggestion that all these proteins share common substrate-binding sites on either side of the membrane each of which contain a conserved Lys residue.

Molecular architecture of the thylakoid membrane: lipid diffusion space for plastoquinone

We have determined the stoichiometric composition of membrane components (lipids and proteins) in spinach thylakoids and have derived the molecular area occupied by these components. From this analysis, the lipid phase diffusion space, the fraction of lipids located in the first protein solvation shell (boundary lipids), and the plastoquinone (PQ) concentration are derived. On the basis of these stoichiometric data, we have analyzed the motion of PQ between photosystem (PS) II and cytochrome (cyt.) bf complexes in this highly protein obstructed membrane (protein area about 70%) using percolation theory. This analysis reveals an inefficient diffusion process. We propose that distinct structural features of the thylakoid membrane (grana formation, microdomains) could help to minimize these inefficiencies and ensure a non-rate limiting PQ diffusion process. A large amount of published evidence supports the idea that higher protein associations exist, especially in grana thylakoids. From the quantification of the boundary lipid fraction (about 60%), we conclude that protein complexes involved in these associations should be spaced by lipids. Lipid-spaced protein aggregations in thylakoids are qualitatively different to previously characterized associations (multisubunit complexes, supercomplexes). We derive a hierarchy of protein and lipid interactions in the thylakoid membrane.

Does complexity constrain organelle evolution?

The evolution of eukaryotes was punctuated by invasions of the bacteria that have evolved to mitochondria and plastids. These bacterial endosymbionts founded major eukaryotic lineages by enabling them to carry out aerobic respiration and oxygenic photosynthesis. Yet, having evolved as free-living organisms, they were at first poorly adapted organelles. Although mitochondria and plastids have integrated within the physiology of eukaryotic cells, this integration has probably been constrained by the high level of complexity of their bacterial ancestors and the inability of gradual evolutionary processes to drastically alter complex systems. Here, I review complex processes that directly involve translation of plastid mRNAs and how they could constrain transfer to the nucleus of the genes encoding them.

The paradox of plastid transit peptides: conservation of function despite divergence in primary structure

Transit peptides are N-terminal extensions that facilitate the targeting and translocation of cytosolically synthesized precursors into plastids via a post-translational mechanism. With the complete Arabidopsis genome in hand, it is now evident that transit peptides direct more than 3500 different proteins into the plastid during the life of a typical plant. Deciphering a common mechanism for how this multitude of targeting sequences function has been hampered by the realization that at a primary sequence level, transit peptides are highly divergent in length, composition, and organization. This review addresses recent findings on several of the diverse functions that transit peptides must perform, including direct interaction with envelope lipids, association with a cis-acting guidance complex, recognition by envelope receptors, insertion into the Toc/Tic translocon, interaction with molecular motors, and finally, recognition/cleavage by the stromal processing peptidase. In addition to higher plants, transit peptides also direct the import of proteins into complex plastids derived from secondary endosymbiosis. An emerging concept suggests that transit peptides contain multiple domains that provide either distinct or possibly overlapping functions. Although still poorly characterized, evolutionary processes could yield transit peptides with alternative domain organizations.

Three distinct lipid kinase activities are present in spinach chloroplast envelope membranes: phosphatidylinositol phosphorylation is sensitive to wortmannin and not dependent on chloroplast ATP

Chloroplast envelope membranes display properties that are important in lipid synthesis, regulation of metabolites, and protein transport, as well as in signal transduction. The recent discovery showing that phosphorylation of lipids occurs in envelope membranes provides a new approach for understanding the role of chloroplast lipids in these processes. The present investigation shows that three major lipid kinase activities are at least present in envelope membranes. These activities greatly depend on external conditions, such as pH, ATP concentrations, temperature, and chloroplast ATP and wortmannin sensitivity. Two types of phosphorylated lipid couples displayed similar intrinsic responses toward these biochemical parameters, namely phosphatidic acid (PA) and its lysoderivative (LPA) and monogalactosyl-phosphate-diacylglycerol (MGpDG) and its lysoderivative (LMGpDG), but not phosphatidylinositol-monophosphate (PIP) and its lysoderivative (LPIP). Phosphorylation of phosphatidylinositol was not dependent on chloroplast ATP, but was sensitive toward wortmannin in intact chloroplasts and outer envelope membrane vesicles.

