Recognition and envelope translocation of chloroplast preproteins

Genetic Mapping of the Gmpgl3 Mutant Reveals the Function of GmTic110a in Soybean Chloroplast Development

The generation of oxygen and organic matter in plants mainly depends on photosynthesis, which directly affects plant growth and development. The chloroplast is the main organelle in which photosynthesis occurs. In this study, a Glycine max pale green leaf 3-1 (Gmpgl3-1) mutant was isolated from the soybean mutagenized population. The Gmpgl3-1 mutant presented with decreased chlorophyll contents, reduced chloroplast stroma thylakoids, reduced yields, and decreased numbers of pods per plant. Bulked segregant analysis (BSA) together with map-based cloning revealed a single-nucleotide non-synonymous mutation at the 341st nucleotide of the first exon of the chloroplast development-related GmTic110a gene. The phenotype of the knockout plants was the same as that of the mutant. The GmTic110a gene was highly expressed in the leaves at various developmental stages, and its protein was localized to the inner chloroplast membrane. Split luciferase complementation assays and coimmunoprecipitation (co-IP) experiments revealed that GmTic110a interacted with GmTic20, GmTic40a, and GmTic40b in tobacco leaves. These results indicated that the GmTic110a gene plays an important role in chloroplast development.

Chloroplast: The Emerging Battlefield in Plant-Microbe Interactions

Higher plants and some algae convert the absorbed light into chemical energy through one of the most important organelles, chloroplast, for photosynthesis and store it in the form of organic compounds to supply their life activities. However, more and more studies have shown that the role of chloroplasts is more than a factory for photosynthesis. In the process of light conversion to chemical energy, any damage to the components of chloroplast may affect the photosynthesis efficiency and promote the production of by-products, reactive oxygen species, that are mainly produced in the chloroplasts. Substantial evidence show that chloroplasts are also involved in the battle of plants and microbes. Chloroplasts are important in integrating a variety of external environmental stimuli and regulate plant immune responses by transmitting signals to the nucleus and other cell compartments through retrograde signaling pathways. Besides, chloroplasts can also regulate the biosynthesis and signal transduction of phytohormones, including salicylic acid and jasmonic acid, to affect the interaction between the plants and microbes. Since chloroplasts play such an important role in plant immunity, correspondingly, chloroplasts have become the target of pathogens. Different microbial pathogens target the chloroplast and affect its functions to promote their colonization in the host plants.

A non-structural protein encoded by Rice Dwarf Virus targets to the nucleus and chloroplast and inhibits local RNA silencing

RNA silencing is a potent antiviral mechanism in plants and animals. As a counter-defense, many viruses studied to date encode one or more viral suppressors of RNA silencing (VSR). In the latter case, how different VSRs encoded by a virus function in silencing remains to be fully understood. We previously showed that the nonstructural protein Pns10 of a Phytoreovirus, Rice dwarf virus (RDV), functions as a VSR. Here we present evidence that another nonstructural protein, Pns11, also functions as a VSR. While Pns10 was localized in the cytoplasm, Pns11 was localized both in the nucleus and chloroplasts. Pns11 has two bipartite nuclear localization signals (NLSs), which were required for nuclear as well as chloroplastic localization. The NLSs were also required for the silencing activities of Pns11. This is the first report that multiple VSRs encoded by a virus are localized in different subcellular compartments, and that a viral protein can be targeted to both the nucleus and chloroplast. These findings may have broad significance in studying the subcellular targeting of VSRs and other viral proteins in viral-host interactions.

Differentiation of chromoplasts and other plastids in plants

Plant cells are characterized by a unique group of interconvertible organelles called plastids, which are descended from prokaryotic endosymbionts. The most studied plastid type is the chloroplast, which carries out the ancestral plastid function of photosynthesis. During the course of evolution, plastid activities were increasingly integrated with cellular metabolism and functions, and plant developmental processes, and this led to the creation of new types of non-photosynthetic plastids. These include the chromoplast, a carotenoid-rich organelle typically found in flowers and fruits. Here, we provide an introduction to non-photosynthetic plastids, and then review the structures and functions of chromoplasts in detail. The role of chromoplast differentiation in fruit ripening in particular is explored, and the factors that govern plastid development are examined, including hormonal regulation, gene expression, and plastid protein import. In the latter process, nucleus-encoded preproteins must pass through two successive protein translocons in the outer and inner envelope membranes of the plastid; these are known as TOC and TIC (translocon at the outer/inner chloroplast envelope), respectively. The discovery of SP1 (suppressor of ppi1 locus1), which encodes a RING-type ubiquitin E3 ligase localized in the plastid outer envelope membrane, revealed that plastid protein import is regulated through the selective targeting of TOC complexes for degradation by the ubiquitin-proteasome system. This suggests the possibility of engineering plastid protein import in novel crop improvement strategies.

