Interaction of the targeting sequence of chloroplast precursors with Hsp70 molecular chaperones

Protective effect of Yangxue Jiedu Soup against psoriasis-like lesions by regulating TLR4/NF-κB signaling pathway mediated by secretion of exosome HSP70

Psoriasis is a common chronic inflammatory hypertrophic skin disease characterized by abnormal proliferation and differentiation of keratinocyte and immune T cell. The pathogenesis of psoriasis has not been fully elucidated and there is no effective therapy in clinic. As a traditional Chinese medicine formula, Yangxue Jiedu Soup (YJS) has been used to treat inflammatory diseases caused by Yin Deficiency and Blood Dryness. The purpose of present study was to investigate the therapeutic effect and molecular mechanism of YJS on psoriasis model mice. Results showed that YJS effectively inhibited the hypertrophy, erythema and scales of psoriasis-like lesions to alleviate the pathological changes of skin lesions, and further decreased the production of TNF-α, IL-6, IL-1β, IFN-γ, IL-17 and IL-23. Meanwhile, YJS also significantly reduced keratinocyte proliferation and maintained immune system balance by inhibiting the expression of PCNA, Ki-67, CD4 + and CD8 + in psoriasis mice. Moreover, the results further indicated that YJS could inhibit TLR4 activation and NF-κB p65 nuclear transfer by suppressing HSP70 secretion to attenuate the inflammatory response in IMQ-induced mice, which provided a theoretical basis for the clinical use of YJS in the treatment of psoriasis.

A new catalytic mechanism of bacterial ferredoxin-NADP+ reductases due to a particular NADP+ binding mode

Ferredoxin-NADP+ reductases (FNRs) are ubiquitous flavoenzymes involved in redox metabolisms. FNRs catalyze the reversible electron transfer between NADP(H) and ferredoxin or flavodoxin. They are classified as plant- and mitochondrial-type FNR. Plant-type FNRs are divided into plastidic and bacterial classes. The plastidic FNRs show turnover numbers between 20 and 100 times higher than bacterial enzymes and these differences have been related to their physiological functions. We demonstrated that purified Escherichia coli FPR (EcFPR) contains tightly bound NADP+ , which does not occur in plastidic type FNRs. The three-dimensional structure of EcFPR evidenced that NADP+ interacts with three arginines (R144, R174, and R184) which could generate a very high affinity and structured site. These arginines are conserved in other bacterial FNRs but not in the plastidic enzymes. We have cross-substituted EcFPR arginines with residues present in analogous positions in the Pisum sativum FNR (PsFNR) and replaced these amino acids by arginines in PsFNR. We analyzed all proteins by structural, kinetic, and stability studies. We found that EcFPR mutants do not contain bound NADP+ and showed increased Km for this nucleotide. The EcFPR activity was inhibited by NADP+ but this behavior disappeared as arginines were removed. A NADP+ analog of the nicotinamide portion produced an activating effect on EcFPR and promoted the NADP+ release. Our results give evidence for a new model of NADP+ binding and catalysis in bacterial FNRs.We propose that this tight NADP+ binding constitutes an essential catalytic and regulatory mechanism of bacterial FNRs involved in redox homeostasis.

Engineered Accumulation of Bicarbonate in Plant Chloroplasts: Known Knowns and Known Unknowns

Heterologous synthesis of a biophysical CO2-concentrating mechanism (CCM) in plant chloroplasts offers significant potential to improve the photosynthetic efficiency of C3 plants and could translate into substantial increases in crop yield. In organisms utilizing a biophysical CCM, this mechanism efficiently surrounds a high turnover rate Rubisco with elevated CO2 concentrations to maximize carboxylation rates. A critical feature of both native biophysical CCMs and one engineered into a C3 plant chloroplast is functional bicarbonate (HCO3 -) transporters and vectorial CO2-to-HCO3 - converters. Engineering strategies aim to locate these transporters and conversion systems to the C3 chloroplast, enabling elevation of HCO3 - concentrations within the chloroplast stroma. Several CCM components have been identified in proteobacteria, cyanobacteria, and microalgae as likely candidates for this approach, yet their successful functional expression in C3 plant chloroplasts remains elusive. Here, we discuss the challenges in expressing and regulating functional HCO3 - transporter, and CO2-to-HCO3 - converter candidates in chloroplast membranes as an essential step in engineering a biophysical CCM within plant chloroplasts. We highlight the broad technical and physiological concerns which must be considered in proposed engineering strategies, and present our current status of both knowledge and knowledge-gaps which will affect successful engineering outcomes.

Functional analysis of the Chloroplast GrpE (CGE) proteins from Arabidopsis thaliana

The function of proteins depends on specific partners that regulate protein folding, degradation and protein-protein interactions, such partners are the chaperones and cochaperones. In chloroplasts, proteins belonging to several families of chaperones have been identified: chaperonins (Cpn60s), Hsp90s (Hsp90-5/Hsp90C), Hsp100s (Hsp93/ClpC) and Hsp70s (cpHsc70s). Several lines of evidence have demonstrated that cpHsc70 chaperones are involved in molecular processes like protein import, protein folding and oligomer formation that impact important physiological aspects in plants such as thermotolerance and thylakoid biogenesis. Despite the vast amount of data existing around the function of cpHcp70s chaperones, very little attention has been paid to the roles of DnaJ and GrpE cochaperones in the chloroplast. In this study, we performed a phylogenetic analysis of the chloroplastic GrpE (CGE) proteins from 71 species. Based on their phylogenetic relationships and on a motif enrichment analysis, we propose a classification system for land plants' CGEs, which include two independent groups with specific primary structure traits. Furthermore, using in vivo assays we determined that the two CGEs from A. thaliana (AtCGEs) complement the mutant phenotype displayed by a knockout E. coli strain defective in the bacterial grpE gene. Moreover, we determined in planta that the two AtCGEs are bona fide chloroplastic proteins, which form the essential homodimers needed to establish direct physical interactions with the cpHsc70-1 chaperone. Finally, we found evidence suggesting that AtCGE1 is involved in specific physiological phenomena in A. thaliana, such as the chloroplastic response to heat stress, and the correct oligomerization of the photosynthesis-related LHCII complex.

Functional characterization of chloroplast transit peptide in the small subunit of Rubisco in maize

Functions of domains or motifs, which are encoded by the transit peptide (TP) of the precursor of the small subunit of Rubisco (prSSU), have been investigated intensively in dicots. Functional characterization of the prSSU TP, however, is still understudied in maize. In this study, we found that the TP of maize prSSU1 did not function fully in chloroplast targeting in Arabidopsis or vice versa, indicating the divergent function of TPs in chloroplast targeting between maize and Arabidopsis. Through deletion or substitution assays, we found that the N-terminal region of maize or Arabidopsis prSSU1 was necessary and sufficient for importing specifically the fused-green fluorescent protein (GFP) into each corresponding chloroplast. Finally, we found that the first-five amino acids and MM motif in the N-terminal domain of the maize TP played an essential role in maize chloroplast targeting. Thus, our analyses demonstrate that the N-terminal domain of the prSSU1 TP is the key determinant in chloroplast targeting between maize and Arabidopsis. Our study highlights the unique properties of the maize prSSU1 TP in chloroplast targeting, thus helping to understand the role of N-terminal domain in chloroplast targeting across species. It will help to manipulate chloroplast transit peptides (cTPs) for crop bioengineering.