Diesters of glycosylglycerols active in cancer chemoprevention

Enzymatic transesterification, mediated by Pseudomonas cepacia lipase (lipase PS), led to the pure 1,6'-diacylderivatives of 2-O-beta-D-glucosyl-sn-glycerol and 2-O-beta-D-galactosyl-sn-glycerol, the acyl chains being derived from short-medium length fatty acids. A study of the in vitro inhibitory effects of these diacylderivatives on Epstein-Barr virus early antigen activation induced by the tumour promoter 12-O-tetradecanoylphorbol-13-acetate revealed that maximum activity was reached for the hexanoyl chain and that the introduction of a second acyl chain did not significantly modify the inhibitory potential referring to the corresponding 1- or 6'-monoesters.

Dual targeting of spinach protoporphyrinogen oxidase II to mitochondria and chloroplasts by alternative use of two in-frame initiation codons

Protoporphyrinogen oxidase (Protox) is the final enzyme in the common pathway of chlorophyll and heme biosynthesis. Two Protox isoenzymes have been described in tobacco, a plastidic and a mitochondrial form. We isolated and sequenced spinach Protox cDNA, which encodes a homolog of tobacco mitochondrial Protox (Protox II). Alignment of the deduced amino acid sequence between Protox II and other tobacco mitochondrial Protox homologs revealed a 26-amino acid N-terminal extension unique to the spinach enzyme. Immunoblot analysis of spinach leaf extract detected two proteins with apparent molecular masses of 57 and 55 kDa in chloroplasts and mitochondria, respectively. In vitro translation experiments indicated that two translation products (59 and 55 kDa) are produced from Protox II mRNA, using two in-frame initiation codons. Transport experiments using green fluorescent protein-fused Protox II suggested that the larger and smaller translation products (Protox IIL and IIS) target exclusively to chloroplasts and mitochondria, respectively.

Protein import: the hitchhikers guide into chloroplasts

Plastids originated from an endosymbiotic event between an early eukaryotic host cell and an ancestor of today's cyanobacteria. During the events by which the engulfed endosymbiont was transformed into a permanent organelle, many genes were transferred from the plastidal genome to the nucleus of the host cell. Proteins encoded by these genes are synthesised in the cytosol and subsequently translocated into the plastid. Therefore they contain an N-terminal cleavable transit sequence that is necessary for translocation. The sequence is plastid-specific, thus preventing mistargeting into other organelles. Receptors embedded into the outer envelope of the plastid recognise the transit sequences, and precursor proteins are translocated into the chloroplast by a proteinaceous import machinery located in both the outer and inner envelopes. Inside the stroma the transit sequences are cleaved off and the proteins are further routed to their final locations within the plastid.

Lipid phosphorylation in chloroplast envelopes. Evidence for galactolipid CTP-dependent kinase activities

Lipid phosphorylation takes place within the chloroplast envelope. In addition to phosphatidic acid, phosphatidylinositol phosphate, and their corresponding lyso-derivatives, we found that two novel lipids underwent phosphorylation in envelopes, particularly in the presence of carrier-free [gamma-(32)P]ATP. These two lipids incorporated radioactive phosphate in chloroplasts in the presence of [gamma-(32)P]ATP or [(32)P]P(i) and light. Interestingly, these two lipids were preferentially phosphorylated in envelope membranes in the presence [gamma-(32)P]CTP, as the phosphoryl donor, or [gamma-(32)P]ATP, when supplemented with CDP and nucleoside diphosphate kinase II. The lipid kinase activity involved in this reaction was specifically inhibited in the presence of cytosine 5'-O-(thiotriphosphate) (CTPgammaS) and sensitive to CTP chase, thereby showing that both lipids are phosphorylated by an envelope CTP-dependent lipid kinase. The lipids were identified as phosphorylated galactolipids by using an acid hydrolysis procedure that generated galactose 6-phosphate. CTPgammaS did not affect the import of the small ribulose-bisphosphate carboxylase/oxygenase subunit into chloroplasts, the possible physiological role of this novel CTP-dependent galactolipid kinase activity in the chloroplast envelope is discussed.

