Calcium regulation of chloroplast protein import

Understanding protein import in diverse non-green plastids

The spectacular diversity of plastids in non-green organs such as flowers, fruits, roots, tubers, and senescing leaves represents a Universe of metabolic processes in higher plants that remain to be completely characterized. The endosymbiosis of the plastid and the subsequent export of the ancestral cyanobacterial genome to the nuclear genome, and adaptation of the plants to all types of environments has resulted in the emergence of diverse and a highly orchestrated metabolism across the plant kingdom that is entirely reliant on a complex protein import and translocation system. The TOC and TIC translocons, critical for importing nuclear-encoded proteins into the plastid stroma, remain poorly resolved, especially in the case of TIC. From the stroma, three core pathways (cpTat, cpSec, and cpSRP) may localize imported proteins to the thylakoid. Non-canonical routes only utilizing TOC also exist for the insertion of many inner and outer membrane proteins, or in the case of some modified proteins, a vesicular import route. Understanding this complex protein import system is further compounded by the highly heterogeneous nature of transit peptides, and the varying transit peptide specificity of plastids depending on species and the developmental and trophic stage of the plant organs. Computational tools provide an increasingly sophisticated means of predicting protein import into highly diverse non-green plastids across higher plants, which need to be validated using proteomics and metabolic approaches. The myriad plastid functions enable higher plants to interact and respond to all kinds of environments. Unraveling the diversity of non-green plastid functions across the higher plants has the potential to provide knowledge that will help in developing climate resilient crops.

Sustainable Production of Pigments from Cyanobacteria

Pigments are intensely coloured compounds used in many industries to colour other materials. The demand for naturally synthesised pigments is increasing and their production can be incorporated into circular bioeconomy approaches. Natural pigments are produced by bacteria, cyanobacteria, microalgae, macroalgae, plants and animals. There is a huge unexplored biodiversity of prokaryotic cyanobacteria which are microscopic phototrophic microorganisms that have the ability to capture solar energy and CO2 and use it to synthesise a diverse range of sugars, lipids, amino acids and biochemicals including pigments. This makes them attractive for the sustainable production of a wide range of high-value products including industrial chemicals, pharmaceuticals, nutraceuticals and animal-feed supplements. The advantages of cyanobacteria production platforms include comparatively high growth rates, their ability to use freshwater, seawater or brackish water and the ability to cultivate them on non-arable land. The pigments derived from cyanobacteria and microalgae include chlorophylls, carotenoids and phycobiliproteins that have useful properties for advanced technical and commercial products. Development and optimisation of strain-specific pigment-based cultivation strategies support the development of economically feasible pigment biorefinery scenarios with enhanced pigment yields, quality and price. Thus, this chapter discusses the origin, properties, strain selection, production techniques and market opportunities of cyanobacterial pigments.

Vacuolal and Peroxisomal Calcium Ion Transporters in Yeasts and Fungi: Key Role in the Translocation of Intermediates in the Biosynthesis of Fungal Metabolites

Highlights

The intracellular calcium content plays a key role in the expression of genes involved in the biosynthesis and secretion of fungal metabolites.

The cytosolic calcium concentration in fungi is maintained by influx through the cell membrane and by release from store organelles.

Some MSF transporters, e.g., PenV of Penicillium chrysogenum and CefP of Acremonium chrysogenum belong to the TRP calcium ion channels.

A few of the numerous calcium ion transporters existing in organelles of different filamentous fungi have been characterized at the functional and subcellular localization levels.

The cytosolic calcium signal seems to be transduced by the calcitonin/calcineurin cascade controlling the expression of many fungal genes.

Abstract

The intracellular calcium content in fungal cells is influenced by a large number of environmental and nutritional factors. Sharp changes in the cytosolic calcium level act as signals that are decoded by the cell gene expression machinery, resulting in several physiological responses, including differentiation and secondary metabolites biosynthesis. Expression of the three penicillin biosynthetic genes is regulated by calcium ions, but there is still little information on the role of this ion in the translocation of penicillin intermediates between different subcellular compartments. Using advanced information on the transport of calcium in organelles in yeast as a model, this article reviews the recent progress on the transport of calcium in vacuoles and peroxisomes and its relation to the translocation of biosynthetic intermediates in filamentous fungi. The Penicillium chrysogenum PenV vacuole transporter and the Acremonium chrysogenum CefP peroxisomal transporter belong to the transient receptor potential (TRP) class CSC of calcium ion channels. The PenV transporter plays an important role in providing precursors for the biosynthesis of the tripeptide δ-(-α-aminoadipyl-L-cysteinyl-D-valine), the first intermediate of penicillin biosynthesis in P. chrysogenum. Similarly, CefP exerts a key function in the conversion of isopenicillin N to penicillin N in peroxisomes of A. chrysogenum. These TRP transporters are different from other TRP ion channels of Giberella zeae that belong to the Yvc1 class of yeast TRPs. Recent advances in filamentous fungi indicate that the cytosolic calcium concentration signal is connected to the calcitonin/calcineurin signal transduction cascade that controls the expression of genes involved in the subcellular translocation of intermediates during fungal metabolite biosynthesis. These advances open new possibilities to enhance the expression of important biosynthetic genes in fungi.