A novel method for removing contaminant Hsp70 molecular chaperones from recombinant proteins

The production of recombinant proteins in bacteria has increased significantly in recent years, becoming a common tool for both research and the industrial production of proteins. One of the requirements of this methodology is to obtain the desired protein without contaminants. However, this goal cannot always be readily achieved. Multiple strategies have been developed to improve the quality of the desired protein product. Nevertheless, contamination with molecular chaperones is one of the recalcitrant problems that still affects the quality of the obtained proteins. The ability of chaperones to bind to unfolded proteins or to regions where the polypeptide chain is exposed make the removal of the contamination during purification challenging to achieve. This work aimed to develop a strategy to remove contaminating DnaK, one of the homologous Hsp70 molecular chaperones found in Escherichia coli, from purified recombinant proteins. For this purpose, we developed a methodology that captures the DnaK from the contaminating proteins by co-incubation with a GST-cleanser protein that has free functional binding sites for the chaperone. The cleanser protein can then be easily removed together with the captured DnaK. Here, we demonstrated the utility of our system by decontaminating a Histidine-tagged recombinant protein in a batch process. The addition of the GST-cleanser protein in the presence of ATP-Mg eliminates the DnaK contamination substantially. Thus, our decontaminant strategy results versatile and straightforward and can be applied to proteins obtained with different expression and purifications systems as well as to small samples or large volume preparations.

Tomato LrgB regulates heat tolerance and the assimilation and partitioning of carbon

The impact of extreme and sustained high temperatures on plant growth has become increasingly prominent. Heat shock cognate 70-kDa proteins play an important role in plant heat tolerance. In this study, we identified and characterized the tomato ortholog of LrgB (SlLrgB), and demonstrate that it interacts with Hsc70.1. Similar to other genes that encode chloroplast-localized proteins, the expression of SlLrgB is upregulated in green tissues and suppressed by heat shock. Functional analyses utilizing transgenic plants indicate that SlLrgB contributes to chlorophyll metabolism. Both the overexpression and the RNA interference-mediated suppression of SlLrgB led to chlorotic leaves, reduced plant height, smaller size and decreases in pigment levels in ripening fruits. However, the starch levels in the SlLrgB-RNAi lines were significantly increased and the heat tolerance of SlLrgB-RNAi was obvious elevated. Downregulating the expression of Hsc70.1 by VIGS in tomato led to retarded growth, chlorotic leaves, and increased expression of SlLrgB. Based on these data, we suggest that SlLrgB regulates chlorophyll metabolism and the assimilation and partitioning of carbon. We also suggest that Hsc70.1 and SlLrgB contribute to heat tolerance and that Hsc70.1 negatively regulates SlLrgB.

Evidence that Hsc70 Is Associated with Cucumber Necrosis Virus Particles and Plays a Role in Particle Disassembly

Uncoating of a virus particle to expose its nucleic acid is a critical aspect of the viral multiplication cycle, as it is essential for the establishment of infection. In the present study, we investigated the role of plant HSP70 homologs in the uncoating process of Cucumber necrosis virus (CNV), a nonenveloped positive-sense single-stranded RNA [(+)ssRNA] virus having a T=3 icosahedral capsid. We have found through Western blot analysis and mass spectrometry that the HSP70 homolog Hsc70-2 copurifies with CNV particles. Virus overlay and immunogold labeling assays suggest that Hsc70-2 is physically bound to virions. Furthermore, trypsin digestion profiles suggest that the bound Hsc70-2 is partially protected by the virus, indicating an intimate association with particles. In investigating a possible role of Hsc70-2 in particle disassembly, we showed that particles incubated with Hsp70/Hsc70 antibody produce fewer local lesions than those incubated with prebleed control antibody on Chenopodium quinoa In conjunction, CNV virions purified using CsCl and having undetectable amounts of Hsc70-2 produce fewer local lesions. We also have found that plants with elevated levels of HSP70/Hsc70 produce higher numbers of local lesions following CNV inoculation. Finally, incubation of recombinant Nicotiana benthamiana Hsc70-2 with virus particles in vitro leads to conformational changes or partial disassembly of capsids as determined by transmission electron microscopy, and particles are more sensitive to chymotrypsin digestion. This is the first report suggesting that a cellular Hsc70 chaperone is involved in disassembly of a plant virus.

IMPORTANCE

Virus particles must disassemble and release their nucleic acid in order to establish infection in a cell. Despite the importance of disassembly in the ability of a virus to infect its host, little is known about this process, especially in the case of nonenveloped spherical RNA viruses. Previous work has shown that host HSP70 homologs play multiple roles in the CNV infection cycle. We therefore examined the potential role of these cellular components in the CNV disassembly process. We show that the HSP70 family member Hsc70-2 is physically associated with CNV virions and that HSP70 antibody reduces the ability of CNV to establish infection. Statistically significantly fewer lesions are produced when virions having undetectable HSc70-2 are used as an inoculum. Finally incubation of Hsc70-2 with CNV particles results in conformational changes in particles. Taken together, our data point to an important role of the host factor Hsc70-2 in CNV disassembly.

Import of Soluble Proteins into Chloroplasts and Potential Regulatory Mechanisms

Chloroplasts originated from an endosymbiotic event in which a free-living cyanobacterium was engulfed by an ancestral eukaryotic host. During evolution the majority of the chloroplast genetic information was transferred to the host cell nucleus. As a consequence, proteins formerly encoded by the chloroplast genome are now translated in the cytosol and must be subsequently imported into the chloroplast. This process involves three steps: (i) cytosolic sorting procedures, (ii) binding to the designated receptor-equipped target organelle and (iii) the consecutive translocation process. During import, proteins have to overcome the two barriers of the chloroplast envelope, namely the outer envelope membrane (OEM) and the inner envelope membrane (IEM). In the majority of cases, this is facilitated by two distinct multiprotein complexes, located in the OEM and IEM, respectively, designated TOC and TIC. Plants are constantly exposed to fluctuating environmental conditions such as temperature and light and must therefore regulate protein composition within the chloroplast to ensure optimal functioning of elementary processes such as photosynthesis. In this review we will discuss the recent models of each individual import stage with regard to short-term strategies that plants might use to potentially acclimate to changes in their environmental conditions and preserve the chloroplast protein homeostasis.

Once upon a Time - Chloroplast Protein Import Research from Infancy to Future Challenges

Protein import into chloroplasts has been a focus of research for several decades. The first publications dealing with this fascinating topic appeared in the 1970s. From the initial realization that many plastid proteins are being encoded for in the nucleus and require transport into their target organelle to the identification of import components in the cytosol, chloroplast envelopes, and stroma, as well as elucidation of some mechanistic details, more fascinating aspects are still being unraveled. With this overview, we present a survey of the beginnings of chloroplast protein import research, the first steps on this winding road, and end with a glimpse into the future.