Solute pores, ion channels, and metabolite transporters in the outer and inner envelope membranes of higher plant plastids

All plant cells contain plastids. Various reactions are located exclusively within these unique organelles, requiring the controlled exchange of a wide range of solutes, ions, and metabolites. In recent years, several proteins involved in import and/or export of these compounds have been characterized using biochemical and electrophysiological approaches, and in addition have been identified at the molecular level. Several solute channels have been identified in the outer envelope membrane. These porin-like proteins in the outer envelope membrane were formerly thought to be quite unspecific, but have now been shown to exhibit significant substrate specificity and to be highly regulated. Therefore, the inter-envelope membrane space is not as freely accessible as previously thought. Transport proteins in the inner envelope membrane have been characterized in more detail. It has been proved unequivocally that a family of proteins (including triose phosphate-/phosphoenolpyruvate-, and glucose 6-phosphate-specific transporters) permit the exchange of inorganic phosphate and phosphorylated intermediates. A new type of plastidic 2-oxoglutarate/malate transporter has been identified and represents the first carrier with 12 putative transmembrane domains, to be located in the inner envelope membrane. The plastidic ATP/ADP transporter also contains 12 putative transmembrane domains and possesses striking structural similarity to ATP/ADP transporters found in intracellular, human pathogenic bacteria.

Limonene synthase, the enzyme responsible for monoterpene biosynthesis in peppermint, is localized to leucoplasts of oil gland secretory cells

Circumstantial evidence based on ultrastructural correlation, specific labeling, and subcellular fractionation studies indicates that at least the early steps of monoterpene biosynthesis occur in plastids. (4S)-Limonene synthase, which is responsible for the first dedicated step of monoterpene biosynthesis in mint species, appears to be translated as a preprotein bearing a long plastidial transit peptide. Immunogold labeling using polyclonal antibodies raised to the native enzyme demonstrated the specific localization of limonene synthase to the leucoplasts of peppermint (Mentha x piperita) oil gland secretory cells during the period of essential oil production. Labeling was shown to be absent from all other plastid types examined, including the basal and stalk cell plastids of the secretory phase glandular trichomes. Furthermore, in vitro translation of the preprotein and import experiments with isolated pea chloroplasts were consistent in demonstrating import of the nascent protein to the plastid stroma and proteolytic processing to the mature enzyme at this site. These experiments confirm that the leucoplastidome of the oil gland secretory cells is the exclusive location of limonene synthase, and almost certainly the preceding steps of monoterpene biosynthesis, in peppermint leaves. However, succeeding steps of monoterpene metabolism in mint appear to occur outside the leucoplasts of oil gland cells.

The evolutionary origin of the protein-translocating channel of chloroplastic envelope membranes: identification of a cyanobacterial homolog

The known envelope membrane proteins of the chloroplastic protein import apparatus lack sequence similarity to proteins of other eukaryotic or prokaryotic protein transport systems. However, we detected a putative homolog of the gene encoding Toc75, the protein-translocating channel from the outer envelope membrane of pea chloroplasts, in the genome of the cyanobacterium Synechocystis sp. PCC 6803. We investigated whether the low sequence identity of 21% reflects a structural and functional relationship between the two proteins. We provide evidence that the cyanobacterial protein is also localized in the outer membrane. From this information and the similarity of the predicted secondary structures, we conclude that Toc75 and the cyanobacterial protein, referred to as SynToc75, are structural homologs. synToc75 is essential, as homozygous null mutants were not recovered after directed mutagenesis. Sequence analysis indicates that SynToc75 belongs to a family of outer membrane proteins from Gram-negative bacteria whose function is not yet known. However, we demonstrate that these proteins are related to a specific group of prokaryotic secretion channels that transfer virulence factors, such as hemolysins and adhesins, across the outer membrane.

Positive charges determine the topology and functionality of the transmembrane domain in the chloroplastic outer envelope protein Toc34

The chloroplastic outer envelope protein Toc34 is inserted into the membrane by a COOH-terminal membrane anchor domain in the orientation Ncyto-Cin. The insertion is independent of ATP and a cleavable transit sequence. The cytosolic domain of Toc34 does not influence the insertion process and can be replaced by a different hydrophilic reporter peptide. Inversion of the COOH-terminal, 45-residue segment, including the membrane anchor domain (Toc34Cinv), resulted in an inverted topology of the protein, i.e., Nin-Ccyto. A mutual exchange of the charged amino acid residues NH2- and COOH-proximal of the hydrophobic alpha-helix indicates that a double-positive charge at the cytosolic side of the transmembrane alpha-helix is the sole determinant for its topology. When the inverted COOH-terminal segment was fused to the chloroplastic precursor of the ribulose-1,5-bisphosphate carboxylase small subunit (pS34Cinv), it engaged the transit sequence-dependent import pathway. The inverted peptide domain of Toc34 functions as a stop transfer signal and is released out of the outer envelope protein translocation machinery into the lipid phase. Simultaneously, the NH2-terminal part of the hybrid precursor remained engaged in the inner envelope protein translocon, which could be reversed by the removal of ATP, demonstrating that only an energy-dependent force but no further ionic interactions kept the precursor in the import machinery.

