A method for isolating a high yield of Arabidopsis chloroplasts capable of efficient import of precursor proteins

Similar Protein Phosphatases Control Starch Metabolism in Plants and Glycogen Metabolism in Mammals

We report that protein phosphorylation is involved in the control of starch metabolism in Arabidopsis leaves at night. sex4 (starch excess 4) mutants, which have strongly reduced rates of starch metabolism, lack a protein predicted to be a dual specificity protein phosphatase. We have shown that this protein is chloroplastic and can bind to glucans and have presented evidence that it acts to regulate the initial steps of starch degradation at the granule surface. Remarkably, the most closely related protein to SEX4 outside the plant kingdom is laforin, a glucan-binding protein phosphatase required for the metabolism of the mammalian storage carbohydrate glycogen and implicated in a severe form of epilepsy (Lafora disease) in humans.

Evidence for distinct mechanisms of starch granule breakdown in plants

The aim of this work was to understand the initial steps of starch breakdown inside chloroplasts. In the non-living endosperm of germinating cereal grains, starch breakdown is initiated by alpha-amylase secreted from surrounding cells. However, loss of alpha-amylase from Arabidopsis does not prevent chloroplastic starch breakdown (Yu, T.-S., Zeeman, S. C., Thorneycroft, D., Fulton, D. C., Dunstan, H., Lue, W.-L., Hegemann, B., Tung, S.-Y., Umemoto, T., Chapple, A., Tsai, D.-L., Wang, S.-M, Smith, A. M., Chen, J., and Smith, S. M. (2005) J. Biol. Chem. 280, 9773-9779), implying that other enzymes must attack the starch granule. Here, we present evidence that the debranching enzyme isoamylase 3 (ISA3) acts at the surface of the starch granule. Atisa3 mutants have more leaf starch and a slower rate of starch breakdown than wild-type plants. The amylopectin of Atisa3 contains many very short branches and ISA3-GFP localizes to granule-like structures inside chloroplasts. We suggest that ISA3 removes short branches from the granule surface. To understand how some starch is still degraded in Atisa3 mutants we eliminated a second debranching enzyme, limit dextrinase (pullulanase-type). Atlda mutants are indistinguishable from the wild type. However, the Atisa3/Atlda double mutant has a more severe starch-excess phenotype and a slower rate of starch breakdown than Atisa3 single mutants. The double mutant accumulates soluble branched oligosaccharides (limit dextrins) that are undetectable in the wild-type and the single mutants. Together these results suggest that glucan debranching occurs primarily at the granule surface via ISA3, but in its absence soluble branched glucans are debranched in the stroma via limit dextrinase. Consistent with this model, chloroplastic alpha-amylase AtAMY3, which could release soluble branched glucans, is induced in Atisa3 and in the Atisa3/Atlda double mutant.

A Salvage Pathway for Phytol Metabolism in Arabidopsis

Chlorophyll is the most abundant photosynthetic pigment in higher plants. During senescence, chlorophyll is hydrolyzed, resulting in the release of free phytol and chlorophyllide. Although the degradation of chlorophyllide has been studied in depth, the metabolic fate of phytol in plants is less clear. Here, we provide evidence that phytol can be incorporated into chlorophyll, tocopherol, and lipid esters by Arabidopsis seedlings. Phytol is phosphorylated to phytyl-phosphate and phytyl-diphosphate by two successive kinase activities associated with chloroplast envelope membranes of Arabidopsis. Although phytol kinase is CTP-dependent, the second kinase reaction, phytyl-phosphate kinase, shows broader specificity for CTP, GTP, UTP, and ATP. Therefore, in addition to de novo synthesis from geranylgeranyl-diphosphate, phosphorylation of free phytol represents an alternative route for phytyl-diphosphate production as the precursor for chloroplast prenyl lipid synthesis. Lipid esters are produced after feeding phytol to Arabidopsis seedlings, and they also accumulate in large amounts in leaves during senescence. The predominant phytyl ester that accumulates during senescence is hexadecatrienoic acid phytyl ester. Fatty acid phytyl ester synthesis by protein extracts of Arabidopsis is stimulated in the presence of phytol- and acyl-CoA esters. Thus, Arabidopsis contains a distinct enzymatic machinery for redirecting free phytol released from chlorophyll degradation into chloroplast lipid metabolism.

