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

Probing the in situ volumes of Arabidopsis leaf plastids using three-dimensional confocal and scanning electron microscopy

Leaf plastids harbor a plethora of biochemical reactions including photosynthesis, one of the most important metabolic pathways on Earth. Scientists are eager to unveil the physiological processes within the organelle but also their interconnection with the rest of the plant cell. An increasingly important feature of this venture is to use experimental data in the design of metabolic models. A remaining obstacle has been the limited in situ volume information of plastids and other cell organelles. To fill this gap for chloroplasts, we established three microscopy protocols delivering in situ volumes based on: (i) chlorophyll fluorescence emerging from the thylakoid membrane, (ii) a CFP marker embedded in the envelope, and (iii) calculations from serial block-face scanning electron microscopy (SBFSEM). The obtained data were corroborated by comparing wild-type data with two mutant lines affected in the plastid division machinery known to produce small and large mesophyll chloroplasts, respectively. Furthermore, we also determined the volume of the much smaller guard cell plastids. Interestingly, their volume is not governed by the same components of the division machinery which defines mesophyll plastid size. Based on our three approaches, the average volume of a mature Col-0 wild-type mesophyll chloroplasts is 93 μm3 . Wild-type guard cell plastids are approximately 18 μm3 . Lastly, our comparative analysis shows that the chlorophyll fluorescence analysis can accurately determine chloroplast volumes, providing an important tool to research groups without access to transgenic marker lines expressing genetically encoded fluorescence proteins or costly SBFSEM equipment.

Plant mitochondria - past, present and future

The study of plant mitochondria started in earnest around 1950 with the first isolations of mitochondria from animal and plant tissues. The first 35 years were spent establishing the basic properties of plant mitochondria and plant respiration using biochemical and physiological approaches. A number of unique properties (compared to mammalian mitochondria) were observed: (i) the ability to oxidize malate, glycine and cytosolic NAD(P)H at high rates; (ii) the partial insensitivity to rotenone, which turned out to be due to the presence of a second NADH dehydrogenase on the inner surface of the inner mitochondrial membrane in addition to the classical Complex I NADH dehydrogenase; and (iii) the partial insensitivity to cyanide, which turned out to be due to an alternative oxidase, which is also located on the inner surface of the inner mitochondrial membrane, in addition to the classical Complex IV, cytochrome oxidase. With the appearance of molecular biology methods around 1985, followed by genomics, further unique properties were discovered: (iv) plant mitochondrial DNA (mtDNA) is 10-600 times larger than the mammalian mtDNA, yet it only contains approximately 50% more genes; (v) plant mtDNA has kept the standard genetic code, and it has a low divergence rate with respect to point mutations, but a high recombinatorial activity; (vi) mitochondrial mRNA maturation includes a uniquely complex set of activities for processing, splicing and editing (at hundreds of sites); (vii) recombination in mtDNA creates novel reading frames that can produce male sterility; and (viii) plant mitochondria have a large proteome with 2000-3000 different proteins containing many unique proteins such as 200-300 pentatricopeptide repeat proteins. We describe the present and fairly detailed picture of the structure and function of plant mitochondria and how the unique properties make their metabolism more flexible allowing them to be involved in many diverse processes in the plant cell, such as photosynthesis, photorespiration, CAM and C4 metabolism, heat production, temperature control, stress resistance mechanisms, programmed cell death and genomic evolution. However, it is still a challenge to understand how the regulation of metabolism and mtDNA expression works at the cellular level and how retrograde signaling from the mitochondria coordinates all those processes.

Methods for studying protein targeting to and within the chloroplast

Distinct protein complements impart each of the chloroplast's three membranes and three aqueous spaces with specific functions essential for plant growth and development. Chloroplasts capture light energy, synthesize macromolecular building blocks and specialized metabolites, and communicate environmental signals to the nucleus. Establishing and maintaining these processes requires approximately 3000 proteins derived from nuclear genes, constituting approximately 95% of the chloroplast proteome. These proteins are imported into chloroplasts from the cytosol, sorted to the correct subcompartment, and assembled into functioning complexes. In vitro import assays can reconstitute these processes in isolated chloroplasts. We describe methods for monitoring in vitro protein import using Pisum sativum chloroplasts and for protease protection, fractionation, and native protein electrophoresis that are commonly combined with the import assay. These techniques facilitate investigation of the import and sorting processes, of where a protein resides, and of how that protein functions.