A SecY homologue is required for the elaboration of the chloroplast thylakoid membrane and for normal chloroplast gene expression

Results of in vitro and genetic studies have provided evidence for four pathways by which proteins are targeted to the chloroplast thylakoid membrane. Although these pathways are initially engaged by distinct substrates and involve some distinct components, an unresolved issue has been whether multiple pathways converge on a common translocation pore in the membrane. A homologue of eubacterial SecY called cpSecY is localized to the thylakoid membrane. Since SecY is a component of a protein-translocating pore in bacteria, cpSecY likely plays an analogous role. To explore the role of cpSecY, we obtained maize mutants with transposon insertions in the corresponding gene. Null cpSecY mutants exhibit a severe loss of thylakoid membrane, differing in this regard from mutants lacking cpSecA. Therefore, cpSecY function is not limited to a translocation step downstream of cpSecA. The phenotype of cpSecY mutants is also much more pleiotropic than that of double mutants in which both the cpSecA- and DeltapH-dependent thylakoid-targeting pathways are disrupted. Therefore, cpSecY function is likely to extend beyond any role it might play in these targeting pathways. CpSecY mutants also exhibit a defect in chloroplast translation, revealing a link between chloroplast membrane biogenesis and chloroplast gene expression.

Cloning and functional expression of AtCOQ3, the Arabidopsis homologue of the yeast COQ3 gene, encoding a methyltransferase from plant mitochondria involved in ubiquinone biosynthesis

A mutant of Saccharomyces cerevisiae deleted for the COQ3 gene was constructed. COQ3 encodes a 3,4-dihydroxy-5-hexaprenylbenzoate (DHHB) methyltransferase that catalyses the fourth step in the biosynthesis of ubiquinone from p-hydroxybenzoic acid. A full length cDNA encoding a homologue of DHHB-methyltransferase was cloned from an Arabidopsis thaliana cDNA library by functional complementation of a yeast coq3 deletion mutant. The Arabidopsis thaliana cDNA (AtCOQ3) was able to restore the respiration ability and ubiquinone synthesis of the mutant. The product of the 1372 bp cDNA contained 322 amino acids and had a molecular mass of 35,360 Da. The predicted amino acid sequence contained all consensus regions for S-adenosyl methionine methyltransferases and presented 26% identity with Saccharomyces cerevisiae DHHB-methyltransferase and 38% identity with the rat protein, as well as with a bacterial (Escherichia coli and Salmonella typhimurium) methyltransferase encoded by the UBIG gene. Southern analysis showed that the Arabidopsis thaliana enzyme was encoded by a single nuclear gene. The NH2-terminal part of the cDNA product contained features consistent with a putative mitochondrial transit sequence. The cDNA in Escherichia coli was overexpressed and antibodies were raised against the recombinant protein. Western blot analysis of Arabidopsis thaliana and pea protein extracts indicated that the AtCOQ3 gene product is localized within mitochondrial membranes. This result suggests that at least this step of ubiquinone synthesis takes place in mitochondria.

Low density membranes are associated with RNA-binding proteins and thylakoids in the chloroplast of Chlamydomonas reinhardtii

Chloroplast subfractions were tested with a UV cross-linking assay for proteins that bind to the 5' untranslated region of the chloroplast psbC mRNA of the green alga Chlamydomonas reinhardtii. These analyses revealed that RNA-binding proteins of 30-32, 46, 47, 60, and 80 kD are associated with chloroplast membranes. The buoyant density and the acyl lipid composition of these membranes are compatible with their origin being the inner chloroplast envelope membrane. However, unlike previously characterized inner envelope membranes, these membranes are associated with thylakoids. One of the membrane-associated RNA-binding proteins appears to be RB47, which has been reported to be a specific activator of psbA mRNA translation. These results suggest that translation of chloroplast mRNAs encoding thylakoid proteins occurs at either a subfraction of the chloroplast inner envelope membrane or a previously uncharacterized intra-chloroplast compartment, which is physically associated with thylakoids.