TypiCal but DeliCate Ca++re: Dissecting the Essence of Calcium Signaling Network as a Robust Response Coordinator of Versatile Abiotic and Biotic Stimuli in Plants

Plant growth, development, and ultimately crop productivity are largely impacted by the interaction of plants with different abiotic and biotic factors throughout their life cycle. Perception of different abiotic stresses, such as salt, cold, drought, heat, and heavy metals, and interaction with beneficial and harmful biotic agents by plants lead to transient, sustained, or oscillatory changes of [calcium ion, Ca²⁺]_cyt within the cell. Significant progress has been made in the decoding of Ca²⁺ signatures into downstream responses to modulate differential developmental and physiological responses in the whole plant. Ca²⁺ sensor proteins, mainly calmodulins (CaMs), calmodulin-like proteins (CMLs), and others, such as Ca²⁺-dependent protein kinases (CDPKs), calcineurin B-like proteins (CBLs), and calmodulin-binding transcription activators (CAMTAs) have played critical roles in coupling the specific stress stimulus with an appropriate response. This review summarizes the current understanding of the Ca²⁺ influx and efflux system in plant cells and various Ca²⁺ binding protein-mediated signal transduction pathways that are delicately orchestrated to mitigate abiotic and biotic stresses. The probable interactions of different components of Ca²⁺ sensor relays and Ca²⁺ sensor responders in response to various external stimuli have been described diagrammatically focusing on established pathways and latest developments. Present comprehensive insight into key components of the Ca²⁺ signaling toolkit in plants can provide an innovative framework for biotechnological manipulations toward crop improvability in near future.

The Role of Light-Dark Regulation of the Chloroplast ATP Synthase

The chloroplast ATP synthase catalyzes the light-driven synthesis of ATP and is activated in the light and inactivated in the dark by redox-modulation through the thioredoxin system. It has been proposed that this down-regulation is important for preventing wasteful hydrolysis of ATP in the dark. To test this proposal, we compared the effects of extended dark exposure in Arabidopsis lines expressing the wild-type and mutant forms of ATP synthase that are redox regulated or constitutively active. In contrast to the predictions of the model, we observed that plants with wild-type redox regulation lost photosynthetic capacity rapidly in darkness, whereas those expressing redox-insensitive form were far more stable. To explain these results, we propose that in wild-type plants, down-regulation of ATP synthase inhibits ATP hydrolysis, leading to dissipation of thylakoid proton motive force (pmf) and subsequent inhibition of protein transport across the thylakoid through the twin arginine transporter (Tat)-dependent and Sec-dependent import pathways, resulting in the selective loss of specific protein complexes. By contrast, in mutants with a redox-insensitive ATP synthase, pmf is maintained by ATP hydrolysis, thus allowing protein transport to maintain photosynthetic activities for extended periods in the dark. Hence, a basal level of Tat-dependent, as well as, Sec-dependent import activity, in the dark helps replenishes certain components of the photosynthetic complexes and thereby aids in maintaining overall complex activity. However, the influence of a dark pmf on thylakoid protein import, by itself, could not explain all the effects we observed in this study. For example, we also observed in wild type plants a large transient buildup of thylakoid pmf and nonphotochemical exciton quenching upon sudden illumination of dark adapted plants. Therefore, we conclude that down-regulation of the ATP synthase is probably not related to preventing loss of ATP per se. Instead, ATP synthase redox regulation may be impacting a number of cellular processes such as (1) the accumulation of chloroplast proteins and/or ions or (2) the responses of photosynthesis to rapid changes in light intensity. A model highlighting the complex interplay between ATP synthase regulation and pmf in maintaining various chloroplast functions in the dark is presented. Significance Statement: We uncover an unexpected role for thioredoxin modulation of the chloroplast ATP synthase in regulating the dark-stability of the photosynthetic apparatus, most likely by controlling thylakoid membrane transport of proteins and ions.