NaOH low-salt method for chloroplast isolation and highly pure cpDNA preparation from Aeluropus littoralis

PREMISE OF THE STUDY

High-yield pure chloroplast DNA (cpDNA) is necessary for whole genome sequencing. Chloroplast extraction with traditional high-salt methods causes damage to nuclei and destroys the integrity of organelles, which leads to high genomic contamination from the nucleus and mitochondria. To overcome this issue, we modified a traditional high-salt method to obtain a new approach called the NaOH low-salt method (NLS).

METHODS AND RESULTS

The NLS method is based on the mild alkaline lysis of plant cells, followed by homogenization with ultrasonic waves and fractionation under reduced osmotic pressure. Results showed that this modified protocol worked efficiently to extract the intact chloroplast from Aeluropus littoralis and other grasses to obtain high-quality pure cpDNA, which was confirmed by fluorescent microscopy, qPCR, and Illumina paired-end sequencing analysis.

CONCLUSIONS

Compared with high-salt methods, the NLS method has proven robust for extraction of intact chloroplasts and preparation of high-yield pure cpDNA from grasses.

Isolation of Chromoplasts and Suborganellar Compartments from Tomato and Bell Pepper Fruit

Tomato is a model for fruit development and ripening. The isolation of intact plastids from this organism is therefore important for metabolic and proteomic analyses. Pepper, a species from the same family, is also of interest since it allows isolation of intact chromoplasts in large amounts. Here, we provide a detailed protocol for the isolation of tomato plastids at three fruit developmental stages, namely, nascent chromoplasts from the mature green stage, chromoplasts from an intermediate stage, and fully differentiated red chromoplasts. The method relies on sucrose density gradient centrifugations. It yields high purity organelles suitable for proteome analyses. Enzymatic and microscopy assays are summarized to assess purity and intactness. A method is also described for subfractionation of pepper chromoplast lipoprotein structures.

Multi-functional roles for the polypeptide transport associated domains of Toc75 in chloroplast protein import

Toc75 plays a central role in chloroplast biogenesis in plants as the membrane channel of the protein import translocon at the outer envelope of chloroplasts (TOC). Toc75 is a member of the Omp85 family of bacterial and organellar membrane insertases, characterized by N-terminal POTRA (polypeptide-transport associated) domains and C-terminal membrane-integrated β-barrels. We demonstrate that the Toc75 POTRA domains are essential for protein import and contribute to interactions with TOC receptors, thereby coupling preprotein recognition at the chloroplast surface with membrane translocation. The POTRA domains also interact with preproteins and mediate the recruitment of molecular chaperones in the intermembrane space to facilitate membrane transport. Our studies are consistent with the multi-functional roles of POTRA domains observed in other Omp85 family members and demonstrate that the domains of Toc75 have evolved unique properties specific to the acquisition of protein import during endosymbiotic evolution of the TOC system in plastids.

DOI:http://dx.doi.org/10.7554/eLife.12631.001

Chloroplasts are a hallmark feature of plant cells and the sites of photosynthesis – the process in which plants harness the energy in sunlight for their own needs. The first chloroplasts arose when a photosynthetic bacterium was engulfed by another host cell, and most of the original bacterial genes have been transferred to the host cell’s nucleus during the evolution of land plants. As a result, modern chloroplasts need to import the thousands of proteins encoded by these genes from the rest of the cell.

The chloroplast protein import system relies on a protein transporter in the chloroplast membrane that evolved from a family of bacterial transporters. However, the bacterial transporters were initially involved in protein export, and it was not known how the activity of these transporters adapted to move proteins in the opposite direction.