Proteomic Analysis Reveals the Leaf Color Regulation Mechanism in Chimera Hosta "Gold Standard" Leaves

Leaf color change of variegated leaves from chimera species is regulated by fine-tuned molecular mechanisms. Hosta "Gold Standard" is a typical chimera Hosta species with golden-green variegated leaves, which is an ideal material to investigate the molecular mechanisms of leaf variegation. In this study, the margin and center regions of young and mature leaves from Hosta "Gold Standard", as well as the leaves from plants after excess nitrogen fertilization were studied using physiological and comparative proteomic approaches. We identified 31 differentially expressed proteins in various regions and development stages of variegated leaves. Some of them may be related to the leaf color regulation in Hosta "Gold Standard". For example, cytosolic glutamine synthetase (GS1), heat shock protein 70 (Hsp70), and chloroplastic elongation factor G (cpEF-G) were involved in pigment-related nitrogen synthesis as well as protein synthesis and processing. By integrating the proteomics data with physiological results, we revealed the metabolic patterns of nitrogen metabolism, photosynthesis, energy supply, as well as chloroplast protein synthesis, import and processing in various leaf regions at different development stages. Additionally, chloroplast-localized proteoforms involved in nitrogen metabolism, photosynthesis and protein processing implied that post-translational modifications were crucial for leaf color regulation. These results provide new clues toward understanding the mechanisms of leaf color regulation in variegated leaves.

Eukaryotic Hsp70 chaperones in the intermembrane space of chloroplasts

MAIN CONCLUSION

Multiple eukaryotic Hsp70 typically localized in the cytoplasm are also distributed to the intermembrane space of chloroplasts and might thereby represent the missing link in energizing protein translocation. Protein translocation into organelles is a central cellular process that is tightly regulated. It depends on signals within the preprotein and on molecular machines catalyzing the process. Molecular chaperones participate in transport and translocation of preproteins into organelles to control folding and to provide energy for the individual steps. While most of the processes are explored and the components are identified, the transfer of preproteins into and across the intermembrane space of chloroplasts is not yet understood. The existence of an energy source in this compartment is discussed, because the required transit peptide length for successful translocation into chloroplasts is shorter than that found for mitochondria where energy is provided exclusively by matrix chaperones. Furthermore, a cytosolic-type Hsp70 homologue was proposed as component of the chloroplast translocon in the intermembrane space energizing the initial translocation. The molecular identity of such intermembrane space localized Hsp70 remained unknown, which led to a controversy concerning its existence. We identified multiple cytosolic Hsp70s by mass spectrometry on isolated, thermolysin-treated Medicago sativa chloroplasts. The localization of these Hsp70s of M. sativa or Arabidopsis thaliana in the intermembrane space was confirmed by a self-assembly GFP-based in vivo system. The localization of cytosolic Hsp70s in the stroma of chloroplasts or different mitochondrial compartments could not be observed. Similarly, we could not identify any cytosolic Hsp90 in the intermembrane space of chloroplast. With respect to our results we discuss the possible targeting and function of the Hsp70 found in the intermembrane space.

Cucumber Necrosis Virus Recruits Cellular Heat Shock Protein 70 Homologs at Several Stages of Infection

RNA viruses often depend on host factors for multiplication inside cells due to the constraints of their small genome size and limited coding capacity. One such factor that has been exploited by several plant and animal viruses is heat shock protein 70 (HSP70) family homologs which have been shown to play roles for different viruses in viral RNA replication, viral assembly, disassembly, and cell-to-cell movement. Using next generation sequence analysis, we reveal that several isoforms of Hsp70 and Hsc70 transcripts are induced to very high levels during cucumber necrosis virus (CNV) infection of Nicotiana benthamiana and that HSP70 proteins are also induced by at least 10-fold. We show that HSP70 family protein homologs are co-opted by CNV at several stages of infection. We have found that overexpression of Hsp70 or Hsc70 leads to enhanced CNV genomic RNA, coat protein (CP), and virion accumulation, whereas downregulation leads to a corresponding decrease. Hsc70-2 was found to increase solubility of CNV CP in vitro and to increase accumulation of CNV CP independently of viral RNA replication during coagroinfiltration in N. benthamiana. In addition, virus particle assembly into virus-like particles in CP agroinfiltrated plants was increased in the presence of Hsc70-2. HSP70 was found to increase the targeting of CNV CP to chloroplasts during infection, reinforcing the role of HSP70 in chloroplast targeting of host proteins. Hence, our findings have led to the discovery of a highly induced host factor that has been co-opted to play multiple roles during several stages of the CNV infection cycle.

IMPORTANCE

Because of the small size of its RNA genome, CNV is dependent on interaction with host cellular components to successfully complete its multiplication cycle. We have found that CNV induces HSP70 family homologs to a high level during infection, possibly as a result of the host response to the high levels of CNV proteins that accumulate during infection. Moreover, we have found that CNV co-opts HSP70 family homologs to facilitate several aspects of the infection process such as viral RNA, coat protein and virus accumulation. Chloroplast targeting of the CNV CP is also facilitated, which may aid in CNV suppression of host defense responses. Several viruses have been shown to induce HSP70 during infection and others to utilize HSP70 for specific aspects of infection such as replication, assembly, and disassembly. We speculate that HSP70 may play multiple roles in the infection processes of many viruses.

The similarity between N-terminal targeting signals for protein import into different organelles and its evolutionary relevance

The proper distribution of proteins between the cytosol and various membrane-bound compartments is crucial for the functionality of eukaryotic cells. This requires the cooperation between protein transport machineries that translocate diverse proteins from the cytosol into these compartments and targeting signal(s) encoded within the primary sequence of these proteins that define their cellular destination. The mechanisms exerting protein translocation differ remarkably between the compartments, but the predominant targeting signals for mitochondria, chloroplasts and the ER share the N-terminal position, an α-helical structural element and the removal from the core protein by intraorganellar cleavage. Interestingly, similar properties have been described for the peroxisomal targeting signal type 2 mediating the import of a fraction of soluble peroxisomal proteins, whereas other peroxisomal matrix proteins encode the type 1 targeting signal residing at the extreme C-terminus. The structural similarity of N-terminal targeting signals poses a challenge to the specificity of protein transport, but allows the generation of ambiguous targeting signals that mediate dual targeting of proteins into different compartments. Dual targeting might represent an advantage for adaptation processes that involve a redistribution of proteins, because it circumvents the hierarchy of targeting signals. Thus, the co-existence of two equally functional import pathways into peroxisomes might reflect a balance between evolutionary constant and flexible transport routes.