MscS-like Proteins Control Plastid Size and Shape in Arabidopsis thaliana

BACKGROUND

Mechanosensitive (MS) ion channels provide a mechanism for the perception of mechanical stimuli such as sound, touch, and osmotic pressure. The bacterial MS ion channel MscS opens in response to increased membrane tension and serves to protect against cellular lysis during osmotic downshock. MscS-like proteins are found widely in bacterial and archaeal species and have also been identified in fission yeast and plants. None of the eukaryotic members of the family have yet been characterized.

RESULTS

Here, we characterize two MscS-like (MSL) proteins from Arabidopsis thaliana, MSL2 and MSL3. MSL3 can rescue the osmotic-shock sensitivity of a bacterial mutant lacking MS-ion-channel activity, suggesting that it functions as a mechanosensitive ion channel. Arabidopsis plants harboring insertional mutations in both MSL3 and MSL2 show abnormalities in the size and shape of plastids, which are plant-specific endosymbiotic organelles responsible for photosynthesis, gravity perception, and numerous metabolic reactions. MSL2-GFP and MSL3-GFP are localized to discrete foci on the plastid envelope and colocalize with the plastid division protein AtMinE.

CONCLUSIONS

Our data support a model wherein MSL2 and MSL3 control plastid size, shape, and perhaps division during normal plant development by altering ion flux in response to changes in membrane tension. We propose that MscS family members have evolved new roles in plants since the endosymbiotic event that gave rise to plastids.

A molecular-genetic study of the Arabidopsis Toc75 gene family

Toc75 (translocon at the outer envelope membrane of chloroplasts, 75 kD) is the protein translocation channel at the outer envelope membrane of plastids and was first identified in pea (Pisum sativum) using biochemical approaches. The Arabidopsis (Arabidopsis thaliana) genome contains three Toc75-related sequences, termed atTOC75-I, atTOC75-III, and atTOC75-IV, which we studied using a range of molecular, genetic, and biochemical techniques. Expression of atTOC75-III is strongly regulated and at its highest level in young, rapidly expanding tissues. By contrast, atTOC75-IV is expressed uniformly throughout development and at a much lower level than atTOC75-III. The third sequence, atTOC75-I, is a pseudogene that is not expressed due to a gypsy/Ty3 transposon insertion in exon 1, and numerous nonsense, frame-shift, and splice-junction mutations. The expressed genes, atTOC75-III and atTOC75-IV, both encode integral envelope membrane proteins. Unlike atToc75-III, the smaller atToc75-IV protein is not processed upon targeting to the envelope, and its insertion does not require ATP at high concentrations. The atTOC75-III gene is essential for viability, since homozygous atToc75-III knockout mutants (termed toc75-III) could not be identified, and aborted seeds were observed at a frequency of approximately 25% in the siliques of self-pollinated toc75-III heterozygotes. Homozygous toc75-III embryos were found to abort at the two-cell stage. Homozygous atToc75-IV knockout plants (termed toc75-IV) displayed no obvious visible phenotypes. However, structural abnormalities were observed in the etioplasts of toc75-IV seedlings and atTOC75-IV overexpressing lines, and toc75-IV plants were less efficient at deetiolation than wild type. These results suggest some role for atToc75-IV during growth in the dark.

Arabidopsis tic110 is essential for the assembly and function of the protein import machinery of plastids

The translocon at the inner envelope membrane of chloroplasts (Tic) plays a central role in plastid biogenesis by coordinating the sorting of nucleus-encoded preproteins across the inner membrane and coordinating the interactions of preproteins with the processing and folding machineries of the stroma. Despite these activities, the precise roles of known Tic proteins in translocation, sorting, and preprotein maturation have not been defined. In this report, we examine the in vivo function of a major Tic component, Tic110. We demonstrate that Arabidopsis thaliana Tic110 (atTic110) is essential for plastid biogenesis and plant viability. The downregulation of atTic110 expression results in the reduced accumulation of a wide variety of plastid proteins. The expression of dominant negative mutants of atTic110 disrupts assembly of Tic complexes and the translocation of preproteins across the inner envelope membrane. Together, these data suggest that Tic110 plays a general role in the import of nuclear-encoded preproteins as a common component of Tic complexes.