Paila et al. set out to better understand the chloroplast protein import system and produced mutated forms of the transporter in the model plant Arabidopsis thaliana. These experiments revealed that a part of the transporter that is conserved in many other organisms, the “protein transport associated domains”, has been adapted for three key roles in protein import. First, this part of the transporter interacts with the other components of the import system that make the transporter more selective and control which direction the proteins are transported. Second, the domains interact with proteins during transport to help move them across the chloroplast membrane. Finally, the domains recruit other molecules called chaperones, which stop the protein from aggregating or misfolding during the transport process. These activities are similar to those for the bacterial export transporters, but clearly evolved to allow transport in the opposite direction – that is, to import proteins into chloroplasts.

The next challenges are to explain how proteins destined for chloroplasts are recognized and transported through the chloroplast’s membrane.

DOI:http://dx.doi.org/10.7554/eLife.12631.002

Comparison of methods for extracting thylakoid membranes of Arabidopsis plants

Robust and reproducible methods for extracting thylakoid membranes are required for the analysis of photosynthetic processes in higher plants such as Arabidopsis. Here, we compare three methods for thylakoid extraction using two different buffers. Method I involves homogenizing the plant material with a metal/glass blender; method II involves manually grinding the plant material in ice-cold grinding buffer with a mortar and method III entails snap-freezing followed by manual grinding with a mortar, after which the frozen powder is thawed in isolation buffer. Thylakoid membrane samples extracted using each method were analyzed with respect to protein and chlorophyll content, yields relative to starting material, oxygen-evolving activity, protein complex content and phosphorylation. We also examined how the use of fresh and frozen thylakoid material affected the extracts' contents of protein complexes. The use of different extraction buffers did not significantly alter the protein content of the extracts in any case. Method I yielded thylakoid membranes with the highest purity and oxygen-evolving activity. Method III used low amounts of starting material and was capable of capturing rapid phosphorylation changes in the sample at the cost of higher levels of contamination. Method II yielded thylakoid membrane extracts with properties intermediate between those obtained with the other two methods. Finally, frozen and freshly isolated thylakoid membranes performed identically in blue native-polyacrylamide gel electrophoresis experiments conducted in order to separate multimeric protein supracomplexes.

Chloroplasts extend stromules independently and in response to internal redox signals

A fundamental mystery of plant cell biology is the occurrence of "stromules," stroma-filled tubular extensions from plastids (such as chloroplasts) that are universally observed in plants but whose functions are, in effect, completely unknown. One prevalent hypothesis is that stromules exchange signals or metabolites between plastids and other subcellular compartments, and that stromules are induced during stress. Until now, no signaling mechanisms originating within the plastid have been identified that regulate stromule activity, a critical missing link in this hypothesis. Using confocal and superresolution 3D microscopy, we have shown that stromules form in response to light-sensitive redox signals within the chloroplast. Stromule frequency increased during the day or after treatment with chemicals that produce reactive oxygen species specifically in the chloroplast. Silencing expression of the chloroplast NADPH-dependent thioredoxin reductase, a central hub in chloroplast redox signaling pathways, increased chloroplast stromule frequency, whereas silencing expression of nuclear genes related to plastid genome expression and tetrapyrrole biosynthesis had no impact on stromules. Leucoplasts, which are not photosynthetic, also made more stromules in the daytime. Leucoplasts did not respond to the same redox signaling pathway but instead increased stromule formation when exposed to sucrose, a major product of photosynthesis, although sucrose has no impact on chloroplast stromule frequency. Thus, different types of plastids make stromules in response to distinct signals. Finally, isolated chloroplasts could make stromules independently after extraction from the cytoplasm, suggesting that chloroplast-associated factors are sufficient to generate stromules. These discoveries demonstrate that chloroplasts are remarkably autonomous organelles that alter their stromule frequency in reaction to internal signal transduction pathways.