Non-native, N-terminal Hsp70 molecular motor recognition elements in transit peptides support plastid protein translocation

Previously, we identified the N-terminal domain of transit peptides (TPs) as a major determinant for the translocation step in plastid protein import. Analysis of Arabidopsis TP dataset revealed that this domain has two overlapping characteristics, highly uncharged and Hsp70-interacting. To investigate these two properties, we replaced the N-terminal domains of the TP of the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase and its reverse peptide with a series of unrelated peptides whose affinities to the chloroplast stromal Hsp70 have been determined. Bioinformatic analysis indicated that eight out of nine peptides in this series are not similar to the TP N terminus. Using in vivo and in vitro protein import assays, the majority of the precursors containing Hsp70-binding elements were targeted to plastids, whereas none of the chimeric precursors lacking an N-terminal Hsp70-binding element were targeted to the plastids. Moreover, a pulse-chase assay showed that two chimeric precursors with the most uncharged peptides failed to translocate into the stroma. The ability of multiple unrelated Hsp70-binding elements to support protein import verified that the majority of TPs utilize an N-terminal Hsp70-binding domain during translocation and expand the mechanistic view of the import process. This work also indicates that synthetic biology may be utilized to create de novo TPs that exceed the targeting activity of naturally occurring sequences.

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

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

The selective biotin tagging and thermolysin proteolysis of chloroplast outer envelope proteins reveals information on protein topology and association into complexes

The understanding of chloroplast function requires the precise localization of proteins in each of its sub-compartments. High-sensitivity mass spectrometry has allowed the inventory of proteins in thylakoid, stroma, and envelope fractions. Concerning membrane association, proteins can be either integral or peripheral or even soluble proteins bound transiently to a membrane complex. We sought a method providing information at the surface of the outer envelope membrane (OEM), based on specific tagging with biotin or proteolysis using thermolysin, a non-membrane permeable protease. To evaluate this method, envelope, thylakoid, and stroma proteins were separated by two-dimensional electrophoresis and analyzed by immunostaining and mass spectrometry. A short selection of proteins associated to the chloroplast envelope fraction was checked after superficial treatments of intact chloroplasts. We showed that this method could allow the characterization of OEM embedded proteins facing the cytosol, as well as peripheral and soluble proteins associated via tight or lose interactions. Some stromal proteins were associated with biotinylated spots and analyzes are still needed to determine whether polypeptides were tagged prior import or if they co-migrated with OEM proteins. This method also suggests that some proteins associated with the inner envelope membrane (IEM) might need the integrity of a trans-envelope (IEM-OEM) protein complex (e.g., division ring-forming components) or at least an intact OEM partner. Following this evaluation, proteomic analyzes should be refined and the putative role of inter-membrane space components stabilizing trans-envelope complexes demonstrated. For future comprehensive studies, perspectives include the dynamic analyses of OEM proteins and IEM-OEM complexes in various physiological contexts and using virtually any other purified membrane organelle.

Transit peptide design and plastid import regulation

Import of most nuclear encoded proteins into plastids is directed by an N-terminal transit peptide. Early studies suggested that transit peptides are interchangeable between precursor proteins. However, emerging evidence shows that different transit peptides contain different motifs specifying their preference for certain plastid types or ages. In this opinion article, we propose a 'multi-selection and multi-order' (M&M) model for transit peptide design, describing each transit peptide as an assembly of motifs for interacting with selected translocon components. These interactions determine the preference of the precursor for a particular plastid type or age. Furthermore, the order of the motifs varies among transit peptides, explaining why no consensus sequences have been identified through linear sequence comparison of all transit peptides as one group.

Chloroplastic Hsp100 chaperones ClpC2 and ClpD interact in vitro with a transit peptide only when it is located at the N-terminus of a protein

BACKGROUND

Clp/Hsp100 chaperones are involved in protein quality control. They act as independent units or in conjunction with a proteolytic core to degrade irreversibly damaged proteins. Clp chaperones from plant chloroplasts have been also implicated in the process of precursor import, along with Hsp70 chaperones. They are thought to pull the precursors in as the transit peptides enter the organelle. How Clp chaperones identify their substrates and engage in their processing is not known. This information may lie in the position, sequence or structure of the Clp recognition motifs.

RESULTS

We tested the influence of the position of the transit peptide on the interaction with two chloroplastic Clp chaperones, ClpC2 and ClpD from Arabidopsis thaliana (AtClpC2 and AtClpD). The transit peptide of ferredoxin-NADP+ reductase was fused to either the N- or C-terminal end of glutathione S-transferase. Another fusion with the transit peptide interleaved between two folded proteins was used to probe if AtClpC2 and AtClpD could recognize tags located in the interior of a polypeptide. We also used a mutated transit peptide that is not targeted by Hsp70 chaperones (TP1234), yet it is imported at a normal rate. The fusions were immobilized on resins and the purified recombinant chaperones were added. After a washing protocol, the amount of bound chaperone was assessed. Both AtClpC2 and AtClpD interacted with the transit peptides when they were located at the N-terminal position of a protein, but not when they were allocated to the C-terminal end or at the interior of a polypeptide.

CONCLUSIONS

AtClpC2 and AtClpD have a positional preference for interacting with a transit peptide. In particular, the localization of the signal sequence at the N-terminal end of a protein seems mandatory for interaction to take place. Our results have implications for the understanding of protein quality control and precursor import in chloroplasts.

Molecular chaperone involvement in chloroplast protein import

Chloroplasts are organelles of endosymbiotic origin that perform essential functions in plants. They contain about 3000 different proteins, the vast majority of which are nucleus-encoded, synthesized in precursor form in the cytosol, and transported into the chloroplasts post-translationally. These preproteins are generally imported via envelope complexes termed TOC and TIC (Translocon at the Outer/Inner envelope membrane of Chloroplasts). They must navigate different cellular and organellar compartments (e.g., the cytosol, the outer and inner envelope membranes, the intermembrane space, and the stroma) before arriving at their final destination. It is generally considered that preproteins are imported in a largely unfolded state, and the whole process is energy-dependent. Several chaperones and cochaperones have been found to mediate different stages of chloroplast import, in similar fashion to chaperone involvement in mitochondrial import. Cytosolic factors such as Hsp90, Hsp70 and 14-3-3 may assist preproteins to reach the TOC complex at the chloroplast surface, preventing their aggregation or degradation. Chaperone involvement in the intermembrane space has also been proposed, but remains uncertain. Preprotein translocation is completed at the trans side of the inner membrane by ATP-driven motor complexes. A stromal Hsp100-type chaperone, Hsp93, cooperates with Tic110 and Tic40 in one such motor complex, while stromal Hsp70 is proposed to act in a second, parallel complex. Upon arrival in the stroma, chaperones (e.g., Hsp70, Cpn60, cpSRP43) also contribute to the folding, assembly or onward intraorganellar guidance of the proteins. In this review, we focus on chaperone involvement during preprotein translocation at the chloroplast envelope. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.