Calcineurin B-like Protein CBL10 Directly Interacts with TOC34 (Translocon of the Outer Membrane of the Chloroplasts) and Decreases Its GTPase Activity in Arabidopsis

As calcium sensor relays in plants, calcineurin B-like (CBL) proteins provide an important contribution to decoding Ca2+ signatures elicited by a variety of abiotic stresses. Currently, it is well known that CBLs perceive and transmit the Ca2+ signals mainly to a group of serine/threonine protein kinases called CBL-interacting protein kinases (CIPKs). In this study, we report that the CBL10 member of this family has a novel interaction partner besides the CIPK proteins. Yeast two-hybrid screening with CBL10 as bait identified an Arabidopsis cDNA clone encoding a TOC34 protein, which is a member of the TOC (Translocon of the Outer membrane of the Chloroplasts) complex and possesses the GTPase activity. Further analyses showed that in addition to CBL10, CBL7 also interacts with TOC34 at much lower strength in the yeast two-hybrid system. However, the rest of the CBL family members failed to interact with TOC34. Bimolecular fluorescence complementation (BiFC) analysis verified that the CBL10-TOC34 interaction occurs at the outer membrane of chloroplasts in vivo. In addition, we also demonstrated that CBL10 physically associates with TOC34 in vitro, resulting in a significant decrease in the GTPase activity of the TOC34 protein. Taken together, our findings clearly indicate that a member of the CBL family, CBL10, can modulate not only the CIPK members but also TOC34, allowing the CBL family to relay the Ca2+ signals in more diverse ways than currently known.

The calmodulin-like proteins AtCML4 and AtCML5 are single-pass membrane proteins targeted to the endomembrane system by an N-terminal signal anchor sequence

Calmodulins (CaMs) are important mediators of Ca(2+) signals that are found ubiquitously in all eukaryotic organisms. Plants contain a unique family of calmodulin-like proteins (CMLs) that exhibit greater sequence variance compared to canonical CaMs. The Arabidopsis thaliana proteins AtCML4 and AtCML5 are members of CML subfamily VII and possess a CaM domain comprising the characteristic double pair of EF-hands, but they are distinguished from other members of this subfamily and from canonical CaMs by an N-terminal extension of their amino acid sequence. Transient expression of yellow fluorescent protein-tagged AtCML4 and AtCML5 under a 35S-promoter in Nicotiana benthamiana leaf cells revealed a spherical fluorescence pattern. This pattern was confirmed by transient expression in Arabidopsis protoplasts under the native promoter. Co-localization analyses with various endomembrane marker proteins suggest that AtCML4 and AtCML5 are localized to vesicular structures in the interphase between Golgi and the endosomal system. Further studies revealed AtCML5 to be a single-pass membrane protein that is targeted into the endomembrane system by an N-terminal signal anchor sequence. Self-assembly green fluorescent protein and protease protection assays support a topology with the CaM domain exposed to the cytosolic surface and not the lumen of the vesicles, indicating that AtCML5 could sense Ca(2+) signals in the cytosol. Phylogenetic analysis suggests that AtCML4 and AtCML5 are closely related paralogues originating from a duplication event within the Brassicaceae family. CML4/5-like proteins seem to be universally present in eudicots but are absent in some monocots. Together these results show that CML4/5-like proteins represent a flowering plant-specific subfamily of CMLs with a potential function in vesicle transport within the plant endomembrane system.

Calcium-dependent regulation of photosynthesis

The understanding of calcium as a second messenger in plants has been growing intensively over the last decades. Recently, attention has been drawn to the organelles, especially the chloroplast but focused on the stromal Ca2+ transients in response to environmental stresses. Herein we will expand this view and discuss the role of Ca2+ in photosynthesis. Moreover we address of how Ca2+ is delivered to chloroplast stroma and thylakoids. Thereby, new light is shed on the regulation of photosynthetic electron flow and light-dependent metabolism by the interplay of Ca2+, thylakoid acidification and redox status. This article is part of a Special Issue entitled: Chloroplast biogenesis.

Automated nutrient screening system enables high-throughput optimisation of microalgae production conditions

BACKGROUND

Microalgae provide an excellent platform for the production of high-value-products and are increasingly being recognised as a promising production system for biomass, animal feeds and renewable fuels.

RESULTS

Here, we describe an automated screen, to enable high-throughput optimisation of 12 nutrients for microalgae production. Its miniaturised 1,728 multiwell format allows multiple microalgae strains to be simultaneously screened using a two-step process. Step 1 optimises the primary elements nitrogen and phosphorous. Step 2 uses Box-Behnken analysis to define the highest growth rates within the large multidimensional space tested (Ca, Mg, Fe, Mn, Zn, Cu, B, Se, V, Si) at three levels (-1, 0, 1). The highest specific growth rates and maximum OD750 values provide a measure for continuous and batch culture.

CONCLUSION

The screen identified the main nutrient effects on growth, pairwise nutrient interactions (for example, Ca-Mg) and the best production conditions of the sampled statistical space providing the basis for a targeted full factorial screen to assist with optimisation of algae production.