Visualisation of plastid outgrowths in potato (Solanum tuberosum L.) tubers by carboxyfluorescein diacetate staining

We describe two types of plastid outgrowths visualised in potato tubers after carboxyfluorescein diacetate staining. Probable esterase activity of the outgrowths has been demonstrated for the first time ever. Plastid outgrowths were observed in the phelloderm and storage parenchyma cells of red potato (S. tuberosum L. cv. Rosalinde) tubers after administration of carboxyfluorescein diacetate stain. Endogenous esterases cleaved off acetic groups to release membrane-unpermeable green fluorescing carboxyfluorescein which accumulated differentially in particular cell compartments. The intensive green fluorescence of carboxyfluorescein exhibited highly branched stromules (stroma-filled plastid tubular projections of the plastid envelope) and allowed distinguishing them within cytoplasmic strands of the phelloderm cells. Stromules (1) were directed towards the nucleus or (2) penetrated the whole cells through the cytoplasmic bands of highly vacuolated phelloderm cells. Those directed towards the nucleus were flattened and adhered to the nuclear envelope. Stromule-like interconnections between two parts of the same plastids (isthmuses) were also observed. We also documented the formation of another type of the stroma-filled plastid outgrowths, referred to here as protrusions, which differed from previously defined stromules in both morphology and esterase activity. Unlike stromules, the protrusions were found to be associated with developmental processes leading to starch accumulation in the storage parenchyma cells. These results strongly suggest that stromules and protrusions exhibit esterase activity. This has been demonstrated for the first time. Morphological and biochemical features as well as possible functions of stromules and protrusions are discussed below.

Plants salvage deoxyribonucleosides in mitochondria

Deoxyribonucleoside kinases phosphorylate deoxyribonucleosides into the corresponding 5'-monophosphate deoxyribonucleosides to supply the cell with nucleic acid precursors. In mitochondrial fractions of the model plant Arabidopsis thaliana, we detected deoxyadenosine and thymidine kinase activities, while the cytosol fraction contained six-fold lower activity and chloroplasts contained no measurable activities. In addition, a mitochondrial fraction isolated from the potato Solanum tuberosum contained thymidine kinase and deoxyadenosine kinase activities. We conclude that an active salvage of deoxyribonucleosides in plants takes place in their mitochondria. In general, the observed localization of the plant dNK activities in the mitochondrion suggests that plants have a different organization of the deoxyribonucleoside salvage compared to mammals.

Protein import into isolated pea root leucoplasts

Leucoplasts are important organelles for the synthesis and storage of starch, lipids and proteins. However, molecular mechanism of protein import into leucoplasts and how it differs from that of import into chloroplasts remain unknown. We used pea seedlings for both chloroplast and leucoplast isolations to compare within the same species. We further optimized the isolation and import conditions to improve import efficiency and to permit a quantitative comparison between the two plastid types. The authenticity of the import was verified using a mitochondrial precursor protein. Our results show that, when normalized to Toc75, most translocon proteins are less abundant in leucoplasts than in chloroplasts. A precursor shown to prefer the receptor Toc132 indeed had relatively more similar import efficiencies between chloroplasts and leucoplasts compared to precursors that prefer Toc159. Furthermore we found two precursors that exhibited very high import efficiency into leucoplasts. Their transit peptides may be candidates for delivering transgenic proteins into leucoplasts and for analyzing motifs important for leucoplast import.

Quantitative proteomic analysis of intact plastids

Plastids are specialized cell organelles in plant cells that are differentiated into various forms including chloroplasts, chromoplasts, and amyloplasts, and fulfill important functions in maintaining the overall cell metabolism and sensing environmental factors such as sunlight. It is therefore important to grasp the mechanisms of differentiation and functional changes of plastids in order to enhance the understanding of vegetality. In this chapter, details of a method for the extraction of intact plastids that makes analysis possible while maintaining the plastid functions are provided; in addition, a quantitative shotgun method for analyzing the composition and changes in the content of proteins in plastids as a result of environmental impacts is described.