Chloroplast envelope protein targeting fidelity is independent of cytosolic components in dual organelle assays

The general mechanisms of intracellular protein targeting are well established, and depend on a targeting sequence in the protein, which is recognized by a targeting factor. Once a membrane protein is delivered to the correct organelle its targeting sequence can be recognized by receptors and a translocase, leading to membrane insertion. However, the relative contribution of each step for generating fidelity and efficiency of the overall process has not been systematically addressed. Here, we use tail-anchored (TA) membrane proteins in cell-free competitive targeting assays to chloroplasts to show that targeting can occur efficiently and with high fidelity in the absence of all cytosolic components, suggesting that chloroplast envelope protein targeting is primarily dependent on events at the outer envelope. Efficiency of targeting was increased by the addition of complete cytosol, and by Hsp70 or Hsp90, depending on the protein, but none of these cytosolic components influenced the fidelity of targeting. Our results suggest that the main role of targeting factors in chloroplast localization is to increase targeting efficiency by maintaining recognition competency at the outer envelope.

Cytosolic HSP90 cochaperones HOP and FKBP interact with freshly synthesized chloroplast preproteins of Arabidopsis

Most chloroplast and mitochondrial proteins are synthesized in the cytosol of the plant cell and have to be imported into the organelles post-translationally. Molecular chaperones play an important role in preventing protein aggregation of freshly translated preproteins and assist in maintaining the preproteins in an import competent state. Preproteins can associate with HSP70, HSP90, and 14-3-3 proteins in the cytosol. In this study, we analyzed a large set of wheat germ-translated chloroplast preproteins with respect to their chaperone binding. Our results demonstrate that the formation of distinct 14-3-3 or HSP90 containing preprotein complexes is a common feature in post-translational protein transport in addition to preproteins that seem to interact solely with HSP70. We were able to identify a diverse and extensive class of preproteins as HSP90 substrates, thus providing a tool for the investigation of HSP90 client protein association. The analyses of chimeric HSP90 and 14-3-3 binding preproteins with exchanged transit peptides indicate an involvement of both the transit peptide and the mature part of the proteins, in HSP90 binding. We identified two partner components of the HSP90 cycle, which were present in the preprotein containing high-molecular-weight complexes, the HSP70/HSP90 organizing protein HOP, as well as the immunophilin FKBP73. The results establish chloroplast preproteins as a general class of HSP90 client proteins in plants using HOP and FKBP as novel cochaperones.

Small heat shock protein Hsp17.8 functions as an AKR2A cofactor in the targeting of chloroplast outer membrane proteins in Arabidopsis

Plastid proteins that are encoded by the nuclear genome and synthesized in the cytosol undergo posttranslational targeting to plastids. Ankyrin repeat protein 2A (AKR2A) and AKR2B were recently shown to be involved in the targeting of proteins to the plastid outer envelope. However, it remains unknown whether other factors are involved in this process. In this study, we investigated a factor involved in AKR2A-mediated protein targeting to chloroplasts in Arabidopsis (Arabidopsis thaliana). Hsp17.8, a member of the class I (CI) cytosolic small heat shock proteins (sHsps), was identified in interactions with AKR2A. The interaction between Hsp17.8 and AKR2A was further confirmed by coimmunoprecipitation experiments. The carboxyl-terminal ankyrin repeat domain of AKR2A was responsible for AKR2A binding to Hsp17.8. Other CI cytosolic sHsps also interact with AKR2A to varying degrees. Additionally, Hsp17.8 binds to chloroplasts in vitro and enhances AKR2A binding to chloroplasts. HSP17.8 was expressed under normal growth conditions, and its expression increased after heat shock. Hsp17.8 exists as a dimer under normal physiological conditions, and it is converted to high oligomeric complexes, ranging from 240 kD to greater than 480 kD, after heat shock. High levels of Hsp17.8 together with AKR2A resulted in increased plastid targeting of Outer Envelope Protein7 (OEP7), a plastid outer envelope protein expressed as a green fluorescent protein fusion protein. In contrast, artificial microRNA suppression of HSP17.8 and closely related CI cytosolic sHSPs in protoplasts resulted in a reduction of OEP7:green fluorescent protein targeting to plastids. Based on these data, we propose that Hsp17.8 functions as an AKR2A cofactor in targeting membrane proteins to plastid outer membranes under normal physiological conditions.

Insights into the Clp/HSP100 chaperone system from chloroplasts of Arabidopsis thaliana

HSP100 proteins are molecular chaperones involved in protein quality control. They assist in protein (un)folding, prevent aggregation, and are thought to participate in precursor translocation across membranes. Caseinolytic proteins ClpC and ClpD from plant chloroplasts belong to the HSP100 family. Their role has hitherto been investigated by means of physiological studies and reverse genetics. In the present work, we employed an in vitro approach to delve into the structural and functional characteristics of ClpC2 and ClpD from Arabidopsis thaliana (AtClpC2 and AtClpD). They were expressed in Escherichia coli and purified to near-homogeneity. The proteins were detected mainly as dimers in solution, and, upon addition of ATP, the formation of hexamers was observed. Both proteins exhibited basal ATPase activity (K(m), 1.42 mm, V(max), 0.62 nmol/(min × μg) for AtClpC2 and K(m) ∼19.80 mm, V(max) ∼0.19 nmol/(min × μg) for AtClpD). They were able to reactivate the activity of heat-denatured luciferase (∼40% for AtClpC2 and ∼20% for AtClpD). The Clp proteins tightly bound a fusion protein containing a model transit peptide. This interaction was detected by binding assays, where the chaperones were selectively trapped by the transit peptide-containing fusion, immobilized on glutathione-agarose beads. Association of HSP100 proteins to import complexes with a bound transit peptide-containing fusion was also observed in intact chloroplasts. The presented data are useful to understand protein quality control and protein import into chloroplasts in plants.

Exploring the positional importance of aromatic residues and lysine in the interactions of peptides with the Plasmodium falciparum Hsp70-1

Plasmodium falciparum harbors an essential relict plastid called the apicoplast that is involved in several important biosynthetic processes. Over 500 nuclear encoded proteins are imported into the organelle that is now recognized as an important therapeutic target. These proteins contain an N-terminal transit peptide sequence essential for apicoplast targeting during which the P. falciparum Hsp70-1 plays an important role. In the present study, we have focused on the in vitro interactions of PfHsp70-1 with synthetic peptides endowed with transit peptide like features. The peptides exhibit higher affinity for PfHsp70-1 in the presence of ADP compared to ATP. The results highlight the positional importance of selected residues in the designed peptides for affinity. They suggest that better peptide affinity for the protein requires a Lys at second position, retention of aromatic residue at the last position, and absence of acidic residues at any position in the transit peptides. Overall, the present work is the first in vitro fluorescence-based study of PfHsp70-1 with peptides possessing transit peptide-like features.