A Protochlorophyllide (Pchlide) a Oxygenase for Plant Viability

Higher plants contain a small, 5-member family of Rieske non-heme oxygenases that comprise the inner plastid envelope protein TIC55, phaeophorbide a oxygenasee (PAO), chlorophyllide a oxygenase (CAO), choline monooxygenase, and a 52 kDa protein (PTC52) associated with the precursor NADPH:protochlorophyllide (Pchlide) oxidoreductase A (pPORA) A translocon (PTC). Some of these chloroplast proteins have documented roles in chlorophyll biosynthesis (CAO) and degradation (PAO and TIC55), whereas the function of PTC52 remains unresolved. Biochemical evidence provided here identifies PTC52 as Pchlide a oxygenase of the inner plastid envelope linking Pchlide b synthesis to pPORA import. Protochlorophyllide b is the preferred substrate of PORA and its lack no longer allows pPORA import. The Pchlide b-dependent import pathway of pPORA thus operates in etiolated seedlings and is switched off during greening. Using dexamethasone-induced RNA interference (RNAi) we tested if PTC52 is involved in controlling both, pPORA import and Pchlide homeostasis in planta. As shown here, RNAi plants deprived of PTC52 transcript and PTC52 protein were unable to import pPORA and died as a result of excess Pchlide a accumulation causing singlet oxygen formation during greening. In genetic studies, no homozygous ptc52 knock-out mutants could be obtained presumably as a result of embryo lethality, suggesting a role for PTC52 in the initial greening of plant embryos. Phylogenetic studies identified PTC52-like genes amongst unicellular photosynthetic bacteria and higher plants, suggesting that the biochemical function associated with PTC52 may have an ancient evolutionary origin. PTC52 also harbors conserved motifs with bacterial oxygenases such as the terminal oxygenase component of 3-ketosteroid 9-alpha-hydroxylase (KshA) from Rhodococcus rhodochrous. 3D-modeling of PTC52 structure permitted the prediction of amino acid residues that contribute to the substrate specificity of this enzyme. In vitro-mutagenesis was used to test the predicted PTC52 model and provide insights into the reaction mechanism of this Rieske non-heme oxygenase.

Chloroplast: the Trojan horse in plant-virus interaction

The chloroplast is one of the most dynamic organelles of a plant cell. It carries out photosynthesis, synthesizes major phytohormones, plays an active part in the defence response and is crucial for interorganelle signalling. Viruses, on the other hand, are extremely strategic in manipulating the internal environment of the host cell. The chloroplast, a prime target for viruses, undergoes enormous structural and functional damage during viral infection. Indeed, large proportions of affected gene products in a virus-infected plant are closely associated with the chloroplast and the process of photosynthesis. Although the chloroplast is deficient in gene silencing machinery, it elicits the effector-triggered immune response against viral pathogens. Virus infection induces the organelle to produce an extensive network of stromules which are involved in both viral propagation and antiviral defence. From studies over the last few decades, the involvement of the chloroplast in the regulation of plant-virus interaction has become increasingly evident. This review presents an exhaustive account of these facts, with their implications for pathogenicity. We have attempted to highlight the intricacies of chloroplast-virus interactions and to explain the existing gaps in our current knowledge, which will enable virologists to utilize chloroplast genome-based antiviral resistance in economically important crops.

The endoplasmic reticulum is a hub to sort proteins toward unconventional traffic pathways and endosymbiotic organelles

The discovery that much of the extracellular proteome in eukaryotic cells consists of proteins lacking a signal peptide, which cannot therefore enter the secretory pathway, has led to the identification of alternative protein secretion routes bypassing the Golgi apparatus. However, proteins harboring a signal peptide for translocation into the endoplasmic reticulum can also be transported along these alternative routes, which are still far from being well elucidated in terms of the molecular machineries and subcellular/intermediate compartments involved. In this review, we first try to provide a definition of all the unconventional protein secretion pathways in eukaryotic cells, as those pathways followed by proteins directed to an 'external space' bypassing the Golgi, where 'external space' refers to the extracellular space plus the lumen of the secretory route compartments and the inner space of mitochondria and plastids. Then, we discuss the role of the endoplasmic reticulum in sorting proteins toward unconventional traffic pathways in plants. In this regard, various unconventional pathways exporting proteins from the endoplasmic reticulum to the vacuole, plasma membrane, apoplast, mitochondria, and plastids are described, including the short routes followed by the proteins resident in the endoplasmic reticulum.

P5-2 of rice black-streaked dwarf virus is a non-structural protein targeted to chloroplasts

The genome segment S5 of rice black-streaked dwarf virus (genus Fijivirus, family Reoviridae) is functionally bicistronic in infected plants. It has a conserved second ORF (P5-2) partially overlapping the major ORF in a different reading frame, but its function remains unknown. P5-2 was detected in infected plants, but not in purified viral particles by Western blotting, indicating that it is a non-structural protein. In immunoelectron microscopy, polyclonal antibodies against P5-2 specifically labelled chloroplasts of infected rice plants. When P5-2 fused with green fluorescent protein was transiently expressed in leaves of Nicotiana benthamiana, fluorescence was also co-localized with chloroplasts. Experiments with deletion mutants of P5-2 showed that its N-terminal part was responsible for its targeting to chloroplasts.

Functions of plastid protein import and the ubiquitin-proteasome system in plastid development

Plastids, such as chloroplasts, are widely distributed endosymbiotic organelles in plants and algae. Apart from their well-known functions in photosynthesis, they have roles in processes as diverse as signal sensing, fruit ripening, and seed development. As most plastid proteins are produced in the cytosol, plastids have developed dedicated translocon machineries for protein import, comprising the TOC (translocon at the outer envelope membrane of chloroplasts) and TIC (translocon at the inner envelope membrane of chloroplasts) complexes. Multiple lines of evidence reveal that protein import via the TOC complex is actively regulated, based on the specific interplay between distinct receptor isoforms and diverse client proteins. In this review, we summarize recent advances in our understanding of protein import regulation, particularly in relation to control by the ubiquitin-proteasome system (UPS), and how such regulation changes plastid development. The diversity of plastid import receptors (and of corresponding preprotein substrates) has a determining role in plastid differentiation and interconversion. The controllable turnover of TOC components by the UPS influences the developmental fate of plastids, which is fundamentally linked to plant development. Understanding the mechanisms by which plastid protein import is controlled is critical to the development of breakthrough approaches to increase the yield, quality and stress tolerance of important crop plants, which are highly dependent on plastid development. This article is part of a Special Issue entitled: Chloroplast Biogenesis.