Isolation of the plant cytosolic fraction for proteomic analysis

The cytosol is the fluid portion of the cell that is not partitioned by membranes. It contains a highly diverse collection of substances and is central to many essential cellular processes ranging from signal transduction, metabolite production and transport, protein biosynthesis and degradation to stress response and defense. Despite its importance, only a few proteomic studies have been performed on the plant cytosol. This is largely due to difficulties in isolating relatively pure samples from plant material free of disrupted organelle material. In this chapter we outline methods for isolating the cytosolic fraction from Arabidopsis cell cultures and seedlings and provide guidance on assessing purity for analysis by mass spectrometry.

Stromal protein degradation is incomplete in Arabidopsis thaliana autophagy mutants undergoing natural senescence

Background

Degradation of highly abundant stromal proteins plays an important role in the nitrogen economy of the plant during senescence. Lines of evidence supporting proteolysis within the chloroplast and outside the chloroplast have been reported. Two extra-plastidic degradation pathways, chlorophagy and Rubisco Containing Bodies, rely on cytoplasmic autophagy.

Results

In this work, levels of three stromal proteins (Rubisco large subunit, chloroplast glutamine synthetase and Rubisco activase) and one thylakoid protein (the major light harvesting complex protein of photosystem II) were measured during natural senescence in WT and in two autophagy T-DNA insertion mutants (atg5 and atg7). Thylakoid-localized protein decreased similarly in all genotypes, but stromal protein degradation was incomplete in the two atg mutants. In addition, degradation of two stromal proteins was observed in chloroplasts isolated from mid-senescence leaves.

Conclusions

These data suggest that autophagy does contribute to the complete proteolysis of stromal proteins, but does not play a major degenerative role. In addition, support for in organello degradation is provided.

Preparation of stroma, thylakoid membrane, and lumen fractions from Arabidopsis thaliana chloroplasts for proteomic analysis

For many studies regarding important chloroplast processes such as oxygenic photosynthesis, fractionation of the total chloroplast proteome is a necessary first step. Here, we describe a method for isolating the stromal, the thylakoid membrane, and the thylakoid lumen subchloroplast fractions from Arabidopsis thaliana leaf material. All three fractions can be isolated sequentially from the same plant material in a single day preparation. The isolated fractions are suitable for various proteomic analyses such as simple mapping studies or for more complex experiments such as differential expression analysis using two-dimensional difference gel electrophoresis (2D-DIGE) or mass spectrometry (MS)-based techniques. Besides this, the obtained fractions can also be used for many other purposes such as immunological assays, enzymatic activity assays, and studies of protein complexes by native-polyacrylamide gel electrophoresis (native-PAGE).

Chloroplast to chromoplast transition in tomato fruit: spectral confocal microscopy analyses of carotenoids and chlorophylls in isolated plastids and time-lapse recording on intact live tissue

BACKGROUND AND AIMS

There are several studies suggesting that tomato (Solanum lycopersicum) chromoplasts arise from chloroplasts, but there is still no report showing the fluorescence of both chlorophylls and carotenoids in an intermediate plastid, and no video showing this transition phase.

METHODS

Pigment fluorescence within individual plastids, isolated from tomato fruit using sucrose gradients, was observed at different ripening stages, and an in situ real-time recording of pigment fluorescence was performed on live tomato fruit slices.

KEY RESULTS

At the mature green and red stages, homogenous fractions of chloroplasts and chromoplasts were obtained, respectively. At the breaker stage, spectral confocal microscopy showed that intermediate plastids contained both chlorophylls and carotenoids. Furthermore, an in situ real-time recording (a) showed that the chloroplast to chromoplast transition was synchronous for all plastids of a single cell; and (b) confirmed that all chromoplasts derived from pre-existing chloroplasts.

CONCLUSIONS

These results give details of the early steps of tomato chromoplast biogenesis from chloroplasts, with the formation of intermediate plastids containing both carotenoids and chlorophylls. They provide information at the sub-cellular level on the synchronism of plastid transition and pigment changes.