Arabidopsis stromal 70-kDa heat shock proteins are essential for chloroplast development

70 kDa heat shock proteins (Hsp70s) act as molecular chaperones involved in essential cellular processes such as protein folding and protein transport across membranes. They also play a role in the cell's response to a wide range of stress conditions. The Arabidopsis family of Hsp70s homologues includes two highly conserved proteins, cpHsc70-1 and cpHsc70-2 which are both imported into chloroplasts (Su and Li in Plant Physiol 146:1231-1241, 2008). Here, we demonstrate that YFP-fusion proteins of both cpHsc70-1 and cpHsc70-2 are predominantly stromal, though low levels were detected in the thylakoid membrane. Both genes are ubiquitously expressed at high levels in both seedlings and adult plants. We further show that both cpHsc70-1 and cpHsc70-2 harbour ATPase activity which is essential for Hsp70 chaperone activity. A previously described T-DNA insertion line for cpHsc70-1 (DeltacpHsc70-1) has variegated cotyledons, malformed leaves, growth retardation, impaired root growth and sensitivity to heat shock treatment. In addition, under stress conditions, this mutant also exhibits unusual sepals, and malformed flowers and sucrose concentrations as low as 1% significantly impair growth. cpHsc70-1/cpHsc70-2 double-mutants are lethal. However, we demonstrate through co-suppression and artificial microRNA (amiRNA) approaches that transgenic plants with severely reduced levels of both genes have a white and stunted phenotype. Interestingly, chloroplasts in these plants have an unusual morphology and contain few or no thylakoid membranes. Our data show that cpHsc70-1 and cpHsc70-2 are essential ATPases, have overlapping roles and are required for normal plastid structure.

Protein import into chloroplasts--how chaperones feature into the game

Chloroplasts originated from an endosymbiotic event, in which an ancestral photosynthetic cyanobacterium was engulfed by a mitochondriate eukaryotic host cell. During evolution, the endosymbiont lost its autonomy by means of a massive transfer of genetic information from the prokaryotic genome to the host nucleus. Consequently, the development of protein import machineries became necessary for the relocation of proteins that are now nuclear-encoded and synthesized in the cytosol but destined for the chloroplast. Organelle biogenesis and maintenance requires a tight coordination of transcription, translation and protein import between the host cell and the organelle. This review focuses on the translocation complexes in the outer and inner envelope membrane with a special emphasis on the role of molecular chaperones. This article is part of a Special Issue entitled Protein translocation across or insertion into membranes.

Studying the functional genomics of stress responses in loblolly pine with the Expresso microarray experiment management system

Conception, design, and implementation of cDNA microarray experiments present a variety of bioinformatics challenges for biologists and computational scientists. The multiple stages of data acquisition and analysis have motivated the design of Expresso, a system for microarray experiment management. Salient aspects of Expresso include support for clone replication and randomized placement; automatic gridding, extraction of expression data from each spot, and quality monitoring; flexible methods of combining data from individual spots into information about clones and functional categories; and the use of inductive logic programming for higher-level data analysis and mining. The development of Expresso is occurring in parallel with several generations of microarray experiments aimed at elucidating genomic responses to drought stress in loblolly pine seedlings. The current experimental design incorporates 384 pine cDNAs replicated and randomly placed in two specific microarray layouts. We describe the design of Expresso as well as results of analysis with Expresso that suggest the importance of molecular chaperones and membrane transport proteins in mechanisms conferring successful adaptation to long-term drought stress.

On the impact of precursor unfolding during protein import into chloroplasts

Protein translocation across membranes is a fundamental cellular process. The majority of the proteins of organelles such as mitochondria and chloroplasts is synthesized in the cytosol and subsequently imported in a post-translational manner. The precursor proteins have to be unfolded at least for translocation, but it has also been assumed that they are unfolded during transport to the organelle in the cytosol. Unfolding is governed by chaperones and the translocon itself. At the same time, chaperones provide the energy for the import process. The energetic properties of the chloroplast translocon were studied by import of the Ig-like module of the muscle protein titin fused to the transit peptide of the chloroplast targeted oxygen evolving complex subunit of 33 kDa (OE33). Our results suggest that p(OE33)titin is folded prior to import and that translocation is initiated by unfolding after having bound to the translocon at the chloroplast surface. Using a set of stabilizing and destabilizing mutants of titin previously analyzed by atomic force microscopy and as passenger for mitochondrial translocation, we studied the unfolding force provided by the chloroplast translocon. Based on these results, a model for translocation is discussed.

A stromal heat shock protein 70 system functions in protein import into chloroplasts in the moss Physcomitrella patens

Heat shock protein 70s (Hsp70s) are encoded by a multigene family and are located in different cellular compartments. They have broad-ranging functions, including involvement in protein trafficking, prevention of protein aggregation, and assistance in protein folding. Hsp70s work together with their cochaperones, J domain proteins and nucleotide exchange factors (e.g., GrpEs), in a functional cycle of substrate binding and release accompanied by ATP hydrolysis. We have taken advantage of the gene targeting capability of the moss Physcomitrella patens to investigate the functions of chloroplast Hsp70s. We identified four Hsp70 genes and two GrpE cochaperone homolog genes (CGE) in moss that encode chloroplast proteins. Disruption of one of the Hsp70 genes, that for Hsp70-2, caused lethality, and protein import into heat-shocked chloroplasts isolated from temperature-sensitive hsp70-2 mutants was appreciably impaired. Whereas the double cge null mutant was not viable, we recovered a cge1 null/cge2 knock down mutant in which Hsp70-2 was upregulated. Chloroplasts isolated from this mutant demonstrated a defect in protein import. In addition, two different precursors staged as early import intermediates could be immunoprecipitated with an Hsp70-2-specific antibody. This immunoprecipitate also contained Hsp93 and Tic40, indicating that it represents a precursor still in the Toc/Tic translocon. Together, these data indicate that a stromal Hsp70 system plays a crucial role in protein import into chloroplasts.

Protein transport into chloroplasts

Most proteins in chloroplasts are encoded by the nuclear genome and synthesized as precursors with N-terminal targeting signals called transit peptides. Novel machinery has evolved to specifically import these proteins from the cytosol into chloroplasts. This machinery consists of more than a dozen components located in and around the chloroplast envelope, including a pair of GTPase receptors, a beta-barrel-type channel across the outer membrane, and an AAA(+)-type motor in the stroma. How individual components assemble into functional subcomplexes and the sequential steps of the translocation process are being mapped out. An increasing number of noncanonical import pathways, including a pathway with initial transport through the endomembrane system, is being revealed. Multiple levels of control on protein transport into chloroplasts have evolved, including the development of two receptor subfamilies, one for photosynthetic proteins and one for housekeeping proteins. The functions or expression levels of some translocon components are further adjusted according to plastid type, developmental stage, and metabolic conditions.