α-Amylase Is Not Required for Breakdown of Transitory Starch in Arabidopsis Leaves

The Arabidopsis thaliana genome encodes three alpha-amylase-like proteins (AtAMY1, AtAMY2, and AtAMY3). Only AtAMY3 has a predicted N-terminal transit peptide for plastidial localization. AtAMY3 is an unusually large alpha-amylase (93.5 kDa) with the C-terminal half showing similarity to other known alpha-amylases. When expressed in Escherichia coli, both the whole AtAMY3 protein and the C-terminal half alone show alpha-amylase activity. We show that AtAMY3 is localized in chloroplasts. The starch-excess mutant of Arabidopsis sex4, previously shown to have reduced plastidial alpha-amylase activity, is deficient in AtAMY3 protein. Unexpectedly, T-DNA knock-out mutants of AtAMY3 have the same diurnal pattern of transitory starch metabolism as the wild type. These results show that AtAMY3 is not required for transitory starch breakdown and that the starch-excess phenotype of the sex4 mutant is not caused simply by deficiency of AtAMY3 protein. Knock-out mutants in the predicted non-plastidial alpha-amylases AtAMY1 and AtAMY2 were also isolated, and these displayed normal starch breakdown in the dark as expected for extraplastidial amylases. Furthermore, all three AtAMY double knock-out mutant combinations and the triple knock-out degraded their leaf starch normally. We conclude that alpha-amylase is not necessary for transitory starch breakdown in Arabidopsis leaves.

Evidence That the Plastid Translocon Tic40 Components Possess Modulating Capabilities

The transport of proteins into the plastid is a process that faces changing cellular needs such as the situation found in different plant organs or developing tissues. The plastid translocon must therefore be responsive to the changing cell environment to deliver efficiently different arrays of structurally diverse proteins. Although the Tic40-related envelope proteins appear to be translocon components designed to address the varying needs of protein translocation, details of their involvement remain elusive. This study was thus designed to combine plant-based experiments and yeast mitochondrion-based approaches for unveiling clues related to how the Tic40 components may behave during the protein translocation process. The main findings related to how Tic40 proteins may work are: 1) natural fluctuations are apparent in developing tissues, in different organs of the same plant, and in different species; 2) transgenic Arabidopsis seedlings can tolerate functionally a wide range of variations in Tic40 levels, from partial suppression to excessive production; 3) the Tic40 proteins themselves exhibit configurational changes in their association with yeast mitochondria in response to different carbon sources; 4) the presence of Tic40 proteins in yeast mitochondria influences regulatory aspects of the mitochondrial translocon; and 5) the Tic40 proteins associate with mitochondrial translocon components involved in regulatory-like events. The combined data provide evidence that Tic40 proteins possess modulating capabilities.

Toc12, a novel subunit of the intermembrane space preprotein translocon of chloroplasts

Translocation of proteins across membranes is essential for the biogenesis of each cell and is achieved by proteinaceous complexes. We analyzed the translocation complex of the intermembrane space from chloroplasts and identified a 12-kDa protein associated with the Toc machinery. Toc12 is an outer envelope protein exposing a soluble domain into the intermembrane space. Toc12 contains a J-domain and stimulates the ATPase activity of DnaK. The conformational stability and the ability to stimulate Hsp70 are dependent on a disulfide bridge within the loop region of the J-domain, suggesting a redox-regulated activation of the chaperone. Toc12 is associated with Toc64 and Tic22. Its J-domain recruits the Hsp70 of outer envelope membrane to the intermembrane space translocon and facilitates its interaction to the preprotein.

A stromal Hsp100 protein is required for normal chloroplast development and function in Arabidopsis

Molecular chaperones are required for the translocation of many proteins across organellar membranes, presumably by providing energy in the form of ATP hydrolysis for protein movement. In the chloroplast protein import system, a heat shock protein 100 (Hsp100), known as Hsp93, is hypothesized to be the chaperone providing energy for precursor translocation, although there is little direct evidence for this hypothesis. To learn more about the possible function of Hsp93 during protein import into chloroplasts, we isolated knockout mutant lines that contain T-DNA disruptions in either atHSP93-V or atHSP93-III, which encode the two Arabidopsis (Arabidopsis thaliana) homologs of Hsp93. atHsp93-V mutant plants are much smaller and paler than wild-type plants. In addition, mutant chloroplasts contain less thylakoid membrane when compared to the wild type. Plastid protein composition, however, seems to be largely unaffected in atHsp93-V knockout plants. Chloroplasts isolated from the atHsp93-V knockout mutant line are still able to import a variety of precursor proteins, but the rate of import of some of these precursors is significantly reduced. These results indicate that atHsp93-V has an important, but not essential, role in the biogenesis of Arabidopsis chloroplasts. In contrast, knockout mutant plants for atHsp93-III, the second Arabidopsis Hsp93 homolog, had a visible phenotype identical to the wild type, suggesting that atHsp93-III may not play as important a role as atHsp93-V in chloroplast development and/or function.