Energetic manipulation of chloroplast protein import and the use of chemical cross-linkers to map protein-protein interactions

Most chloroplast proteins are synthesized in the cytosol as preproteins with N-terminal cleavable transit peptides and are imported into the organelle through the TOC-TIC translocon system. Import involves a complex set of recognition and membrane translocation steps that ensure the fidelity and unidirectional transport of the polypeptide across the double-membrane chloroplast envelope. To understand the mechanism of import, the molecular interactions and energetics of each step must be defined. Here, we describe the methods for capturing intermediates in the import process through the manipulation of the energy state of chloroplasts, and the use of two different chemical cross-linking approaches to examine the molecular interactions that mediate the import process and to assess the assembly state of the translocons. These approaches can be employed to identify sequential protein-protein interactions, and thereby dissect the pathway and roles of import components during protein import into chloroplasts.

Rapid isolation of Arabidopsis chloroplasts and their use for in vitro protein import assays

In vitro chloroplast protein import assays have been performed since the late 1970s, initially with plant species (e.g., pea and spinach) that readily provide an abundant source of starting material and also, subsequently, a good yield of chloroplasts for import assays. However, the sequencing of the Arabidopsis genome paved the way for an additional model system that is more amenable to genetic analysis, as a complement to the more biochemically orientated models such as pea and spinach. A prerequisite for this change was an efficient and reliable protocol for the isolation of chloroplasts for use in protein import assays, enabling biochemical approaches to be combined with the genetic potential of the plant. The method described here was developed as a rapid and low-cost procedure that can be accessed by everyone due to its simplicity. Despite its rapidity and simplicity, the method yields highly pure chloroplasts, and in addition works well with mutant plants that exhibit pale or chlorotic phenotypes. The protocol is also optimized for work with material from young plants (10-14 days old), when protein import is believed to be at its peak, and so plant growth can be conducted in vitro on Murashige and Skoog medium. The isolation method has been used not only for protein import assays, but also for proteomic analysis and further subfractionation studies.

One-carbon metabolism in plants: characterization of a plastid serine hydroxymethyltransferase

SHMT (serine hydroxymethyltransferase; EC 2.1.2.1) catalyses reversible hydroxymethyl group transfer from serine to H4PteGlun (tetrahydrofolate), yielding glycine and 5,10-methylenetetrahydrofolate. In plastids, SHMTs are thought to catalytically direct the hydroxymethyl moiety of serine into the metabolic network of H4PteGlun-bound one-carbon units. Genes encoding putative plastid SHMTs were found in the genomes of various plant species. SHMT activity was detected in chloroplasts in pea (Pisum sativum) and barley (Hordeum vulgare), suggesting that plastid SHMTs exist in all flowering plants. The Arabidopsis thaliana genome encodes one putative plastid SHMT (AtSHMT3). Its cDNA was cloned by reverse transcription-PCR and the encoded recombinant protein was produced in Escherichia coli. Evidence that AtSHMT3 is targeted to plastids was found by confocal microscopy of A. thaliana protoplasts transformed with proteins fused to enhanced green fluorescent protein. Characterization of recombinant AtSHMT3 revealed that substrate affinity for and the catalytic efficiency of H4PteGlu1-8 increase with n, and that H4PteGlu1-8 inhibit AtSHMT3. 5-Methyltetrahydrofolate and 5-formyltetrahydrofolate with one and five glutamate residues inhibited AtSHMT3-catalysed hydroxymethyl group transfer from serine to H4PteGlu6, with the pentaglutamylated inhibitors being more effective. Calculations revealed inhibition with 5-methyltetrahydrofolate or 5-formyltetrahydrofolate resulting in little reduction in AtSHMT3 activity under folate concentrations estimated for plastids.