Targeting of nucleus-encoded proteins to chloroplasts in plants

Most chloroplast proteins are encoded in the nucleus and synthesized on free, cytosolic ribosomes in precursor form. Each precursor has an amino-terminal extension called a transit peptide, which directs the protein through a post-translational targeting pathway and is removed upon arrival inside the organelle. This 'protein import' process is mediated by the coordinate action of two multiprotein complexes, one in each of the envelope membranes: the TOC and TIC (Translocon at the Outer/ Inner envelope membrane of Chloroplasts) machines. Many components of these complexes have been identified biochemically in pea; these include transit peptide receptors, channel proteins, and molecular chaperones. Intriguingly, the Arabidopsis genome encodes multiple, homologous genes for receptor components of the TOC complex. Careful analysis indicated that the different receptor isoforms operate in different import pathways with distinct precursor recognition specificities. These 'substrate-specific' import pathways might play a role in the differentiation of different plastid types, and/or act to prevent deleterious competition effects between abundant and nonabundant precursors. Until recently, all proteins destined for internal chloroplast compartments were thought to possess a cleavable transit peptide, and to engage the TOC/TIC machinery. New studies using proteomics and other approaches have revealed that this is far from true. Remarkably, a significant number of chloroplast proteins are transported via a pathway that involves the endoplasmic reticulum and Golgi apparatus. Other recent reports have elucidated an intriguing array of protein targeting routes leading to the envelope membranes themselves.

Arabidopsis nuclear-encoded plastid transit peptides contain multiple sequence subgroups with distinctive chloroplast-targeting sequence motifs

The N-terminal transit peptides of nuclear-encoded plastid proteins are necessary and sufficient for their import into plastids, but the information encoded by these transit peptides remains elusive, as they have a high sequence diversity and lack consensus sequences or common sequence motifs. Here, we investigated the sequence information contained in transit peptides. Hierarchical clustering on transit peptides of 208 plastid proteins showed that the transit peptide sequences are grouped to multiple sequence subgroups. We selected representative proteins from seven of these multiple subgroups and confirmed that their transit peptide sequences are highly dissimilar. Protein import experiments revealed that each protein contained transit peptide-specific sequence motifs critical for protein import into chloroplasts. Bioinformatics analysis identified sequence motifs that were conserved among members of the identified subgroups. The sequence motifs identified by the two independent approaches were nearly identical or significantly overlapped. Furthermore, the accuracy of predicting a chloroplast protein was greatly increased by grouping the transit peptides into multiple sequence subgroups. Based on these data, we propose that the transit peptides are composed of multiple sequence subgroups that contain distinctive sequence motifs for chloroplast targeting.

Transit peptide diversity and divergence: A global analysis of plastid targeting signals

Proteins are targeted to plastids by N-terminal transit peptides, which are recognized by protein import complexes in the organelle membranes. Historically, transit peptide properties have been defined from vascular plant sequences, but recent large-scale genome sequencing from the many plastid-containing lineages across the tree of life has provided a much broader representation of targeted proteins. This includes the three lineages containing primary plastids (plants and green algae, rhodophytes and glaucophytes) and also the seven major lineages that contain secondary plastids, "secondhand" plastids derived through eukaryotic endosymbiosis. Despite this extensive spread of plastids throughout Eukaryota, an N-terminal transit peptide has been maintained as an essential plastid-targeting motif. This article provides the first global comparison of transit peptide composition and summarizes conservation of some features, the loss of an ancestral motif from the green lineages including plants, and modifications to transit peptides that have occurred in secondary and even tertiary plastids.

Protein targeting to the malaria parasite plastid

The relict plastid, or apicoplast, of the malaria parasite Plasmodium falciparum is an essential organelle and a promising drug target. Most apicoplast proteins are nuclear encoded and post-translationally targeted into the organelle using a bipartite N-terminal extension, consisting of a typical endomembrane signal peptide and a plant-like transit peptide. Apicoplast protein targeting commences through the parasite's secretory pathway. We review recent experimental evidence suggesting that the apicoplast resides in the mainstream endomembrane system proximal to the Golgi. Further, we explore possible mechanisms for translocation of nuclear-encoded apicoplast proteins across the four bounding membranes. Recent insights into the composition of the transit peptide and how it is cleaved and degraded after use are also examined. Characterization of apicoplast targeting has not only shed light on how this group of parasites mediate intracellular protein trafficking events but also it has helped identify new targets for therapeutics. The distinctive leader sequences of apicoplast proteins make them readily identifiable, allowing assembly of a virtual organelle metabolome from the genome. Such analysis has lead to the identification of several biochemical pathways that are absent from the human host and thus represent novel therapeutic targets for parasitic infection.

Post-translational integration of tail-anchored proteins is facilitated by defined molecular chaperones

Tail-anchored (TA) proteins provide an ideal model for studying post-translational integration at the endoplasmic reticulum (ER) of eukaryotes. There are multiple pathways for delivering TA proteins from the cytosol to the ER membrane yet, whereas an ATP-dependent route predominates, none of the cytosolic components involved had been identified. In this study we have directly addressed this issue and identify novel interactions between a model TA protein and the two cytosolic chaperones Hsp40 and Hsc70. To investigate their function, we have reconstituted the membrane integration of TA proteins using purified components. Remarkably, we find that a combination of Hsc70 and Hsp40 can completely substitute for the ATP-dependent factors present in cytosol. On the basis of this in vitro analysis, we conclude that this chaperone pair can efficiently facilitate the ATP-dependent integration of TA proteins.

15-Deoxyspergualin Primarily Targets the Trafficking of Apicoplast Proteins in Plasmodium falciparum

15-Deoxyspergualin, an immunosuppressant with tumoricidal and antimalarial properties, has been implicated in the inhibition of a diverse array of cellular processes including polyamine synthesis and protein synthesis. Endeavoring to identify the mechanism of antimalarial action of this molecule, we examined its effect on Plasmodium falciparum protein synthesis, polyamine biosynthesis, and transport. 15-Deoxyspergualin stalled protein synthesis in P. falciparum through Hsp70 sequestration and subsequent phosphorylation of the eukaryotic initiation factor eIF2alpha. However, protein synthesis inhibition as well as polyamine depletion were invoked only by high micromolar concentrations of 15-deoxyspergualin, in contrast to the submicromolar concentrations sufficient to inhibit parasite growth. Further investigations demonstrated that 15-deoxyspergualin in the malaria parasite primarily targets the hitherto underexplored process of trafficking of nucleus-encoded proteins to the apicoplast. Our finding that 15-deoxyspergualin kills the malaria parasite by interfering with targeting of nucleus-encoded proteins to the apicoplast not only exposes a chink in the armor of the malaria parasite, but also reveals new realms in our endeavors to study this intriguing biological process.