Respiratory processes in non-photosynthetic plastids

Chlororespiration is a respiratory process located in chloroplast thylakoids which consists in an electron transport chain from NAD(P)H to oxygen. This respiratory chain involves the NAD(P)H dehydrogenase complex, the plastoquinone pool and the plastid terminal oxidase (PTOX), and it probably acts as a safety valve to prevent the over-reduction of the photosynthetic machinery in stress conditions. The existence of a similar respiratory activity in non-photosynthetic plastids has been less studied. Recently, it has been reported that tomato fruit chromoplasts present an oxygen consumption activity linked to ATP synthesis. Etioplasts and amyloplasts contain several electron carriers and some subunits of the ATP synthase, so they could harbor a similar respiratory process. This review provides an update on the study about respiratory processes in chromoplasts, identifying the major gaps that need to be addressed in future research. It also reviews the proteomic data of etioplasts and amyloplasts, which suggest the presence of a respiratory electron transport chain in these plastids.

Pronounced Phenotypic Changes in Transgenic Tobacco Plants Overexpressing Sucrose Synthase May Reveal a Novel Sugar Signaling Pathway

Soluble sugars not only serve as nutrients, but also act as signals for plant growth and development, but how sugar signals are perceived and translated into physiological responses in plants remains unclear. We manipulated sugar levels in transgenic plants by overexpressing sucrose synthase (SuSy), which is a key enzyme believed to have reversible sucrose synthesis and sucrose degradation functions. The ectopically expressed SuSy protein exhibited sucrose-degrading activity, which may change the flux of sucrose demand from photosynthetic to non-photosynthetic cells, and trigger an unknown sucrose signaling pathway that lead to increased sucrose content in the transgenic plants. An experiment on the transition from heterotrophic to autotrophic growth demonstrated the existence of a novel sucrose signaling pathway, which stimulated photosynthesis, and enhanced photosynthetic synthesis of sucrose, which was the direct cause or the sucrose increase. In addition, a light/dark time treatment experiment, using different day length ranges for photosynthesis/respiration showed the carbohydrate pattern within a 24-h day and consolidated the role of sucrose signaling pathway as a way to maintain sucrose demand, and indicated the relationships between increased sucrose and upregulation of genes controlling development of the shoot apical meristem (SAM). As a result, transgenic plants featured a higher biomass and a shorter time required to switch to reproduction compared to those of control plants, indicating altered phylotaxis and more rapid advancement of developmental stages in the transgenic plants.

Folding and self-association of atTic20 in lipid membranes: implications for understanding protein transport across the inner envelope membrane of chloroplasts

Background

The Arabidopsis thaliana protein atTic20 is a key component of the protein import machinery at the inner envelope membrane of chloroplasts. As a component of the TIC complex, it is believed to form a preprotein-conducting channel across the inner membrane.

Results

We report a method for producing large amounts of recombinant atTic20 using a codon-optimized strain of E. coli coupled with an autoinduction method of protein expression. This method resulted in the recombinant protein being directed to the bacterial membrane without the addition of a bacterial targeting sequence. Using biochemical and biophysical approaches, we were able to demonstrate that atTic20 homo-oligomerizes in vitro when solubilized in detergents or reconstituted into liposomes. Furthermore, we present evidence that the extramembranous N-terminus of the mature protein displays characteristics that are consistent with it being an intrinsically disordered protein domain.

Conclusion

Our work strengthens the hypothesis that atTic20 functions similarly to other small α-helical integral membrane proteins, such as Tim23, that are involved in protein transport across membranes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12858-014-0029-y) contains supplementary material, which is available to authorized users.

Light intensity-dependent retrograde signalling in higher plants

Plants are able to acclimate to highly fluctuating light environment and evolved a short- and long-term light acclimatory responses, that are dependent on chloroplasts retrograde signalling. In this review we summarise recent evidences suggesting that the chloroplasts act as key sensors of light intensity changes in a wide range (low, high and excess light conditions) as well as sensors of darkness. They also participate in transduction and synchronisation of systemic retrograde signalling in response to differential light exposure of distinct leaves. Regulation of intra- and inter-cellular chloroplast retrograde signalling is dependent on the developmental and functional stage of the plastids. Therefore, it is discussed in following subsections: firstly, chloroplast biogenic control of nuclear genes, for example, signals related to photosystems and pigment biogenesis during early plastid development; secondly, signals in the mature chloroplast induced by changes in photosynthetic electron transport, reactive oxygen species, hormones and metabolite biosynthesis; thirdly, chloroplast signalling during leaf senescence. Moreover, with a help of meta-analysis of multiple microarray experiments, we showed that the expression of the same set of genes is regulated specifically in particular types of signals and types of light conditions. Furthermore, we also highlight the alternative scenarios of the chloroplast retrograde signals transduction and coordination linked to the role of photo-electrochemical signalling.

Emerging facets of plastid division regulation

Plastids are complex organelles that are integrated into the plant host cell where they differentiate and divide in tune with plant differentiation and development. In line with their prokaryotic origin, plastid division involves both evolutionary conserved proteins and proteins of eukaryotic origin where the host has acquired control over the process. The plastid division apparatus is spatially separated between the stromal and the cytosolic space but where clear coordination mechanisms exist between the two machineries. Our knowledge of the plastid division process has increased dramatically during the past decade and recent findings have not only shed light on plastid division enzymology and the formation of plastid division complexes but also on the integration of the division process into a multicellular context. This review summarises our current knowledge of plastid division with an emphasis on biochemical features, the functional assembly of protein complexes and regulatory features of the overall process.