FRD3 controls iron localization in Arabidopsis

The frd3 mutant of Arabidopsis exhibits constitutive expression of its iron uptake responses and is chlorotic. These phenotypes are consistent with defects either in iron deficiency signaling or in iron translocation and localization. Here we present several experiments demonstrating that a functional FRD3 gene is necessary for correct iron localization in both the root and shoot of Arabidopsis plants. Reciprocal grafting experiments with frd3 and wild-type Arabidopsis plants reveal that the phenotype of a grafted plant is determined by the genotype of the root, not by the genotype of the shoot. This indicates that FRD3 function is root-specific and points to a role for FRD3 in delivering iron to the shoot in a usable form. When grown under certain conditions, frd3 mutant plants overaccumulate iron in their shoot tissues. However, we demonstrate by direct measurement of iron levels in shoot protoplasts that intracellular iron levels in frd3 are only about one-half the levels in wild type. Histochemical staining for iron reveals that frd3 mutants accumulate high levels of ferric iron in their root vascular cylinder, the same tissues in which the FRD3 gene is expressed. Taken together, these results clearly indicate a role for FRD3 in iron localization in Arabidopsis. Specifically, FRD3 is likely to function in root xylem loading of an iron chelator or other factor necessary for efficient iron uptake out of the xylem or apoplastic space and into leaf cells.

The chloroplastic protein translocation channel Toc75 and its paralog OEP80 represent two distinct protein families and are targeted to the chloroplastic outer envelope by different mechanisms

Toc75 is postulated to form the protein translocation channel in the chloroplastic outer envelope membrane. Proteins homologous to Toc75 are present in a wide range of organisms, with the closest homologs occurring in cyanobacteria. Therefore, an endosymbiotic origin of Toc75 has been postulated. Recently, a gene encoding a paralog to Toc75 was identified in Arabidopsis and its product was named atToc75-V. In the present study, we characterized this new Toc75 paralog, and investigated extensively the relationships among Toc75 homologs from higher plants and bacteria in order to gain insights into the evolutionary origin of the chloroplastic protein translocation channel. First, we found that the native molecular weight of atToc75-V is 80 kDa and renamed it (AtOEP80) Arabidopsis thalianaouter envelope protein of 80 kDa. Second, we found that AtOEP80 and Toc75 utilize different mechanisms for their targeting to the chloroplastic envelope. Toc75 is directed with a cleavable bipartite transit peptide partly via the general import pathway, whereas AtOEP80 contains the targeting information within its mature sequence, and its targeting is independent of the general pathway. Third, we undertook phylogenetic analyses of Toc75 homologs from various organisms, and found that Toc75 and OEP80 represent two independent gene families that are most likely derived from cyanobacterial sequences. Our results suggest that Toc75 and OEP80 diverged early in the evolution of plastids from their common ancestor with modern cyanobacteria.

Characterization of Arabidopsis glutamine phosphoribosyl pyrophosphate amidotransferase-deficient mutants

Using a transgene-based screening, we previously isolated several Arabidopsis mutants defective in protein import into chloroplasts. Positional cloning of one of the loci, CIA1, revealed that CIA1 encodes Gln phosphoribosyl pyrophosphate amidotransferase 2 (ATase2), one of the three ATase isozymes responsible for the first committed step of de novo purine biosynthesis. The cia1 mutant had normal green cotyledons but small and albino/pale-green mosaic leaves. Adding AMP, but not cytokinin or NADH, to plant liquid cultures partially complemented the mutant phenotypes. Both ATase1 and ATase2 were localized to chloroplasts. Overexpression of ATase1 fully complemented the ATase2-deficient phenotypes. A T-DNA insertion knockout mutant of the ATase1 gene was also obtained. The mutant was indistinguishable from the wild type. A double mutant of cia1/ATase1-knockout had the same phenotype as cia1, suggesting at least partial gene redundancy between ATase1 and ATase2. Characterizations of the cia1 mutant revealed that mutant leaves had slightly smaller cell size but only half the cell number of wild-type leaves. This phenotype confirms the role of de novo purine biosynthesis in cell division. Chloroplasts isolated from the cia1 mutant imported proteins at an efficiency less than 50% that of wild-type chloroplasts. Adding ATP and GTP to isolated mutant chloroplasts could not restore the import efficiency. We conclude that de novo purine biosynthesis is not only important for cell division, but also for chloroplast biogenesis.