Calcium signaling in plant endosymbiotic organelles: mechanism and role in physiology

Recent studies have demonstrated that chloroplasts and mitochondria evoke specific Ca(2+) signals in response to biotic and abiotic stresses in a stress-dependent manner. The identification of Ca(2+) transporters and Ca(2+) signaling molecules in chloroplasts and mitochondria implies that they play roles in controlling not only intra-organellar functions, but also extra-organellar processes such as plant immunity and stress responses. It appears that organellar Ca(2+) signaling might be more important to plant cell functions than previously thought. This review briefly summarizes what is known about the molecular basis of Ca(2+) signaling in plant mitochondria and chloroplasts.

Role of calcium signaling in the transcriptional regulation of the apicoplast genome of Plasmodium falciparum

Calcium is a universal second messenger that plays an important role in regulatory processes in eukaryotic cells. To understand calcium-dependent signaling in malaria parasites, we analyzed transcriptional responses of Plasmodium falciparum to two calcium ionophores (A23187 and ionomycin) that cause redistribution of intracellular calcium within the cytoplasm. While ionomycin induced a specific transcriptional response defined by up- or downregulation of a narrow set of genes, A23187 caused a developmental arrest in the schizont stage. In addition, we observed a dramatic decrease of mRNA levels of the transcripts encoded by the apicoplast genome during the exposure of P. falciparum to both calcium ionophores. Neither of the ionophores caused any disruptions to the DNA replication or the overall apicoplast morphology. This suggests that the mRNA downregulation reflects direct inhibition of the apicoplast gene transcription. Next, we identify a nuclear encoded protein with a calcium binding domain (EF-hand) that is localized to the apicoplast. Overexpression of this protein (termed PfACBP1) in P. falciparum cells mediates an increased resistance to the ionophores which suggests its role in calcium-dependent signaling within the apicoplast. Our data indicate that the P. falciparum apicoplast requires calcium-dependent signaling that involves a novel protein PfACBP1.

Differential age-dependent import regulation by signal peptides

Gene-specific, age-dependent regulations are common at the transcriptional and translational levels, while protein transport into organelles is generally thought to be constitutive. Here we report a new level of differential age-dependent regulation and show that chloroplast proteins are divided into three age-selective groups: group I proteins have a higher import efficiency into younger chloroplasts, import of group II proteins is nearly independent of chloroplast age, and group III proteins are preferentially imported into older chloroplasts. The age-selective signal is located within the transit peptide of each protein. A group III protein with its transit peptide replaced by a group I transit peptide failed to complement its own mutation. Two consecutive positive charges define the necessary motif in group III signals for older chloroplast preference. We further show that different members of a gene family often belong to different age-selective groups because of sequence differences in their transit peptides. These results indicate that organelle-targeting signal peptides are part of cells' differential age-dependent regulation networks. The sequence diversity of some organelle-targeting peptides is not a result of the lack of selection pressure but has evolved to mediate regulation.

The chloroplast protein import system: from algae to trees

Chloroplasts are essential organelles in the cells of plants and algae. The functions of these specialized plastids are largely dependent on the ~3000 proteins residing in the organelle. Although chloroplasts are capable of a limited amount of semiautonomous protein synthesis - their genomes encode ~100 proteins - they must import more than 95% of their proteins after synthesis in the cytosol. Imported proteins generally possess an N-terminal extension termed a transit peptide. The importing translocons are made up of two complexes in the outer and inner envelope membranes, the so-called Toc and Tic machineries, respectively. The Toc complex contains two precursor receptors, Toc159 and Toc34, a protein channel, Toc75, and a peripheral component, Toc64/OEP64. The Tic complex consists of as many as eight components, namely Tic22, Tic110, Tic40, Tic20, Tic21 Tic62, Tic55 and Tic32. This general Toc/Tic import pathway, worked out largely in pea chloroplasts, appears to operate in chloroplasts in all green plants, albeit with significant modifications. Sub-complexes of the Toc and Tic machineries are proposed to exist to satisfy different substrate-, tissue-, cell- and developmental requirements. In this review, we summarize our understanding of the functions of Toc and Tic components, comparing these components of the import machinery in green algae through trees. We emphasize recent findings that point to growing complexities of chloroplast protein import process, and use the evolutionary relationships between proteins of different species in an attempt to define the essential core translocon components and those more likely to be responsible for regulation. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.

The role of calcium in chloroplasts--an intriguing and unresolved puzzle

More than 70 years of studies have indicated that chloroplasts contain a significant amount of calcium, are a potential storage compartment for this ion, and might themselves be prone to calcium regulation. Many of these studies have been performed on the photosynthetic light reaction as well as CO(2) fixation via the Calvin-Benson-Bassham cycle, and they showed that calcium is required in several steps of these processes. Further studies have indicated that calcium is involved in other chloroplast functions that are not directly related to photosynthesis and that there is a calcium-dependent regulation similar to cytoplasmic calcium signal transduction. Nevertheless, the precise role that calcium has as a functional and regulatory component of chloroplast processes remains enigmatic. Calcium concentrations in different chloroplast subcompartments have been measured, but the extent and direction of intra-plastidal calcium fluxes or calcium transport into and from the cytosol are not yet very well understood. In this review we want to give an overview over the current knowledge on the relationship between chloroplasts and calcium and discuss questions that need to be addressed in future research.