The rice ASR5 protein: a putative role in the response to aluminum photosynthesis disturbance

Under acidic soil conditions, aluminum (Al) becomes available to plants, which must cope with its toxicity by mechanisms involving both internal and external detoxification. Rice is the most Al-tolerant among the crop species, with Al detoxification being managed by both mechanisms. Recently, we focused on ASR (Abscisic acid, Stress and Ripening) gene expression analyses and observed increased ASR5 transcript levels in roots and shoots in response to Al. In addition, ASR5 RNAi knock down plants presented an Al-sensitive phenotype. A proteomic approach showed that ASR5 silencing affected several proteins related to photosynthesis in RNAi rice shoots. Furthermore, an ASR5-GFP fusion in rice protoplasts revealed for the first time a chloroplast localization of this protein. Because it is well known that Al induces photosynthetic dysfunction, here we discuss the hypothesis that ASR5 might be sequestered in the chloroplasts as an inactive transcription factor that could be released to the nucleus in response to Al to regulate genes related to photosynthesis.

The Tic20 gene family: phylogenetic analysis and evolutionary considerations

Tic20 is a polytopic protein of the inner envelope membrane of chloroplasts, and it is proposed to act as a translocation channel during chloroplast protein import. By analysing 29 sequences from diverse organisms, it was evident that Tic20-related proteins form two distinct clades, termed Group 1 and Group 2. The former group includes canonical Tic20 proteins that are essential for chloroplast development, while members of the latter are of unknown function. An increased evolutionary rate, in connection with adaptation to terrestrial life, was detected in Group 1. Interestingly, the sub-cellular (genomic) localization of genes coding for Group 1 proteins differs between evolutionary lineages.

Molecular and genetic analyses of Tic20 homologues in Arabidopsis thaliana chloroplasts

The Tic20 protein was identified in pea (Pisum sativum) as a component of the chloroplast protein import apparatus. In Arabidopsis, there are four Tic20 homologues, termed atTic20-I, atTic20-IV, atTic20-II and atTic20-V, all with predicted topological similarity to the pea protein (psTic20). Analysis of Tic20 sequences from many species indicated that they are phylogenetically unrelated to mitochondrial Tim17-22-23 proteins, and that they form two evolutionarily conserved subgroups [characterized by psTic20/atTic20-I/IV (Group 1) and atTic20-II/V (Group 2)]. Like psTic20, all four Arabidopsis proteins have a predicted transit peptide consistent with targeting to the inner envelope. Envelope localization of each one was confirmed by analysis of YFP fusions. RT-PCR and microarray data revealed that the four genes are expressed throughout development. To assess the functional significance of the genes, T-DNA mutants were identified. Homozygous tic20-I plants had an albino phenotype that correlated with abnormal chloroplast development and reduced levels of chloroplast proteins. However, knockouts for the other three genes were indistinguishable from the wild type. To test for redundancy, double and triple mutants were studied; apart from those involving tic20-I, none was distinguishable from the wild type. The tic20-I tic20-II and tic20-I tic20-V double mutants were albino, like the corresponding tic20-I parent. In contrast, tic20-I tic20-IV double homozygotes could not be identified, due to gametophytic and embryonic lethality. Redundancy between atTic20-I and atTic20-IV was confirmed by complementation analysis. Thus, atTic20-I and atTic20-IV are the major functional Tic20 isoforms in Arabidopsis, with partially overlapping roles. While the Group 2 proteins have been conserved over approximately 1.2 billion (1.2 × 10(9) ) years, they are not essential for normal development.

The complexity and evolution of the plastid-division machinery

Plastids are vital organelles, fulfilling important metabolic functions that greatly influence plant growth and productivity. In order to both regulate and harness the metabolic output of plastids, it is vital that the process of plastid division is carefully controlled. This is essential, not only to ensure persistence in dividing plant cells and that optimal numbers of plastids are obtained in specialized cell types, but also to allow the cell to act in response to developmental signals and environmental changes. How this control is exerted by the host nucleus has remained elusive. Plastids evolved by endosymbiosis and during the establishment of a permanent endosymbiosis they retained elements of the bacterial cell-division machinery. Through evolution the photosynthetic eukaryotes have increased dramatically in complexity, from single-cell green algae to multicellular non-vascular and vascular plants. Reflected with this is an increasing complexity of the division machinery and recent findings also suggest increasing complexity in the molecular mechanisms used by the host cell to control the process of plastid division. In the present paper, we explore the current understanding of the process of plastid division at the molecular and cellular level, with particular respect to the evolution of the division machinery and levels of control exerted on the process.

A forward genetic screen to explore chloroplast protein import in vivo identifies Moco sulfurase, pivotal for ABA and IAA biosynthesis and purine turnover

A genetic screen in Arabidopsis was developed to explore the regulation of chloroplast protein import in vivo using two independent reporters representing housekeeping and photosynthetic pre-proteins. We first used 5-enolpyruvylshikimate 3-phosphate synthase (EPSP synthase*), a key enzyme in the shikimic acid pathway, with a mutation that confers tolerance to the herbicide glyphosate. Because the EPSP synthase* pre-protein must be imported for its function, the loss of glyphosate tolerance provided an initial indication of an import deficiency. Second, the fate of GFP fused to a ferredoxin transit peptide (FD5-GFP) was determined. A class of altered chloroplast import (aci) mutants showed both glyphosate sensitivity and FD5-GFP mislocalized to nuclei. aci2-1 was selected for further study. Yellow fluorescent protein (YFP) fused to the transit peptide of EPSP synthase* or the small subunit of Rubisco was not imported into chloroplasts, but also localized to nuclei during protoplast transient expression. Isolated aci2-1 chloroplasts showed a 50% reduction in pre-protein import efficiency in an in vitro assay. Mutants did not grow photoautotrophically on media without sucrose and were small and dark green in soil. aci2-1 and two alleles code for Moco-sulfurase, which activates the aldehyde oxidases required for the biosynthesis of the plant hormones abscisic acid (ABA) and indole-acetic acid (IAA) and controls purine nucleotide (ATP and GTP) turnover and nitrogen recycling via xanthine dehydrogenase. These enzyme activities were not detected in aci2-1. ABA, IAA and/or purine turnover may play previously unrecognized roles in the regulation of chloroplast protein import in response to developmental, metabolic and environmental cues.