A mutation of the CRUMPLED LEAF gene that encodes a protein localized in the outer envelope membrane of plastids affects the pattern of cell division, cell differentiation, and plastid division in Arabidopsis

We identified a novel mutation of a nuclear-encoded gene, designated as CRUMPLED LEAF (CRL), of Arabidopsis thaliana that affects the morphogenesis of all plant organs and division of plastids. Histological analysis revealed that planes of cell division were distorted in shoot apical meristems (SAMs), root tips, and embryos in plants that possess the crl mutation. Furthermore, we observed that differentiation patterns of cortex and endodermis cells in inflorescence stems and root endodermis cells were disturbed in the crl mutant. These results suggest that morphological abnormalities observed in the crl mutant were because of aberrant cell division and differentiation. In addition, cells of the crl mutant contained a reduced number of enlarged plastids, indicating that the division of plastids was inhibited in the crl. The CRL gene encodes a novel protein with a molecular mass of 30 kDa that is localized in the plastid envelope. The CRL protein is conserved in various plant species, including a fern, and in cyanobacteria, but not in other organisms. These data suggest that the CRL protein is required for plastid division, and it also plays an important role in cell differentiation and the regulation of the cell division plane in plants. A possible function of the CRL protein is discussed.

Uptake of a fluorescent dye as a swift and simple indicator of organelle intactness: import-competent chloroplasts from soil-grown Arabidopsis

We developed a rapid and reliable technique for specifically staining intact chloroplasts using the fluorescent dye carboxyfluorescein diacetate. Intact, import-competent chloroplasts were isolated simply and rapidly from soil-grown Arabidopsis thaliana plants, with yields of 20 +/- 5 micro g chlorophyll per g FW, greater than previously reported yields from soil-grown Arabidopsis. Traditional chloroplast isolation buffers sometimes contain low concentrations (<10 mM) sodium ascorbate as a general-purpose anti-oxidant, but we found that only Arabidopsis chloroplasts isolated in the presence of high concentrations (50-100 mM) of sodium ascorbate in the initial grinding buffer were import-competent.

An outer envelope membrane component of the plastid protein import apparatus plays an essential role in Arabidopsis

Translocon at the outer envelope membrane of chloroplasts, 34 kDa (Toc34) is a GTP-binding component of the protein import apparatus within the outer envelope membrane of plastids. The Arabidopsis genome encodes two homologues of Toc34, designated atToc33 and atToc34. In this report, we describe the identification and characterization of two atToc34 knockout mutants, plastid protein import 3-1 (ppi3-1) and ppi3-2. Aerial tissues of the ppi3 mutants appeared similar to the wild type throughout development, and contained structurally normal chloroplasts that were able to efficiently import the Rubisco small subunit precursor (prSS) in vitro. The absence of an obvious ppi3 phenotype in green tissues presumably reflects the ability of atToc33 to substitute for atToc34 in the mutant, and the relatively high level of expression of the atTOC33 gene in these tissues. In the roots, where atTOC33 is expressed at a much lower level, significant growth defects were observed in both mutants: ppi3 roots were approximately 20-30% shorter than wild-type roots. Attempts to identify a double homozygote lacking atToc34 and atToc33 (by crossing the ppi3 mutants with ppi1, an atToc33 knockout mutant) were unsuccessful, indicating that the function provided by atToc33/atToc34 is essential during early development. Plants that were homozygous for ppi1 and heterozygous for ppi3 displayed a chlorotic phenotype much more severe than that of the ppi1 single mutant. Furthermore, the siliques of these plants contained approximately 25% aborted seeds, indicating that the double homozygous mutation is embryo lethal. The data demonstrate that atToc33/atToc34 performs a central and essential role during plastid protein import, and indicate that the atToc34 isoform is relatively more important for plastid biogenesis in roots.

A cytosolic glucosyltransferase is required for conversion of starch to sucrose in Arabidopsis leaves at night

Maltose is exported from the Arabidopsis chloroplast as the main product of starch degradation at night. To investigate its fate in the cytosol, we characterised plants with mutations in a gene encoding a putative glucanotransferase (disproportionating enzyme; DPE2), a protein similar to the maltase Q (MalQ) gene product involved in maltose metabolism in bacteria. Use of a DPE2 antiserum revealed that the DPE2 protein is cytosolic. Four independent mutant lines lacked this protein and displayed a decreased capacity for both starch synthesis and starch degradation in leaves. They contained exceptionally high levels of maltose, and elevated levels of glucose, fructose and other malto-oligosaccharides. Sucrose levels were lower than those in wild-type plants, especially at the start of the dark period. A glucosyltransferase activity, capable of transferring one of the glucosyl units of maltose to glycogen or amylopectin and releasing the other, was identified in leaves of wild-type plants. Its activity was sufficient to account for the rate of starch degradation. This activity was absent from dpe2 mutant plants. Based on these results, we suggest that DPE2 is an essential component of the pathway from starch to sucrose and cellular metabolism in leaves at night. Its role is probably to metabolise maltose exported from the chloroplast. We propose a pathway for the conversion of starch to sucrose in an Arabidopsis leaf.