A 1-megadalton translocation complex containing Tic20 and Tic21 mediates chloroplast protein import at the inner envelope membrane

Chloroplast protein import is mediated by two hetero-oligomeric protein complexes, the Tic and Toc translocons, which are located in the inner and outer envelope membranes. At the inner membrane, many Tic components have been identified and characterized, but it remains unclear how these Tic proteins are organized to form a protein-conducting channel or whether a stable Tic core complex that binds translocating preproteins exists. Here, we report the identification of a 1-megadalton (MD) translocation complex as an intermediate during protein translocation across the inner membrane in Arabidopsis thaliana and pea (Pisum sativum). This complex can be detected by blue native PAGE using the mild detergent digitonin without any chemical cross-linkers. The preprotein arrested in the 1-MD complex can be chased into its fully translocated form after a subsequent incubation. While Tic20 and Tic21 appear to be involved in the 1-MD complex, Tic110, a well-characterized Tic component, exists as a distinct entity from the complex. Several lines of evidence suggest that the 1-MD complex functions in between the Toc and Tic110-containing complexes, most likely as a protein-conducting channel at the inner envelope.

Percoll-purified and photosynthetically active chloroplasts from Arabidopsis thaliana leaves

The availability of the complete genome sequence of Arabidopsis thaliana and of large collections of insertion mutants paved the way for systematic studies of gene functions in this organism, thus requiring adapting biochemical and physiological tools to this model plant. For physiological analysis of photosynthesis, methods combining high level of chloroplast purity and preservation of the photosynthetic activity were missing. Here, we describe a rapid method (less than 1h) to obtain Percoll-purified and photosynthetically active chloroplasts from Arabidopsis leaves retaining almost 90% of the Vmax of photosynthesis measured in the starting leaves from plants grown under a light intensity of 150mumolphotonm(-2)s(-1) and 80% of their initial photosynthetic rate after 3h of storage.

The role of GTP binding and hydrolysis at the atToc159 preprotein receptor during protein import into chloroplasts

The majority of nucleus-encoded chloroplast proteins are targeted to the organelle by direct binding to two membrane-bound GTPase receptors, Toc34 and Toc159. The GTPase activities of the receptors are implicated in two key import activities, preprotein binding and driving membrane translocation, but their precise functions have not been defined. We use a combination of in vivo and in vitro approaches to study the role of the Toc159 receptor in the import reaction. We show that atToc159-A864R, a receptor with reduced GTPase activity, can fully complement a lethal insertion mutation in the ATTOC159 gene. Surprisingly, the atToc159-A864R receptor increases the rate of protein import relative to wild-type receptor in isolated chloroplasts by stabilizing the formation of a GTP-dependent preprotein binding intermediate. These data favor a model in which the atToc159 receptor acts as part of a GTP-regulated switch for preprotein recognition at the TOC translocon.

New method to quantify erythrophagocytosis by autologous monocytes

BACKGROUND

Anemia is the net result of decreased red blood cell (RBC) production and increased removal of RBCs. Replication and maturation of erythroid precursors and RBC lysis can be measured by standardized in vitro methods and surrogate markers, respectively. In contrast, erythrophagocytosis by autologous phagocytes is more difficult to quantify.

METHODS

We developed a method to assess erythrophagocytosis by autologous monocytes from 5 ml of whole blood. RBCs were labeled with carboxyfluorescein-diacetate-succinimidyl ester (CFDA-SE) and subsequently coincubated with autologous CD14(+) monocytes. Phagocytosis was quantified using flow cytometry. After standardization, the assay was validated in patients with severe malarial anemia (SMA), a condition that is associated with increased erythrophagocytosis.

RESULTS

After labeling, CFDA-SE was stably incorporated into RBCs and no significant leakage leading to contamination of nonlabeled cells was observed. Monocytes ingested opsonized, labeled RBCs seven times more than nonopsonized controls. Erythrophagocytosis was significantly higher in SMA than in healthy controls.

CONCLUSIONS

The established assay showed enhanced autoerythrophagocytosis associated with SMA and hence was able to detect clinically relevant erythrophagocytosis. This novel assay is well suited for rapid quantification of in vitro erythrophagocytosis by autologous monocytes.