Comparative genomic studies suggest that the cyanobacterial endosymbionts of the amoeba Paulinella chromatophora possess an import apparatus for nuclear-encoded proteins

Plastids evolved from free-living cyanobacteria through a process of primary endosymbiosis. The most widely accepted hypothesis derives three ancient lineages of primary plastids, i.e. those of glaucophytes, red algae and green plants, from a single cyanobacterial endosymbiosis. This hypothesis was originally predicated on the assumption that transformations of endosymbionts into organelles must be exceptionally rare because of the difficulty in establishing efficient protein trafficking between a host cell and incipient organelle. It turns out, however, that highly integrated endosymbiotic associations are more common than once thought. Among them is the amoeba Paulinella chromatophora, which harbours independently acquired cyanobacterial endosymbionts functioning as plastids. Sequencing of the Paulinella endosymbiont genome revealed an absence of essential genes for protein trafficking, suggesting their residence in the host nucleus and import of protein products back into the endosymbiont. To investigate this hypothesis, we searched the Paulinella endosymbiont genome for homologues of higher plant translocon proteins that form the import apparatus in two-membrane envelopes of primary plastids. We found homologues of Toc12, Tic21 and Tic32, but genes for other key translocon proteins (e.g. Omp85/Toc75 and Tic20) were missing. We propose that these missing genes were transferred to the Paulinella nucleus and their products are imported and integrated into the endosymbiont envelope membranes, thereby creating an effective protein import apparatus. We further suggest that other bacterial/cyanobacterial endosymbionts found in protists, plants and animals could have evolved efficient protein import systems independently and, therefore, reached the status of true cellular organelles.

Mitochondrial and chloroplastic targeting signals of NADP+-dependent isocitrate dehydrogenase

Many mitochondrial and chloroplast proteins are encoded in the nucleus and subsequently imported into the organelles via active protein transport systems. While usually highly specific, some proteins are dual-targeted to both organelles. In tobacco (Nicotiana tabacum L.), the cDNA encoding the mitochondrial isoform of NADP+-dependent isocitrate dehydrogenase (NADP+-ICDH) contains two translational ATG start sites, suggesting the possibility of tandem targeting signals. In this work, the putative mitochondrial and chloroplastic targeting signals from NADP+-ICDH were fused to a yellow fluorescent protein (YFP) reporter to generate a series of constructs and introduced into tobacco leaves by Agrobacterium-mediated transient transformation. The subsequent sub-cellular locations of the ICDH:YFP fusion proteins were then examined using confocal microscopy. Constructs predicted to be targeted to the chloroplast all localized to the chloroplast. However, this was not the case for all of the constructs that were predicted to be mitochondrial targeted. Although some constructs localized to mitochondria as expected, others appeared to be chloroplast localized. This was attributed to an additional 50 amino acid residues of the mature NADP+-ICDH protein that were present in those constructs, generated from either 'Xanthi' or 'Petit Havana' cultivars of tobacco. The results of this study raise interesting questions regarding the targeting and processing of organellar isoforms of NADP+-ICDH.

A search for factors influencing etioplast-chloroplast transition

Chloroplast biogenesis in angiosperm plants requires the light-dependent transition from an etioplast stage. A key factor in this process is NADPH:protochlorophyllide oxidoreductase A (PORA), which catalyzes the light-dependent reduction of protochlorophyllide to chlorophyllide. In a recent study the chloroplast outer envelope channel OEP16 was described to be involved in etioplast to chloroplast transition by forming the translocation pore for the precursor protein of PORA [Pollmann et al. (2007) Proc Natl Acad Sci USA 104:2019-2023]. This hypothesis was based on the finding that a single OEP16.1 knockout mutant in Arabidopsis thaliana was severely affected during seedling de-etiolation and PORA protein was absent in etioplasts. In contrast, in our study the identical T-DNA insertion line greened normally and showed normal etioplast to chloroplast transition, and mature PORA was present in etioplasts [Philippar et al. (2007) Proc Natl Acad Sci USA 104:678-683]. To address these conflicting results regarding the function of OEP16.1 for PORA import, we analyzed several lines segregating from the original OEP16.1 T-DNA insertion line. Thereby we can unequivocally show that the loss of OEP16.1 neither correlates with impaired PORA import nor causes the observed de-etiolation phenotype. Furthermore, we found that the mutant line contains at least 2 additional T-DNA insertions in the genes for the extracellular polygalacturonase converter AroGP1 and the plastid-localized chorismate mutase CM1. However, detailed examination of the de-etiolation phenotype and a genomewide transcriptional analysis revealed no direct influence of these genes on etioplast to chloroplast transition in Arabidopsis cotyledons.

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.