A re-examination in vivo of the phosphatidylcholine-galactolipid metabolic relationship during plant lipid biosynthesis

It remains unclear how and in what form the lipids synthesized in plant endoplasmic reticulum are exported to chloroplasts and used as precursors for the biosynthesis of plastid galactolipids, which are the most abundant lipids on Earth. Neither the mechanism of transfer nor the nature of the lipids imported into plastids has been elucidated. To characterize events occurring in vivo, the labelling of lipids from 15-day-old leek seedlings (Allium porrum, var. furor) was studied using pulse-chase experiments. During the chase, a substantial decline in the radioactivity incorporated into phosphatidylcholine (and not in other phospholipids) was accompanied by an increase in the label found in galactolipids. The positional distribution of labelled fatty acids in phosphatidylcholine and galactolipids was further studied as a function of the chase time; whereas phosphatidylcholine was preferentially labelled at the sn-2 position, the increase in radioactivity in galactolipids mainly concerned the sn-1 position. These results strongly suggest that the diacylglycerol moiety of phosphatidylcholine might not be integrated as a whole in the galactolipid.

Isolation and characterization of an amino acid-selective channel protein present in the chloroplastic outer envelope membrane

The reconstituted pea chloroplastic outer envelope protein of 16 kDa (OEP16) forms a slightly cation-selective, high-conductance channel with a conductance of Lambda = 1,2 nS (in 1 M KCl). The open probability of OEP16 channel is highest at 0 mV (Popen = 0.8), decreasing exponentially with higher potentials. Transport studies using reconstituted recombinant OEP16 protein show that the OEP16 channel is selective for amino acids but excludes triosephosphates or uncharged sugars. Crosslinking indicates that OEP16 forms a homodimer in the membrane. According to its primary sequence and predicted secondary structure, OEP16 shows neither sequence nor structural homologies to classical porins. The results indicate that the intermembrane space between the two envelope membranes might not be as freely accessible as previously thought.

Membrane lipids of Rhodopseudomonas viridis

In search of the precyanobacterial origin of the typical thylakoid lipids found in cyanobacteria and chloroplasts, we analyzed the polar lipids of the anaerobic phototrophic bacterium Rhodopseudomonas viridis. Glycolipids (monogalactosyl-, digalactosyl- and glucuronosyl diacylglycerol), phospholipids (phosphatidyl choline, -ethanolamine, -glycerol and cardiolipin) and an ornithine lipid were isolated and identified by NMR (1H, 13C, 31P) and mass spectrometry. Positional distribution and pairing of fatty acids in molecular species show small, but significant differences between glyco- and phospholipids. In this context, a new enzymatic method is described for assigning the enantiomeric structure of the diacylglycerol moiety in glyco- and phospholipids. 14C-Labelling studies suggest that monogalactosyl diacylglycerol is formed by galactosylation of diacylglycerol as in chloroplasts and not by glucosylation followed by epimerization as in cyanobacteria. The two 1,6-linked galactopyranose residues of digalactosyl diacylglycerol are both in beta-linkage and thus differ from the corresponding chloroplast lipid with its alpha-beta-sequence. R. viridis does not contain the sulfolipid, and even phosphate starvation does not induce the synthesis of this most characteristic thylakoid lipid, which on the other hand is present in other anaerobic phototrophic bacteria.

A nuclear-coded chloroplastic inner envelope membrane protein uses a soluble sorting intermediate upon import into the organelle

The chloroplastic inner envelope protein of 110 kD (IEP110) is part of the protein import machinery in the pea. Different hybrid proteins were constructed to assess the import and sorting pathway of IEP110. The IEP110 precursor (pIEP110) uses the general import pathway into chloroplasts, as shown by the mutual exchange of presequences with the precursor of the small subunit of ribulose-1,5-bisphosphate carboxylase (pSSU). Sorting information to the chloroplastic inner envelope is contained in an NH2-proximal part of mature IEP110 (110N). The NH2-terminus serves to anchor the protein into the membrane. Large COOH-terminal portions of this protein (80-90 kD) are exposed to the intermembrane space in situ. Successful sorting and integration of IEP110 and the derived constructs into the inner envelope are demonstrated by the inaccessability of processed mature protein to the protease thermolysin but accessibility to trypsin, i.e., the imported protein is exposed to the intermembrane space. A hybrid protein consisting of the transit sequence of SSU, the NH2-proximal part of mature IEP110, and mature SSU (tpSSU-110N-mSSU) is completely imported into the chloroplast stroma, from which it can be recovered as soluble, terminally processed 110NmSSU. The soluble 110N-mSSU then enters a reexport pathway, which results not only in the insertion of 110N-mSSU into the inner envelope membrane, but also in the extrusion of large portions of the protein into the intermembrane space. We conclude that chloroplasts possess a protein reexport machinery for IEPs in which soluble stromal components interact with a membrane-localized translocation machinery.