Characterization of the preprotein translocon at the outer envelope membrane of chloroplasts by blue native PAGE

The preprotein translocon at the outer envelope membrane of chloroplasts (Toc) mediates the recognition and import of nuclear-encoded preproteins into chloroplasts. Two receptor components, Toc159 and Toc34, and the channel Toc75 form the Toc complex. In this study, we have analyzed the molecular architecture and organization of the Toc complex by blue native PAGE (BN-PAGE), which is a high-resolution method for separating membrane protein complexes under non-denaturing conditions. Pea chloroplasts isolated in the presence of a protease inhibitor cocktail were directly solubilized in detergent solution and analyzed by BN-PAGE and size exclusion chromatography. Subsequent immunoblot analyses indicated that the complex composed of Toc75, Toc159 and Toc34 has a molecular mass of 800-1,000 kDa. Limited proteolysis revealed a core of the Toc complex, which was resistant to proteases and detergent treatments. The stoichiometry of the three Toc proteins was calculated as approximately 1 : 3 : 3 between Toc159 : Toc75 : Toc34. We have also analyzed the Toc complex of etioplasts and root plastids. These plastids were found to have essentially the same sized Toc complex as that of the chloroplast.

Reorganization of the alternative pathways of the Arabidopsis respiratory chain by nitrogen supply: opposing effects of ammonium and nitrate

The mitochondrial oxidative phosphorylation system in plants possesses a variety of alternative pathways that decrease respiratory ATP production. These alternative pathways are mediated by three classes of bypass proteins: the type II NAD(P)H dehydrogenases (which circumvent complex I of the electron transport chain), the alternative oxidases (AOXs; which circumvent complexes III and IV) and the uncoupling proteins (which circumvent ATP synthase). We have monitored the expression of all genes encoding respiratory bypass proteins in Arabidopsis thaliana growing with different sources of inorganic nitrogen (N). Resupply of nitrate (NO) to N-limited seedling cultures caused a decrease in the transcript abundance of several type II NAD(P)H dehydrogenase and AOX genes, while resupply of ammonium (NH) led to broad increases in expression in the same gene families. Similar results were observed upon switching between nitrate and ammonium in the absence of N stress. Nitrate signalling was found to be mediated primarily by the nitrate ion itself, whereas ammonium regulation was dependent upon assimilation and affected by changes in apoplastic pH. Corresponding alterations in alternative respiratory pathway capacities were apparent in seedlings supplied with either nitrate or ammonium as an N source and in mitochondria purified from the seedlings. Specifically, AOX capacity and protein abundance, as well as calcium-dependent external NADH oxidation, were substantially elevated after growth on ammonium. The increased capacity of respiratory bypass pathways after switching from nitrate to ammonium was correlated to an overall respiratory increase.

Higher plant plastids and cyanobacteria have folate carriers related to those of trypanosomatids

Cyanobacterial and plant genomes encode proteins with some similarity to the folate and biopterin transporters of the trypanosomatid parasite Leishmania. The Synechocystis slr0642 gene product and its closest Arabidopsis homolog, the At2g32040 gene product, are representative examples. Both have 12 probable transmembrane domains, and the At2g32040 protein has a predicted chloroplast transit peptide. When expressed in Escherichia coli pabA pabB or folE, mutants, which are unable to produce or take up folates, the slr0642 protein and a modified At2g32040 protein (truncated and fused to the N terminus of slr0642) enabled growth on 5-formyltetrahydrofolate or folic acid but not on 5-formyltetrahydrofolate triglutamate, demonstrating that both proteins mediate folate monoglutamate transport. Both proteins also mediate transport of the antifolate analogs methotrexate and aminopterin, as evidenced by their ability to greatly increase the sensitivity of E. coli to these inhibitors. The full-length At2g32040 polypeptide was translocated into isolated pea chloroplasts and, when fused to green fluorescent protein, directed the passenger protein to the envelope of Arabidopsis chloroplasts in transient expression experiments. At2g32040 transcripts were present at similar levels in roots and aerial organs, indicating that the protein occurs in non-green plastids as well as chloroplasts. Insertional inactivation of At2g32040 significantly raised the total folate content of chloroplasts and lowered the proportion of 5-methyltetrahydrofolate but did not discernibly affect growth. These findings establish conservation of function among folate and biopterin transporter family proteins from three kingdoms of life.