In vivo interaction between atToc33 and atToc159 GTP-binding domains demonstrated in a plant split-ubiquitin system

The GTPases atToc33 and atToc159 are pre-protein receptor components of the translocon complex at the outer chloroplast membrane in Arabidopsis. Despite their participation in the same complex in vivo, evidence for their interaction is still lacking. Here, a split-ubiquitin system is engineered for use in plants, and the in vivo interaction of the Toc GTPases in Arabidopsis and tobacco protoplasts is shown. Using the same method, the self-interaction of the peroxisomal membrane protein atPex11e is demonstrated. The finding suggests a more general suitability of the split-ubiquitin system as a plant in vivo interaction assay.

A pentapeptide motif related to a pigment binding site in the major light-harvesting protein of photosystem II, LHCII, governs substrate-dependent plastid import of NADPH:protochlorophyllide oxidoreductase A

NADPH:protochlorophyllide (Pchlide) oxidoreductase (POR) A is the only known example thus far of a nucleus-encoded plastid protein that is imported to its final destination in a substrate-dependent, Pchlide-regulated manner. Previous work has shown that the cytosolic PORA precursor (pPORA) does not utilize the general import site but uses a distinct translocon designated the Pchlide-dependent translocon complex. Here we demonstrate that a pentapeptide motif, threonine-threonine-serine-proline-glycine (TTSPG) in pPORA's transit peptide (transA), is involved in Pchlide-dependent transport. Deletion of this motif from the COOH-terminal end of transA abolished both Pchlide binding and protein import. Incorporation of the TTSPG motif into normally non-Pchlide-responsive transit sequences conferred the pigment binding properties onto the engineered chimeric precursors but was insufficient to render protein import substrate dependent. An additional motif was identified in the NH(2)-terminal part of transA that was needed for binding of the precursor to the Pchlide-dependent translocon complex. Point mutations of the TTSPG motif identified glycine as the Pchlide binding site. By analogy to the major light-harvesting chlorophyll a/b binding protein of photosystem II, we propose that the peptidyl carbonyl oxygen of glycine may bind directly or via a water molecule to the central Mg atom of the pigment.

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.

Monogalactosyldiacylglycerol deficiency in Arabidopsis affects pigment composition in the prolamellar body and impairs thylakoid membrane energization and photoprotection in leaves

Monogalactosyldiacylglycerol (MGDG) is the major lipid constituent of chloroplast membranes and has been proposed to act directly in several important plastidic processes, particularly during photosynthesis. In this study, the effect of MGDG deficiency, as observed in the monogalactosyldiacylglycerol synthase1-1 (mgd1-1) mutant, on chloroplast protein targeting, phototransformation of pigments, and photosynthetic light reactions was analyzed. The targeting of plastid proteins into or across the envelope, or into the thylakoid membrane, was not different from wild-type in the mgd1 mutant, suggesting that the residual amount of MGDG in mgd1 was sufficient to maintain functional targeting mechanisms. In dark-grown plants, the ratio of bound protochlorophyllide (Pchlide, F656) to free Pchlide (F631) was increased in mgd1 compared to the wild type. Increased levels of the photoconvertible pigment-protein complex (F656), which is photoprotective and suppresses photooxidative damage caused by an excess of free Pchlide, may be an adaptive response to the mgd1 mutation. Leaves of mgd1 suffered from a massively impaired capacity for thermal dissipation of excess light due to an inefficient operation of the xanthophyll cycle; the mutant contained less zeaxanthin and more violaxanthin than wild type after 60 min of high-light exposure and suffered from increased photosystem II photoinhibition. This is attributable to an increased conductivity of the thylakoid membrane at high light intensities, so that the proton motive force is reduced and the thylakoid lumen is less acidic than in wild type. Thus, the pH-dependent activation of the violaxanthin de-epoxidase and of the PsbS protein is impaired.

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

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

Protein trafficking to plastids: one theme, many variations

Plastids are a diverse group of essential organelles in plants that include chloroplasts. The biogenesis and maintenance of these organelles relies on the import of thousands of nucleus-encoded proteins. The complexity of plastid structure has resulted in the evolution of at least four general import pathways that target proteins into and across the double membrane of the plastid envelope. Several of these pathways can be further divided into specialty pathways that mediate and regulate the import of specific classes of proteins. The co-ordination of import by these specialized pathways with changes in gene expression is critical for plastid and plant development. Moreover, protein import is acutely regulated in response to physiological and metabolic changes within the cell. In the present review we summarize the current knowledge of the mechanism of import via these pathways and highlight the regulatory mechanisms that integrate the plastid protein-trafficking pathways with the developmental and metabolic state of the plant.

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.

Polyphenol oxidase can cross thylakoids by both the Tat and the Sec-dependent pathways: a putative role for two stromal processing sites

Polyphenol oxidase (PPO; EC 1.10.3.2 or EC 1.14.18.1), a thylakoid-lumen protein encoded by a nuclear gene, plays a role in the defense of plants against both herbivores and pathogens. Although previously reported to be a Tat (twin-arginine-dependent translocation) protein, the import of PPO by isolated chloroplasts was inhibited by azide, a diagnostic inhibitor of the Sec-dependent pathway. Import of PPO inhibited thylakoid translocation of a Tat protein and did not affect translocation of Sec-dependent proteins. In contrast, a pre-accumulated iPPO competed with Sec-dependent but not with Tat proteins. A previously reported second processing step in the stroma removes a twin-Arg that is part of a 'Sec-avoidance' motif in the thylakoid targeting domain of PPO. When the second processing site was mutated, the import of the resulting precursor showed Sec-dependent characteristics. The PPO transit peptide could drive thylakoid translocation of a Tat protein in the dark. Azide inhibited the secretion of a PPO intermediate that lacks a twin-Arg to the periplasm of Escherichia coli, but had no effect on the export of the intermediate containing the twin-Arg. PPO is synthesized in plants in response to wound and pathogen-related signals and it is possible that when the Tat pathway is unable to translocate adequate amounts of newly synthesized PPO, translocation is diverted to the Sec-dependent pathway by processing the intermediate at the second site and removing the twin-Arg.