Cross-talk between calcium signalling and protein phosphorylation at the thylakoid

The role of protein phosphorylation for adjusting chloroplast functions to changing environmental needs is well established, whereas calcium signalling in the chloroplast is only recently becoming appreciated. The work presented here explores the potential cross-talk between calcium signalling and protein phosphorylation in chloroplasts and provides the first evidence for targets of calcium-dependent protein phosphorylation at the thylakoid membrane. Thylakoid proteins were screened for calcium-dependent phosphorylation by 2D gel electrophoresis combined with phospho-specific labelling and PsaN, CAS, and VAR1, among other proteins, were identified repeatedly by mass spectrometry. Subsequently their calcium-dependent phosphorylation was confirmed in kinase assays using the purified proteins and chloroplast extracts. This is the first report on the protein targets of calcium-dependent phosphorylation of thylakoid proteins and provides ground for further studies in this direction.

The Arabidopsis calmodulin-like proteins AtCML30 and AtCML3 are targeted to mitochondria and peroxisomes, respectively

Calmodulin (CaM) is a ubiquitous sensor/transducer of calcium signals in eukaryotic organisms. While CaM mediated calcium regulation of cytosolic processes is well established, there is growing evidence for the inclusion of organelles such as chloroplasts, mitochondria and peroxisomes into the calcium/calmodulin regulation network. A number of CaM-binding proteins have been identified in these organelles and processes such as protein import into chloroplasts and mitochondria have been shown to be governed by CaM regulation. What have been missing to date are the mediators of this regulation since no CaM or calmodulin-like protein (CML) has been identified in any of these organelles. Here we show that two Arabidopsis CMLs, AtCML3 and AtCML30, are localized in peroxisomes and mitochondria, respectively. AtCML3 is targeted via an unusual C-terminal PTS1-like tripeptide while AtCML30 utilizes an N-terminal, non-cleavable transit peptide. Both proteins possess the typical structure of CaMs, with two pairs of EF-hand motifs separated by a short linker domain. They furthermore display common characteristics, such as calcium-dependent alteration of gel mobility and calcium-dependent exposure of a hydrophobic surface. This indicates that they can function in a similar manner as canonical CaMs. The presence of close homologues to AtCML3 and AtCML30 in other plants further indicates that organellar targeting of these CMLs is not a specific feature of Arabidopsis. The identification of peroxisomal and mitochondrial CMLs is an important step in the understanding how these organelles are integrated into the cellular calcium/calmodulin signaling pathways.

Plant organellar calcium signalling: an emerging field

This review provides a comprehensive overview of the established and emerging roles that organelles play in calcium signalling. The function of calcium as a secondary messenger in signal transduction networks is well documented in all eukaryotic organisms, but so far existing reviews have hardly addressed the role of organelles in calcium signalling, except for the nucleus. Therefore, a brief overview on the main calcium stores in plants-the vacuole, the endoplasmic reticulum, and the apoplast-is provided and knowledge on the regulation of calcium concentrations in different cellular compartments is summarized. The main focus of the review will be the calcium handling properties of chloroplasts, mitochondria, and peroxisomes. Recently, it became clear that these organelles not only undergo calcium regulation themselves, but are able to influence the Ca(2+) signalling pathways of the cytoplasm and the entire cell. Furthermore, the relevance of recent discoveries in the animal field for the regulation of organellar calcium signals will be discussed and conclusions will be drawn regarding potential homologous mechanisms in plant cells. Finally, a short overview on bacterial calcium signalling is included to provide some ideas on the question where this typically eukaryotic signalling mechanism could have originated from during evolution.

A toolset of aequorin expression vectors for in planta studies of subcellular calcium concentrations in Arabidopsis thaliana

Calcium has long been acknowledged as one of the most important signalling components in plants. Many abiotic and biotic stimuli are transduced into a cellular response by temporal and spatial changes in cellular calcium concentration and the calcium-sensitive protein aequorin has been exploited as a genetically encoded calcium indicator for the measurement of calcium in planta. The objective of this work was to generate a compatible set of aequorin expression plasmids for the generation of transgenic plant lines to measure changes in calcium levels in different cellular subcompartments. Aequorin was fused to different targeting peptides or organellar proteins as a means to localize it to the cytosol, the nucleus, the plasma membrane, and the mitochondria. Furthermore, constructs were designed to localize aequorin in the stroma as well as the inner and outer surface of the chloroplast envelope membranes. The modular set-up of the plasmids also allows the easy replacement of targeting sequences to include other compartments. An additional YFP-fusion was included to verify the correct subcellular localization of all constructs by laser scanning confocal microscopy. For each construct, pBin19-based binary expression vectors driven by the 35S or UBI10 promoter were made for Agrobacterium-mediated transformation. Stable Arabidopsis lines were generated and initial tests of several lines confirmed their feasibility to measure calcium signals in vivo.