The roles of toc34 and toc75 in targeting the toc159 preprotein receptor to chloroplasts

The Toc complex at the outer envelope of chloroplasts initiates the import of nuclear-encoded preproteins from the cytosol into the organelle. The core of the Toc complex is composed of two receptor GTPases, Toc159 and Toc34, as well as Toc75, a beta-barrel membrane channel. Toc159 is equally distributed between a soluble cytoplasmic form and a membrane-inserted form, suggesting that assembly of the Toc complex is dynamic. In the present study, we used the Arabidopsis thaliana orthologs of Toc159 and Toc34, atToc159 and atToc33, respectively, to investigate the requirements for assembly of the trimeric Toc complex. In addition to its intrinsic GTPase activity, we demonstrate that integration of atToc159 into the Toc complex requires atToc33 GTPase activity. Additionally, we show that the interaction of the two GTPase domains stimulates association of the membrane anchor of atToc159 with the translocon. Finally, we employ reconstituted proteoliposomes to demonstrate that proper insertion of the receptor requires both Toc75 and Toc34. Collectively these data suggest that Toc34 and Toc75 act sequentially to mediate docking and insertion of Toc159 resulting in assembly of the functional translocon.

Proteomic study of the Arabidopsis thaliana chloroplastic envelope membrane utilizing alternatives to traditional two-dimensional electrophoresis

With the completion of the sequencing of the Arabidopsis genome and with the significant increase in the amount of other plant genome and expressed sequence tags (ESTs) data, plant proteomics is rapidly becoming a very active field. We have pursued a high-throughput mass spectrometry-based proteomics approach to identify and characterize membrane proteins localized to the Arabidopsis thaliana chloroplastic envelope membrane. In this study, chloroplasts were prepared from plate- or soil-grown Arabidopsis plants using a novel isolation procedure, and "mixed" envelopes were subsequently isolated using sucrose step gradients. We applied two alternative methodologies, off-line multidimensional protein identification technology (Off-line MUDPIT) and one-dimensional (1D) gel electrophoresis followed by proteolytic digestion and liquid chromatography coupled with tandem mass spectrometry (Gel-C-MS/MS), to identify envelope membrane proteins. This proteomic study enabled us to identify 392 nonredundant proteins.

Tic40, a membrane-anchored co-chaperone homolog in the chloroplast protein translocon

The function of Tic40 during chloroplast protein import was investigated. Tic40 is an inner envelope membrane protein with a large hydrophilic domain located in the stroma. Arabidopsis null mutants of the atTic40 gene were very pale green and grew slowly but were not seedling lethal. Isolated mutant chloroplasts imported precursor proteins at a lower rate than wild-type chloroplasts. Mutant chloroplasts were normal in allowing binding of precursor proteins. However, during subsequent translocation across the inner membrane, fewer precursors were translocated and more precursors were released from the mutant chloroplasts. Cross-linking experiments demonstrated that Tic40 was part of the translocon complex and functioned at the same stage of import as Tic110 and Hsp93, a member of the Hsp100 family of molecular chaperones. Tertiary structure prediction and immunological studies indicated that the C-terminal portion of Tic40 contains a TPR domain followed by a domain with sequence similarity to co-chaperones Sti1p/Hop and Hip. We propose that Tic40 functions as a co-chaperone in the stromal chaperone complex that facilitates protein translocation across the inner membrane.