Three sets of translocation intermediates are formed during the early stage of protein import into chloroplasts

During the early stage of protein import into chloroplasts, precursor proteins synthesized in the cytosol irreversibly bind to chloroplasts to form the early translocation intermediate under stringent energy conditions. Many efforts have been made to identify the components involved in protein import by analyzing the early intermediate. However, the state of the precursor within the intermediate has not been well investigated so far. In this study, an attempt was made to evaluate the extent of translocation of the precursor by determining the state of the precursor in the early intermediate under various conditions and analyzing the fragments generated by limited proteolysis of the precursors docked to chloroplasts. Our results indicate that three different sets of early intermediate are formed based on temperature and the hydrolysis of GTP/ATP. These have been identified based on the size of proteolytic fragments of the precursor as "energy-dependent association," "insertion," and "penetration" states. These findings suggest two individual ATP-hydrolyzing steps during the early stage of protein import, one of which is temperature-sensitive. Our results also demonstrate that translocation through the outer envelope membrane is mainly dependent on internal ATP.

Plastid evolution

The ancestors of modern cyanobacteria invented O(2)-generating photosynthesis some 3.6 billion years ago. The conversion of water and CO(2) into energy-rich sugars and O(2) slowly transformed the planet, eventually creating the biosphere as we know it today. Eukaryotes didn't invent photosynthesis; they co-opted it from prokaryotes by engulfing and stably integrating a photoautotrophic prokaryote in a process known as primary endosymbiosis. After approximately a billion of years of coevolution, the eukaryotic host and its endosymbiont have achieved an extraordinary level of integration and have spawned a bewildering array of primary producers that now underpin life on land and in the water. No partnership has been more important to life on earth. Secondary endosymbioses have created additional autotrophic eukaryotic lineages that include key organisms in the marine environment. Some of these organisms have subsequently reverted to heterotrophic lifestyles, becoming significant pathogens, microscopic predators, and consumers. We review the origins, integration, and functions of the different plastid types with special emphasis on their biochemical abilities, transfer of genes to the host, and the back supply of proteins to the endosymbiont.

Isolation and preparation of chloroplasts from Arabidopsis thaliana plants

A major area of research in the postgenomic era has been the proteomic analysis of various subcellular and suborganellar compartments. The success of these studies is to a large extent dependent upon efficient protocols for the preparation of highly pure organelles or suborganellar components. Here we describe a simple, rapid, and low-cost method for isolating a high yield of Arabidopsis chloroplasts. The method can readily be applied to wild-type plants and different mutants, and at different developmental stages ranging from 10-day-old seedlings to rosette leaves from older plants. The isolated chloroplast fraction is highly pure, with immunologically undetectable contamination from other cellular organelles. Chloroplasts isolated using the method described here have been successfully used for proteomic analysis, as well as in studies on chloroplast protein import and other aspects of chloroplast biology.

Plastid division in an evolutionary context

Plastids are derived from free-living cyanobacteria that were engulfed by eukaryotic host cells through the process of endosymbiosis and, like their cyanobacterial ancestors, divide by binary fission. Over the last decade the continued identification and functional analysis of plastid division components, coupled with ever-increasing genomic resources, have yielded insights into the origins and evolution of the plastid division mechanism in higher plants. Here we review the current understanding of the evolution of the chloroplast division proteins and present a model of how the machinery has developed to execute plastid division in Arabidopsis.

Protein import into chloroplasts: new aspects of a well-known topic

Protein import into plant chloroplasts is a fascinating topic that is being investigated by many research groups. Since the majority of chloroplast proteins are synthesised as precursor proteins in the cytosol, they have to be posttranslationally imported into the organelle. For this purpose, most preproteins are synthesised with an N-terminal presequence, which is both necessary and sufficient for organelle recognition and translocation initiation. The import of preproteins is facilitated by two translocation machineries in the outer and inner envelope of chloroplasts, the Toc and Tic complexes, respectively. Translocation of precursor proteins across the envelope membrane has to be highly regulated to react to the metabolic requirements of the organelle. The aim of this review is to summarise the events that take place at the translocation machineries that are known so far. In addition, we focus in particular on alternative import pathways and the aspect of regulation of protein transport at the outer and inner envelope membrane.

Functional definition of outer membrane proteins involved in preprotein import into mitochondria

The role of plant mitochondrial outer membrane proteins in the process of preprotein import was investigated, as some of the principal components characterized in yeast have been shown to be absent or evolutionarily distinct in plants. Three outer membrane proteins of Arabidopsis thaliana mitochondria were studied: TOM20 (translocase of the outer mitochondrial membrane), METAXIN, and mtOM64 (outer mitochondrial membrane protein of 64 kD). A single functional Arabidopsis TOM20 gene is sufficient to produce a normal multisubunit translocase of the outer membrane complex. Simultaneous inactivation of two of the three TOM20 genes changed the rate of import for some precursor proteins, revealing limited isoform subfunctionalization. Inactivation of all three TOM20 genes resulted in severely reduced rates of import for some but not all precursor proteins. The outer membrane protein METAXIN was characterized to play a role in the import of mitochondrial precursor proteins and likely plays a role in the assembly of beta-barrel proteins into the outer membrane. An outer mitochondrial membrane protein of 64 kD (mtOM64) with high sequence similarity to a chloroplast import receptor was shown to interact with a variety of precursor proteins. All three proteins have domains exposed to the cytosol and interacted with a variety of precursor proteins, as determined by pull-down and yeast two-hybrid interaction assays. Furthermore, inactivation of one resulted in protein abundance changes in the others, suggesting functional redundancy. Thus, it is proposed that all three components directly interact with precursor proteins to participate in early stages of mitochondrial protein import.