The chloroplast calcium sensor CAS is required for photoacclimation in Chlamydomonas reinhardtii

The plant-specific calcium binding protein CAS (calcium sensor) has been localized in chloroplast thylakoid membranes of vascular plants and green algae. To elucidate the function of CAS in Chlamydomonas reinhardtii, we generated and analyzed eight independent CAS knockdown C. reinhardtii lines (cas-kd). Upon transfer to high-light (HL) growth conditions, cas-kd lines were unable to properly induce the expression of LHCSR3 protein that is crucial for nonphotochemical quenching. Prolonged exposure to HL revealed a severe light sensitivity of cas-kd lines and caused diminished activity and recovery of photosystem II (PSII). Remarkably, the induction of LHCSR3, the growth of cas-kd lines under HL, and the performance of PSII were fully rescued by increasing the calcium concentration in the growth media. Moreover, perturbing cellular Ca(2+) homeostasis by application of the calmodulin antagonist W7 or the G-protein activator mastoparan impaired the induction of LHCSR3 expression in a concentration-dependent manner. Our findings demonstrate that CAS and Ca(2+) are critically involved in the regulation of the HL response and particularly in the control of LHCSR3 expression.

Modes of action of microbially-produced phytotoxins

Some of the most potent phytotoxins are synthesized by microbes. A few of these share molecular target sites with some synthetic herbicides, but many microbial toxins have unique target sites with potential for exploitation by the herbicide industry. Compounds from both non-pathogenic and pathogenic microbes are discussed. Microbial phytotoxins with modes of action the same as those of commercial herbicides and those with novel modes of action of action are covered. Examples of the compounds discussed are tentoxin, AAL-toxin, auscaulitoxin aglycone, hydantocidin, thaxtomin, and tabtoxin.

A Survey of Chloroplast Protein Kinases and Phosphatases in Arabidopsis thaliana

Protein phosphorylation is a major mode of regulation of metabolism, gene expression and cell architecture. In chloroplasts, reversible phosphorylation of proteins is known to regulate a number of prominent processes, for instance photosynthesis, gene expression and starch metabolism. The complements of the involved chloroplast protein kinases (cpPKs) and phosphatases (cpPPs) are largely unknown, except 6 proteins (4 cpPKs and 2 cpPPs) which have been experimentally identified so far. We employed combinations of programs predicting N-terminal chloroplast transit peptides (cTPs) to identify 45 tentative cpPKs and 21 tentative cpPPs. However, test sets of 9 tentative cpPKs and 13 tentative cpPPs contain only 2 and 7 genuine cpPKs and cpPPs, respectively, based on experimental subcellular localization of their N-termini fused to the reporter protein RFP. Taken together, the set of enzymes known to be involved in the reversible phosphorylation of chloroplast proteins in A. thaliana comprises altogether now 6 cpPKs and 9 cpPPs, the function of which needs to be determined in future by functional genomics approaches. This includes the calcium-regulated PK CIPK13 which we found to be located in the chloroplast, indicating that calcium-dependent signal transduction pathways also operate in this organelle.

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.

Protein import into chloroplasts: the Tic complex and its regulation

Chloroplasts like mitochondria were derived from an endosymbiontic event. Due to the massive gene transfer to the nucleus during endosymbiosis, only a limited number of chloroplastic proteins are still encoded for in the plastid genome. Most of the nuclear-encoded plastidic proteins are post-translationally translocated back to the chloroplast via the general import pathway through distinct outer and inner envelope membrane protein complexes, the Toc and Tic translocons (Translocon at the outer/inner envelope membrane of chloroplasts). Eight Tic subunits have been described so far, including two potential channel proteins (Tic110 and Tic20), the "motor complex" (Tic40 associated with the stromal chaperone Hsp93) and the "redox regulon" (Tic62, Tic55, and Tic32) involved in regulation of protein import via the metabolic redox status of the chloroplast. Regulation can additionally occur via thioredoxins (Tic110 and Tic55) or via the calcium/calmodulin network (Tic110 and Tic32). In this review we present the current knowledge about the Tic complex focusing on its regulation and addressing some still open questions.