In vitro interaction between a chloroplast transit peptide and chloroplast outer envelope lipids is sequence-specific and lipid class-dependent

Interaction of artificial lipid bilayers (liposomes) with the purified transit peptide (SS-tp) of the precursor form of the small subunit for ribulose-2,5-bisphosphate carboxylase/oxygenase (prSSU) has been studied using a vesicle-disruption assay (calcein dye release) and electron microscopy. Employing purified forms of Escherichia coli-expressed prSSU, mature small subunit, glutathione S-transferase-transit peptide fusion protein, and SS-tp in dye release studies demonstrated that lipid interaction is mediated primarily through the transit peptide. Using chemically synthesized peptides (20-mers), the lipid-interacting domain of the transit peptide was partially mapped to the C-terminal 20 amino acids of the transit peptide. Peptides corresponding to other regions of the transit peptide and control peptides promoted significantly less calcein release. Interaction between the transit peptide and the bilayer was very rapid and could not be resolved by stopped-flow fluorometry with a mixing time of <50 ms. Interaction between the peptides and bilayer was also lipid class-dependent. Disruption occurred only when the bilayer contained the galactolipid monogalactosyldiacylglycerol (MGDG). The extent of bilayer disruption directly correlated with the relative concentration of MGDG in the liposome, with maximum calcein release occurring in 20 mol % MGDG liposomes. Lipid bilayers with greater than 20 mol % MGDG could not be achieved as determined by calcein entrapment. Electron microscopy of the liposomes before and after addition of the transit peptide suggested that the transit peptide induced a dramatic reorganization of lipids. These results are discussed in light of a possible mechanism for the early steps in protein transport that may involve polymorphic changes in the envelope membrane organization to include localized non-bilayer HII structures.

The regulation of enzymes involved in chlorophyll biosynthesis

All living organisms contain tetrapyrroles. In plants, chlorophyll (chlorophyll a plus chlorophyll b) is the most abundant and probably most important tetrapyrrole. It is involved in light absorption and energy transduction during photosynthesis. Chlorophyll is synthesized from the intact carbon skeleton of glutamate via the C5 pathway. This pathway takes place in the chloroplast. It is the aim of this review to summarize the current knowledge on the biochemistry and molecular biology of the C5-pathway enzymes, their regulated expression in response to light, and the impact of chlorophyll biosynthesis on chloroplast development. Particular emphasis will be placed on the key regulatory steps of chlorophyll biosynthesis in higher plants, such as 5-aminolevulinic acid formation, the production of Mg(2+)-protoporphyrin IX, and light-dependent protochlorophyllide reduction.

Strategies for the sustainable production of new biodegradable polyesters in plants: a review

In this study we review relevant pathways with regard to the production of poly(3-hydroxyalkanoates) (PHA) with medium chain length monomers in higher plants. On the basis of what is known of the genetics and the biochemistry of PHA formation in bacteria, and of fatty acid metabolism in various organisms, a number of possibilities for PHA production in model plants and in economically important crop plants are listed. Along with the molecular biology of PHA synthesis and fatty acid metabolism, we discuss theoretical and environmental considerations, metabolic engineering strategies, and plant transformation systems.Key words: polyhydroxyalkanoate, fatty acid, starch, potato, Arabidopsis.

Movement of DNA across the chloroplast envelope: Implications for the transfer of promiscuous DNA

Little is known about the mechanistic basis for the movement of promiscuous nucleic acids across cell membranes. To address this problem we sought conditions that would permit the entry of plasmid DNA into isolated, intact pea chloroplasts. DNA uptake did not occur normally, but was induced by hypotonic treatments, by incubation with millimolar levels of Mg(2+), or by heat shock at 42 °C. These results are consistent with DNA movement being permitted by conditions that transiently alter the permeability of the chloroplast envelope. Plant cells are subject to osmotic tensions and/or conditions inducing polymorphic changes in the membranes, such as those used in the present study, under several environmental stresses. In an evolutionary time frame, these phenomena may provide a mechanism for the transfer of promiscuous nucleic acids between organelles.