Toc64/OEP64 is not essential for the efficient import of proteins into chloroplasts in Arabidopsis thaliana

Toc64/OEP64 was identified biochemically in pea as a putative component of the chloroplast protein import apparatus. In Arabidopsis, three paralogous genes (atTOC64-III, atTOC64-V and atTOC64-I) encode Toc64-related proteins, and these have been reported to localize in chloroplasts, mitochondria and the cytosol, respectively. To assess the role of the atToc64-III protein in chloroplast protein import in an in vivo context, we identified and characterized Arabidopsis knockout mutants. The absence of detectable defects in toc64-III single mutants raised the possibility of redundancy, and prompted us to also identify toc64-V and toc64-I mutants, cross them to toc64-III, and generate double- and triple-mutant combinations. The toc64 mutants were analysed carefully with respect to a variety of criteria, including chlorophyll accumulation, photosynthetic performance, organellar ultrastructure and chloroplast protein accumulation. In each case, the mutant plants were indistinguishable from wild type. Furthermore, the efficiency of chloroplast protein import was not affected by the toc64 mutations, even when a putative substrate of the atToc64-III protein (wheatgerm-translated precursor of the 33 kDa subunit of the oxygen-evolving complex, OE33) was examined. Moreover, under various stress conditions (high light, osmotic stress and cold), the toc64 triple-mutant plants were not significantly different from wild type. These results demonstrate that Toc64/OEP64 is not essential for the efficient import of proteins into chloroplasts in Arabidopsis, and draw into question the functional significance of this component.

The E3 ligase AtCHIP ubiquitylates FtsH1, a component of the chloroplast FtsH protease, and affects protein degradation in chloroplasts

The Arabidopsis E3 ligase AtCHIP was found to interact with FtsH1, a subunit of the chloroplast FtsH protease complex. FtsH1 can be ubiquitylated by AtCHIP in vitro, and the steady-state level of FtsH1 is reduced in AtCHIP-over-expressing plants under high-intensity light conditions, suggesting that the ubiquitylation of FtsH1 by AtCHIP might lead to the degradation of FtsH1 in vivo. Furthermore, the steady-state level of another subunit of the chloroplast FtsH protease complex, FtsH2, is also reduced in AtCHIP-over-expressing plants under high-intensity light conditions, and FtsH2 interacts physically with AtCHIP in vivo, suggesting the possibility that FtsH2 is also a substrate protein for AtCHIP in plant cells. A substrate of FtsH protease in vivo, the photosystem II reaction center protein D1, is not efficiently removed by FtsH in AtCHIP-over-expressing plants under high-intensity light conditions, supporting the assumption that FtsH subunits are substrates of AtCHIP in vivo, and that AtCHIP over-expression may lead to a reduced level of FtsH in chloroplasts. AtCHIP interacts with cytosolic Hsp70 and the precursors of FtsH1 and FtsH2 in the cytoplasm, and Hsp70 also interacts with FtsH1, and these protein-protein interactions appear to be increased under high-intensity light conditions, suggesting that Hsp70 might be partly responsible for the increased degradation of the substrates of Hsp70, such as FtsH1 and FtsH2, in AtCHIP-over-expressing plants under high-intensity light conditions. Therefore, AtCHIP, together with Hsp70, may play an important role in protein quality control in chloroplasts.

Intracellular Signalling: Chloroplast Backchat

Chloroplast-derived signals modulate expression of nuclear genes for chloroplast proteins. GUN1 has recently been identified as a chloroplast-localized pentatricopeptide repeat protein that integrates information from several different signalling pathways.

Functional similarity between the chloroplast translocon component, Tic40, and the human co-chaperone, Hsp70-interacting protein (Hip)

Tic40 is a component of the protein import apparatus of the inner envelope of chloroplasts, but its role in the import mechanism has not been clearly defined. The C terminus of Tic40 shares weak similarity with the C-terminal Sti1 domains of the mammalian Hsp70-interacting protein (Hip) and Hsp70/Hsp90-organizing protein (Hop) co-chaperones. Additionally, Tic40 may possess a tetratricopeptide repeat (TPR) protein-protein interaction domain, another characteristic feature of Hip/Hop co-chaperones. To investigate the functional importance of different parts of the Tic40 protein and to determine whether the homology between Tic40 and co-chaperones is functionally significant, different Tic40 deletion and Tic40:Hip fusion constructs were generated and assessed for complementation activity in the Arabidopsis Tic40 knock-out mutant, tic40. Interestingly, all Tic40 deletion constructs failed to complement tic40, indicating that each part removed is essential for Tic40 function; these included a construct lacking the Sti1-like domain (DeltaSti1), a second lacking a central region, including the putative TPR domain (DeltaTPR), and a third lacking the predicted transmembrane anchor region. Moreover, the DeltaSti1 and DeltaTPR constructs caused strong dominant-negative, albino phenotypes in tic40 transformants, indicating that the truncated Tic40 proteins interfere with the residual chloroplast protein import that occurs in tic40 plants. Remarkably, the Tic40:Hip fusion constructs showed that the Sti1 domain of human Hip is functionally equivalent to the Sti1-like region of Tic40, strongly suggesting a co-chaperone role for the Tic40 protein. Supporting this notion, yeast two-hybrid and bimolecular fluorescence complementation assays demonstrated the in vivo interaction of Tic40 with Tic110, a protein believed to recruit stromal chaperones to protein import sites.