The chloroplast protein import machinery: a review

Plastids are a heterogeneous family of organelles found ubiquitously in plants and algal cells. Most prominent are the chloroplasts, which carry out such essential processes as photosynthesis and the biosynthesis of fatty acids as well as of amino acids. As mitochondria, chloroplasts are derived from a single endosymbiotic event. They are believed to have evolved from an ancient cyanobacterium, which was engulfed by an early eukaryotic ancestor. During evolution the plastid genome has been greatly reduced and most of the genes have been transferred to the host nucleus. Consequently, more than 98% of all plastid proteins are translated on cytosolic ribosomes. They have to be posttranslationally targeted to and imported into the organelle. Targeting is assisted by cytosolic proteins which interact with proteins destined for plastids and thereby keep them in an import competent state. After reaching the target organelle, many proteins have to conquer the barrier of the chloroplast outer and inner envelope. This process is mediated by complex molecular machines in the outer (Toc complex) and inner (Tic complex) envelope of chloroplasts, respectively. Most proteins destined for the compartments inside the chloroplast contain a cleavable N-terminal transit peptide, whereas most of the outer envelope components insert into the membrane without such a targeting peptide.

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.

Breaking the code: Ca2+ sensors in plant signalling

Ca2+ ions play a vital role as second messengers in plant cells during various developmental processes and in response to environmental stimuli. Plants have evolved a diversity of unique proteins that bind Ca2+ using the evolutionarily conserved EF-hand motif. The currently held hypothesis is that these proteins function as Ca2+ sensors by undergoing conformational changes in response to Ca2+-binding that facilitate their regulation of target proteins and thereby co-ordinate various signalling pathways. The three main classes of these EF-hand Ca2+sensors in plants are CaMs [calmodulins; including CMLs (CaM-like proteins)], CDPKs (calcium-dependent protein kinases) and CBLs (calcineurin B-like proteins). In the plant species examined to date, each of these classes is represented by a large family of proteins, most of which have not been characterized biochemically and whose physiological roles remain unclear. In the present review, we discuss recent advances in research on CaMs and CMLs, CDPKs and CBLs, and we attempt to integrate the current knowledge on the different sensor classes into common physiological themes.

Preprotein import into chloroplasts via the Toc and Tic complexes is regulated by redox signals in Pisum sativum

The import of nuclear-encoded preproteins is necessary to maintain chloroplast function. The recognition and transfer of most precursor proteins across the chloroplast envelopes are facilitated by two membrane-inserted protein complexes, the translocons of the chloroplast outer and inner envelope (Toc and Tic complexes, respectively). Several signals have been invoked to regulate the import of preproteins. In our study, we were interested in redox-based import regulation mediated by two signals: regulation based on thiols and on the metabolic NADP+/NADPH ratio. We sought to identify the proteins participating in the regulation of these transport pathways and to characterize the preprotein subgroups whose import is redox-dependent. Our results provide evidence that the formation and reduction of disulfide bridges in the Toc receptors and Toc translocation channel have a strong influence on import yield of all tested preproteins that depend on the Toc complex for translocation. Furthermore, the metabolic NADP+/NADPH ratio influences not only the composition of the Tic complex, but also the import efficiency of most, but not all, preproteins tested. Thus, several Tic subcomplexes appear to participate in the translocation of different preprotein subgroups, and the redox-active components of these complexes likely play a role in regulating transport.

Calcium regulation in endosymbiotic organelles of plants

In plant cells calcium-dependent signaling pathways are involved in a large array of biological processes in response to hormones, biotic/abiotic stress signals and a variety of developmental cues. This is generally achieved through binding of calcium to diverse calcium-sensing proteins, which subsequently control downstream events by activating or inhibiting biochemical reactions. Regulation by calcium is considered as a eukaryotic trait and has not been described for prokaryotes. Nevertheless, there is increasing evidence indicating that organelles of prokaryotic origin, such as chloroplasts and mitochondria, are integrated into the calcium-signaling network of the cell. An important transducer of calcium in these organelles appears to be calmodulin. In this review we want to give an overview over present data showing that endosymbiotic organelles harbour calcium-dependent biological processes with a focus on calmodulin-regulation.

Arabidopsis ATPase family gene 1-like protein 1 is a calmodulin-binding AAA+-ATPase with a dual localization in chloroplasts and mitochondria

Members of the AAA(+)-ATPase superfamily (ATPases associated with various cellular activities) are found in all kingdoms of life and they are involved in very diverse cellular processes, including protein degradation, membrane fusion or cell division. The Arabidopsis genome encodes approximately 140 different proteins that are putative members of this superfamily, although the exact function of most of these proteins remains unknown. Using affinity chromatography on calmodulin-agarose with chloroplast proteins, we purified a 50 kDa protein encoded by AT4G30490 with similarity to the ATPase family gene 1 protein from yeast. Structural analysis showed that the protein possesses a single AAA-domain characteristic for members of the AAA(+)-ATPase superfamily and that this contains all features specific to proteins of the ATPase family gene 1-like subfamily. In vitro pull-down as well as cross-linking assays corroborate calcium-dependent binding of the protein to calmodulin. The calmodulin binding domain could be located to a region of 20 amino acids within the AAA-domain in close proximity to the Walker A motif. Our analysis further showed that the protein is localized in both mitochondria and chloroplasts, further supporting the incorporation of both endosymbiotic organelles into the calcium-signaling network of the cell. Localization of the same calmodulin-binding protein into mitochondria and chloroplasts could be a means to provide a coordinated regulation of processes in both organelles by calcium signals.