A new pathway for the synthesis of the plant sulpholipid, sulphoquinovosyldiacylglycerol

A new pathway is proposed for the biosynthesis of the plant sulpholipid, sulphoquinovosyldiacylglycerol. The pathway begins at UDP-glucose and involves the formation therefrom of UDP-4-ketoglucose-5-ene to which is subsequently added sulphite (or its metabolic equivalent). Evidence consistent with this pathway, rather than with the previously proposed 'sulphoglycolytic' route, was obtained from experiments with pea chloroplast preparations. The evidence included the failure of potential inhibitors of the sulphoglycolytic pathway to alter the rate of synthesis of sulpholipid and the stimulation of the incorporation of 35SO4(2-) into the latter by UTP. Radioactivity was effectively incorporated into sulpholipid from UDP-[14C]glucose and this radiolabelling was stimulated by the addition of methyl alpha-glucose-enide or of an enzyme system known to be forming (although not accumulating) UDP-4-ketoglucose-5-ene. This new pathway is also consistent with other data in the literature.

Acyl-lipid desaturases and their importance in the tolerance and acclimatization to cold of cyanobacteria

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Enzymatic product formation impairs both the chloroplast receptor-binding function as well as translocation competence of the NADPH: protochlorophyllide oxidoreductase, a nuclear-encoded plastid precursor protein

The key enzyme of chlorophyll biosynthesis in higher plants, the light-dependent NADPH:protochlorophyllide oxidoreductase (POR, EC 1.6.99.1), is a nuclear-encoded plastid protein. Its posttranslational transport into plastids of barley depends on the intraplastidic availability of one of its substrates, protochlorophyllide (PChlide). The precursor of POR (pPOR), synthesized from a corresponding full-length barley cDNA clone by coupling in vitro transcription and translation, is enzymatically active and converts PChlide to chlorophyllide (Chlide) in a light- and NADPH-dependent manner. Chlorophyllide formed catalytically remains tightly but noncovalently bound to the precursor protein and stabilizes a transport-incompetent conformation of pPOR. As shown by in vitro processing experiments, the chloroplast transit peptide in the Chlide-pPOR complex appears to be masked and thus is unable to physically interact with the outer plastid envelope membrane. In contrast, the chloroplast transit peptide in the naked pPOR (without its substrates and its product attached to it) and in the pPOR-substrate complexes, such as pPOR-PChlide or pPOR-PChlide-NADPH, seems to react independently of the mature region of the polypeptide, and thus is able to bind to the plastid envelope. When envelope-bound pPOR-PChlide-NADPH complexes were exposed to light during a short preincubation, the enzymatically produced Chlide slowed down the actual translocation step, giving rise to the sequential appearance of two partially processed translocation intermediates. However, ongoing translocation induced by feeding the chloroplasts delta-aminolevulinic acid, a precursor of PChlide, was able to override these two early blocks in translocation, suggesting that the plastid import machinery has a substantial capacity to denature a tightly folded, envelope-bound precursor protein. Together, our results show that pPOR with Chlide attached to it is impaired both in the ATP-dependent step of binding to a receptor protein component of the outer chloroplast envelope membrane, as well as in the PChlide-dependent step of precursor translocation.

Import of a new chloroplast inner envelope protein is greatly stimulated by potassium phosphate

A cDNA clone encoding a major chloroplast inner envelope membrane protein of 96 kDa (IEP96) was isolated and characterized. The protein is synthesized as a larger-molecular-weight precursor (pIEP96) which contains a cleavable N-terminal transit sequence of 50 amino acids. The transit peptide exhibits typical stromal targeting information. It is cleaved in vitro by the stromal processing peptidase, though the mature protein is clearly localized in the inner envelope membrane. Translocation of pIEP96 into chloroplasts is greatly stimulated in the presence of 80 mM potassium phosphate which results in an import efficiency of about 90%. This effect is specific for potassium and phosphate, but cannot be ascribed to a membrane potential across the inner envelope membrane. Protein sequence analysis reveals five stretches of repeats of 26 amino acids in length. The N-terminal 300 amino acids are 45% identical (76% similarity) to the 35 kDa alpha-subunit of acetyl-CoA carboxyl-transferase from Escherichia coli. The C-terminal 500 amino acids share significant similarity (69%) with USOI, a component of the cytoskeleton in yeast.