A 38-amino-acid sequence encompassing the arm domain of the cucumber necrosis virus coat protein functions as a chloroplast transit Peptide in infected plants

Experiments to determine the subcellular location of the coat protein (CP) of the tombusvirus Cucumber necrosis virus (CNV) have been conducted. By confocal microscopy, it was found that an agroinfiltrated CNV CP-green fluorescent protein (GFP) fusion targets chloroplasts in Nicotiana benthamiana leaves and that a 38-amino-acid (aa) region that includes the complete CP arm region plus the first 4 amino acids of the shell domain are sufficient for targeting. Western blot analyses of purified and fractionated chloroplasts showed that the 38-aa region directs import to the chloroplast stroma, suggesting that the CNV arm can function as a chloroplast transit peptide (TP) in plants. Several features of the 38-aa region are similar to features typical of chloroplast TPs, including (i) the presence of an alanine-rich uncharged region near the N terminus, followed by a short region rich in basic amino acids; (ii) a conserved chloroplast TP phosphorylation motif; (iii) the requirement that the CNV 38-aa sequence be present at the amino terminus of the imported protein; and (iv) specific proteolytic cleavage upon import into the chloroplast stroma. In addition, a region just downstream of the 38-aa sequence contains a 14-3-3 binding motif, suggesting that chloroplast targeting requires 14-3-3 binding, as has been suggested for cellular proteins that are targeted to chloroplasts. Chloroplasts of CNV-infected plants were found to contain CNV CP, but only the shell and protruding domain regions were present, indicating that CNV CP enters chloroplasts during infection and that proteolytic cleavage occurs as predicted from agroinfiltration studies. We also found that particles of a CNV CP mutant deficient in externalization of the arm region have a reduced ability to establish infection. The potential biological significance of these findings is discussed.

Crystallization and preliminary X-ray diffraction studies of ferredoxin reductase from Leptospira interrogans

Ferredoxin-NADP+ reductase (FNR) is an FAD-containing enzyme that catalyzes electron transfer between NADP(H) and ferredoxin. Here, results are reported of the recombinant expression, purification and crystallization of FNR from Leptospira interrogans, a parasitic bacterium of animals and humans. The L. interrogans FNR crystals belong to a primitive monoclinic space group and diffract to 2.4 angstroms resolution at a synchrotron source.

Protein import into chloroplasts: an ever-evolving storyThis review is one of a selection of papers published in the Special Issue on Plant Cell Biology

Chloroplasts are but one type of a diverse group of essential organelles that distinguish plant cells and house many critical biochemical pathways, including photosynthesis. The biogenesis of plastids is essential to plant growth and development and relies on the targeting and import of thousands of nuclear-encoded proteins from the cytoplasm. The import of the vast majority of these proteins is dependent on translocons located in the outer and inner envelope membranes of the chloroplast, termed the Toc and Tic complexes, respectively. The core components of the Toc and Tic complexes have been identified within the last 12 years; however, the precise functions of many components are still being elucidated, and new components are still being identified. In Arabidopsis thaliana (and other species), many of the components are encoded by more than one gene, and it appears that the isoforms differentially associate with structurally distinct import complexes. Furthermore, it appears that these complexes represent functionally distinct targeting pathways, and the regulation of import by these separate pathways may play a role in the differentiation and specific functions of distinct plastid types during plant growth and development. This review summarizes these recent discoveries and emphasizes the mechanisms of differential Toc complex assembly and substrate recognition.

Functional characterization of sequence motifs in the transit peptide of Arabidopsis small subunit of rubisco

The transit peptides of nuclear-encoded chloroplast proteins are necessary and sufficient for targeting and import of proteins into chloroplasts. However, the sequence information encoded by transit peptides is not fully understood. In this study, we investigated sequence motifs in the transit peptide of the small subunit of the Rubisco complex by examining the ability of various mutant transit peptides to target green fluorescent protein reporter proteins to chloroplasts in Arabidopsis (Arabidopsis thaliana) leaf protoplasts. We divided the transit peptide into eight blocks (T1 through T8), each consisting of eight or 10 amino acids, and generated mutants that had alanine (Ala) substitutions or deletions, of one or two T blocks in the transit peptide. In addition, we generated mutants that had the original sequence partially restored in single- or double-T-block Ala (A) substitution mutants. Analysis of chloroplast import of these mutants revealed several interesting observations. Single-T-block mutations did not noticeably affect targeting efficiency, except in T1 and T4 mutations. However, double-T mutants, T2A/T4A, T3A/T6A, T3A/T7A, T4A/T6A, and T4A/T7A, caused a 50% to 100% loss in targeting ability. T3A/T6A and T4A/T6A mutants produced only precursor proteins, whereas T2A/T4A and T4A/T7A mutants produced only a 37-kD protein. Detailed analyses revealed that sequence motifs ML in T1, LKSSA in T3, FP and RK in T4, CMQVW in T6, and KKFET in T7 play important roles in chloroplast targeting. In T1, the hydrophobicity of ML is important for targeting. LKSSA in T3 is functionally equivalent to CMQVW in T6 and KKFET in T7. Furthermore, subcellular fractionation revealed that Ala substitution in T1, T3, and T6 produced soluble precursors, whereas Ala substitution in T4 and T7 produced intermediates that were tightly associated with membranes. These results demonstrate that the transit peptide contains multiple motifs and that some of them act in concert or synergistically.

Recognition and envelope translocation of chloroplast preproteins

Plastids are a diverse group of plant organelles that perform essential functions including important steps in many biosynthetic pathways. Chloroplasts are the best characterized type of plastid, and constitute the site of oxygenic photosynthesis in plants, a process essential to all higher life forms. It is well established that the majority (>90%) of chloroplast proteins are nucleus-encoded and must be post-translationally imported into these envelope-bound compartments. Most nucleus-encoded chloroplast proteins are translated in precursor form on cytosolic ribosomes, targeted to the chloroplast surface, and then imported across the double-membrane envelope by translocons in the outer and inner envelope membranes of the chloroplast, termed TOC and TIC, respectively. Recently, significant progress has been made in our understanding of how proteins are targeted to the chloroplast surface and translocated across the chloroplast envelope into the stroma. Evidence suggesting the existence of multiple import pathways at the outer envelope membrane for different classes of precursor proteins has been presented. These pathways appear to utilize similar TOC complexes equipped with different combinations of homologous GTPase receptors, providing preprotein recognition specificity.

NADP-malic enzyme and Hsp70: co-purification of both proteins and modification of NADP-malic enzyme properties by association with Hsp70

Different preparations of antibodies against 62 kDa NADP-malic enzyme (NADP-ME) from purified maize leaves cross-react with a 72 kDa protein from diverse tissues in many species. A 72 kDa protein, suggested to be a non-photosynthetic NADP-ME, has been purified from several plant species. However, to date, a cDNA coding for this putative 72 kDa NADP-ME has not been isolated. The screening of maize and tobacco leaf expression libraries using antibodies against purified 62 kDa NADP-ME allowed the identification of a heat shock protein (Hsp70). In addition, tandem mass spectrometry (MS/MS) studies indicate that along with NADP-ME, a 72 kDa protein, identified as an Hsp70 and reacting with the antibodies, is also purified from maize roots. On the other hand, the screening of a maize root cDNA library revealed the existence of a cDNA that encodes a mature 66 kDa NADP-ME. These results suggest that the 72 kDa protein is not actually an NADP-ME but in fact an Hsp70, at least in maize and tobacco. Probably, NADP-ME-Hsp70 association, taking place at least when preparing crude extracts, can lead to a co-purification of the proteins and can thus explain the cross-reaction of the antibodies. In the present work, we analyse and discuss a probable interaction of NADP-ME with Hsp70.