A polyglycine stretch is necessary for proper targeting of the protein translocation channel precursor to the outer envelope membrane of chloroplasts

Toc75 is a protein translocation channel in the outer envelope membrane of chloroplasts and its presence is essential for the biogenesis of the organelles. Toc75 is the only protein identified so far in the outer membrane of chloroplasts or mitochondria that is synthesized as a larger precursor, preToc75, with a bipartite transit peptide. Its N-terminus targets the protein to the stroma and is removed by the stromal processing peptidase, whereas its C-terminus mediates envelope targeting and is removed by a yet unknown peptidase. Several conserved domains have been identified in the C-terminal portion of the preToc75 transit peptide from six plant species. We evaluated their importance in the biogenesis of Toc75 by means of deletion or site-directed mutagenesis, followed by import experiments using isolated chlroplasts. Among the conserved domains, a polyglycine stretch was found to be necessary for envelope targeting. Substitution of this domain with other stretches of a single amino acid such as alanine caused mistargeting of the protein into the stroma, indicating an important role for this domain. Furthermore, a glutamate at +2 and two alanine residues at -3 and -1 to the second cleavage site were found to be important for processing. A potential mechanism for the biogenesis of Toc75 is discussed.

Protein transport into secondary plastids and the evolution of primary and secondary plastids

Chloroplasts are key organelles in algae and plants due to their photosynthetic abilities. They are thought to have evolved from prokaryotic cyanobacteria taken up by a eukaryotic host cell in a process termed primary endocytobiosis. In addition, a variety of organisms have evolved by subsequent secondary endocytobioses, in which a heterotrophic host cell engulfed a eukaryotic alga. Both processes dramatically enhanced the complexity of the resulting cells. Since the first version of the endosymbiotic theory was proposed more than 100 years ago, morphological, physiological, biochemical, and molecular data have been collected substantiating the emerging picture about the origin and the relationship of individual organisms with different primary or secondary chloroplast types. Depending on their origin, plastids in different lineages may have two, three, or four envelope membranes. The evolutionary success of endocytobioses depends, among other factors, on the specific exchange of molecules between the host and endosymbiont. This raises questions concerning how targeting of nucleus-encoded proteins into the different plastid types occurs and how these processes may have developed. Most studies of protein translocation into plastids have been performed on primary plastids, but in recent years more complex protein-translocation systems of secondary plastids have been investigated. Analyses of transport systems in different algal lineages with secondary plastids reveal that during evolution existing translocation machineries were recycled or recombined rather than being developed de novo. This review deals with current knowledge about the evolution and function of primary and secondary plastids and the respective protein-targeting systems.

In vitro analysis of chloroplast protein import

This unit describes protocols for isolating chloroplasts from pea (Pisum sativum) and Arabidopsis thaliana for the study of nuclear-encoded plastid precursor proteins. Chloroplasts from both preparations are competent for the in vitro import of recombinant preproteins synthesized using in vitro translation systems derived from reticulocyte or wheat germ lysates. These assays can be used to test whether a particular protein is targeted to chloroplasts, for analyzing the suborganellar location of newly imported preproteins, or to study the mechanism of import itself.

The chlorate-resistant and photomorphogenesis-defective mutant cr88 encodes a chloroplast-targeted HSP90

The cr88 mutant of Arabidopsis is a novel chlorate-resistant mutant that displays long hypocotyls in red light, but not in far red or blue light, and is delayed in the greening process. In cotyledons and young leaves, plastids are less developed compared with those of the wild type. In addition, a subset of light-regulated genes are under-expressed in this mutant. To understand the pleiotropic phenotypes of cr88, we isolated the CR88 gene through map-based cloning. We found that CR88 encodes a chloroplast-targeted 90-kDa heat shock protein (HSP90). The CR88 gene is expressed at highest levels during early post-germination stages and in leaves and reproductive organs. It is constitutively expressed but is also light and heat shock inducible. Chloroplast import experiments showed that the protein is localized to the stroma compartment of the chloroplast. The possible function of an HSP90 in the chloroplast and a plausible explanation of the pleiotropic phenotypes observed in cr88 are discussed.

Essential role of the G-domain in targeting of the protein import receptor atToc159 to the chloroplast outer membrane

Two homologous GTP-binding proteins, atToc33 and atToc159, control access of cytosolic precursor proteins to the chloroplast. atToc33 is a constitutive outer chloroplast membrane protein, whereas the precursor receptor atToc159 also exists in a soluble, cytosolic form. This suggests that atToc159 may be able to switch between a soluble and an integral membrane form. By transient expression of GFP fusion proteins, mutant analysis, and biochemical experimentation, we demonstrate that the GTP-binding domain regulates the targeting of cytosolic atToc159 to the chloroplast and mediates the switch between cytosolic and integral membrane forms. Mutant atToc159, unable to bind GTP, does not reinstate a green phenotype in an albino mutant (ppi2) lacking endogenous atToc159, remaining trapped in the cytosol. Thus, the function of atToc159 in chloroplast biogenesis is dependent on an intrinsic GTP-regulated switch that controls localization of the receptor to the chloroplast envelope.