(Galacto) lipid export from envelope to thylakoid membranes in intact chloroplasts. II. A general process with a key role for the envelope in the establishment of lipid asymmetry in thylakoid membranes

The transfer of organelle of newly synthesized lipid molecules from inner envelope to thylakoid membranes, as well as their subsequent transbilayer distribution in these membranes, have been studied in intact chloroplasts isolated from young and mature spinach, young pea and mature lettuce leaves, using a recently developed methodology (Rawyler, A., Meylan, M. and Siegenthaler, P.A. (1992) Biochim. Biophys. Acta 1104, 331-341). Three radiolabelled precursors were used. UDP-[14C]galactose allowed to follow the fate of mono- and digalactosyldiacylglycerol (MGDG and DGDG) made from polyunsaturated, preexisting diacylglycerol (DAG), whereas [14C]acetate and [14C]glycerol 3-phosphate were used to follow the fate of MGDG and phosphatidylglycerol (PG), respectively, after de novo synthesis. MGDG, DGDG and PG molecules assembled at the envelope level were found to be exportable to thylakoids in amounts strictly proportional to the amounts synthesized, provided that the necessary substrates were not limiting. Lipid export was class-selective; under our conditions, as much as 50-80% of the MGDG, 87% of the PG and 20-30% of the DGDG synthesized were exported to thylakoids. However, within the MGDG class labelled from [14C]acetate, there was hardly any selectivity in the export of its various molecular species. For MGDG, the proportionality coefficient, which reflects the efficiency of the export process, was higher in chloroplasts from young than from mature leaves, and higher in spinach than in pea and lettuce. Temperature affected the efficiency of galactolipid export in a class-dependent way. MGDG synthesis and export had similar Q10 values of about 4 in young and 3 in mature spinach leaves, while the Q10 of DGDG export was higher than that of its synthesis. In most cases, the transmembrane distribution of labelled lipids in thylakoids was found to match closely the corresponding distribution of mass, regardless of plant age and species and of incubation time and temperature. In some cases however, small but significant differences occurred between the label and the mass transbilayer distributions of MGDG (labelled molecules more inwardly oriented), DGDG and PG (more outwardly oriented). We propose a general model in which the thylakoid lipid asymmetry is primarily preestablished in the chloroplast envelope by the topography of its lipid-synthesizing enzymes, together with the occurrence of relatively fast lateral diffusion and translocation rates of the newly synthesized lipids. Transient fusions between inner envelope and thylakoid membranes would allow lipid export by lateral diffusion and build the observed lipid asymmetry in the latter.

Molecular cloning of a chloroplastic protein associated with both the envelope and thylakoid membranes

Chloroplasts consist of six morphologically distinct compartments. Each compartment has a specific set of polypeptides that perform distinct biochemical functions. We report here the identification of a membrane-associated protein with a novel localization. This protein was synthesized as a 37 kDa precursor and was processed to a mature protein of 30 kDa after being imported into isolated pea chloroplasts. Fractionation of chloroplasts showed that the 30 kDa mature protein was associated with both of the envelope membranes as well as with thylakoid membranes. Immunocyto-chemical localization of the 30 kDa protein revealed that the protein occurred in clusters in the vicinity of both the envelope and the thylakoid. Possible functions of this 30 kDa protein, inferred from its novel localization pattern, are discussed.

Biogenesis of thylakoid membranes with emphasis on the process in Chlamydomonas

Recent results obtained by electron microscopic and biochemical analyses of greening Chlamydomonas reinhardtii y1 suggest that localized expansion of the plastid envelope is involved in thylakoid biogenesis. Kinetic analyses of the assembly of light-harvesting complexes and development of photosynthetic function when degreened cells of the alga are exposed to light suggest that proteins integrate into membrane at the level of the envelope. Current information, therefore, supports the earlier conclussion that the chloroplast envelope is a major biogenic structure, from which thylakoid membranes emerge. Chloroplast development in Chlamydomonas provides unique opportunities to examine in detail the biogenesis of thylakoids.

Expression of the triose phosphate translocator gene from potato is light dependent and restricted to green tissues

The export of primary photosynthesis products from chloroplasts into the cytoplasm is mediated by the triose phosphate translocator. The transporter is an integral membrane protein localized at the inner envelope of chloroplasts. In order to study the expression of the major chloroplast envelope protein gene E29, which is assumed to function as the translocator, we have isolated corresponding cDNA clones from potato. A full-length clone was sequenced and shown to be highly homologous to the E29 gene from spinach. Expression on the RNA level is restricted to green tissues, is light dependent and cannot be induced by sucrose in darkness. The presence of a single-copy gene argues for the existence of different translocator systems responsible for import and export of carbohydrates in chloroplasts and amyloplasts.