The targeting of the atToc159 preprotein receptor to the chloroplast outer membrane is mediated by its GTPase domain and is regulated by GTP

The multimeric translocon at the outer envelope membrane of chloroplasts (Toc) initiates the recognition and import of nuclear-encoded preproteins into chloroplasts. Two Toc GTPases, Toc159 and Toc33/34, mediate preprotein recognition and regulate preprotein translocation. Although these two proteins account for the requirement of GTP hydrolysis for import, the functional significance of GTP binding and hydrolysis by either GTPase has not been defined. A recent study indicates that Toc159 is equally distributed between a soluble cytoplasmic form and a membrane-inserted form, raising the possibility that it might cycle between the cytoplasm and chloroplast as a soluble preprotein receptor. In the present study, we examined the mechanism of targeting and insertion of the Arabidopsis thaliana orthologue of Toc159, atToc159, to chloroplasts. Targeting of atToc159 to the outer envelope membrane is strictly dependent only on guanine nucleotides. Although GTP is not required for initial binding, the productive insertion and assembly of atToc159 into the Toc complex requires its intrinsic GTPase activity. Targeting is mediated by direct binding between the GTPase domain of atToc159 and the homologous GTPase domain of atToc33, the Arabidopsis Toc33/34 orthologue. Our findings demonstrate a role for the coordinate action of the Toc GTPases in assembly of the functional Toc complex at the chloroplast outer envelope membrane.

A simple method for isolating import-competent Arabidopsis chloroplasts

We present a simple, rapid and low-cost method for isolating a high yield of Arabidopsis chloroplasts that can be used to study chloroplast protein import. Efficiency of chloroplast isolation was dependent upon the ratio between amount of plant tissue and the buffer volume, the size and speed of the homogenisation equipment, and the size of the homogenisation beaker. The import method proved useful when characterising different precursor proteins, developmental stages and import-defective mutants. Time-course experiments enabled the measurement of import rates in the linear range. Compared to protoplastation, this isolation method has significant time and cost savings (approximately 80% and approximately 95%, respectively), and yields chloroplasts with a higher capacity to import proteins.

The ABC transporter SpTUR2 confers resistance to the antifungal diterpene sclareol

PDR5-like proteins represent one group of the ABC superfamily of transporters. Members of this group are present in plants and, due to the function of PDR5-related proteins in fungi in the excretion of xenobiotics (including antifungal agents), it has been proposed that they might play a similar role in plants in the response to and detoxification of herbicides and fungicides. However, until now no functional data has been presented showing an altered plant response to any herbicide or fungicide as a result of manipulating the expression of a PDR5-like gene in plants. In this paper, we show that the plant SpTUR2 PDR5-like ABC transporter is localised to the plasma membrane and that expression of this protein in Arabidopsis leads to the acquisition of resistance to the diterpenoid antifungal agent sclareol. These data both define a possible endogenous substrate for this transporter and highlight the potential of manipulating plant chemical resistance via modulating the expression of specific PDR5-like transporters.

DGD2, an arabidopsis gene encoding a UDP-galactose-dependent digalactosyldiacylglycerol synthase is expressed during growth under phosphate-limiting conditions

The galactolipid digalactosyldiacylglycerol (DGDG), one of the main chloroplast lipids in higher plants, is believed to be synthesized by the galactolipid:galactolipid galactosyltransferase, which transfers a galactose moiety from one molecule of monogalactosyldiacylglycerol (MGDG) to another. Here, we report that Arabidopsis as well as other plant species contain two genes, DGD1 and DGD2, encoding enzymes with DGDG synthase activity. Using MGDG and UDP-galactose as substrates for in vitro assays with DGD2 we could for the first time measure DGDG synthase activity of a heterologously expressed plant cDNA. UDP-galactose, but not MGDG, serves as the galactose donor for DGDG synthesis catalyzed by DGD2, providing clear evidence for the existence of a UDP-galactose-dependent DGDG synthase in higher plants. In in vitro assays, DGD2 was capable of galactosylating DGDG, resulting in the synthesis of an oligogalactolipid tentatively identified as trigalactosyldiacylglycerol. DGD2 mRNA expression in leaves was very low but was strongly induced during growth under phosphate-limiting conditions. This induction correlates with the previously described increase in DGDG during phosphate deprivation. Therefore, in contrast to DGD1, which is responsible for the synthesis of the bulk of DGDG found in chloroplasts, DGD2 apparently is involved in the synthesis of DGDG under specific growth conditions.