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.

Calcium depletion and calmodulin inhibition affect the import of nuclear-encoded proteins into plant mitochondria

Many metabolic processes essential for plant viability take place in mitochondria. Therefore, mitochondrial function has to be carefully balanced in accordance with the developmental stage and metabolic requirements of the cell. One way to adapt organellar function is the alteration of protein composition. Since most mitochondrial proteins are nuclear encoded, fine-tuning of mitochondrial protein content could be achieved by the regulation of protein translocation. Here we present evidence that the import of nuclear-encoded mitochondrial proteins into plant mitochondria is influenced by calcium and calmodulin. In pea mitochondria, the calmodulin inhibitor ophiobolin A as well as the calcium ionophores A23187 and ionomycin inhibit translocation of nuclear-encoded proteins in a concentration-dependent manner, an effect that can be countered by the addition of external calmodulin or calcium, respectively. Inhibition was observed exclusively for proteins translocating into or across the inner membrane but not for proteins residing in the outer membrane or the intermembrane space. Ophiobolin A and the calcium ionophores further inhibit translocation into mitochondria with disrupted outer membranes, but their effect is not mediated via a change in the membrane potential across the inner mitochondrial membrane. Together, our results suggest that calcium/calmodulin influences the import of a subset of mitochondrial proteins at the inner membrane. Interestingly, we could not observe any influence of ophiobolin A or the calcium ionophores on protein translocation into mitochondria of yeast, indicating that the effect of calcium/calmodulin on mitochondrial protein import might be a plant-specific trait.

Import of preproteins into the chloroplast inner envelope membrane

The chloroplast inner envelope membrane contains many integral proteins which differ in the number of alpha-helices that anchor the protein into the bilayer. For most of these proteins it is not known which pathway they engage to reach their final localisation within the membrane. In yeast mitochondria, two distinct sorting/insertion pathways have been described for integral inner membrane proteins, involving the Tim22 and Tim23 translocases. These routes involve on the one hand a conservative sorting, on the other hand a stop-transfer pathway. In this study we performed a systematic characterisation of the import behaviour of seven inner envelope proteins representing different numbers of predicted alpha-helices. We investigated their energy dependence, import rate, involvement of components of the chloroplast general import pathway and distribution between soluble and membrane fractions. Our results show the existence of at least two different families of inner envelope proteins that can be classified due to the occurrence of an intermediate processing form. Each of the proteins we investigated seems to use a stop-transfer pathway for insertion into the inner envelope.

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.

Dual targeting of organellar seryl-tRNA synthetase to maize mitochondria and chloroplasts

Aminoacyl-tRNA synthetases (AARSs) play a critical role in translation and are thus required in three plant protein-synthesizing compartments: cytosol, mitochondria and plastids. A systematic study had previously shown extensive sharing of organellar AARSs from Arabidopsis thaliana, mostly between mitochondria and chloroplasts. However, distribution of AARSs from monocot species, such as maize, has never been experimentally investigated. Here we demonstrate dual targeting of maize seryl-tRNA synthetase, SerZMo, into both mitochondria and chloroplasts using combination of complementary methods, including in vitro import assay, transient expression analysis of green fluorescent protein (GFP) fusions and immunodetection. We also show that SerZMo dual localization is established by the virtue of an ambiguous targeting peptide. Full-length SerZMo protein fused to GFP is targeted to chloroplast stromules, indicating that SerZMo protein performs its function in plastid stroma. The deletion mutant lacking N-terminal region of the ambiguous SerZMo targeting peptide was neither targeted into mitochondria nor chloroplasts, indicating the importance of this region in both mitochondrial and chloroplastic import.

TIC62 redox-regulated translocon composition and dynamics

The preprotein translocon at the inner envelope of chloroplasts (Tic complex) facilitates the import of nuclear-encoded preproteins into the organelle. Seven distinct subunits have been identified so far. For each of those, specific functions have been proposed based on structural prediction or experimental evidence. Three of those subunits possess modules that could act as redox-active regulatory components in the import process. To date, however, the mode of redox regulation of the import process remains enigmatic. To investigate how the chloroplast redox state influences translocon behavior and composition, we studied the Tic component and the putative redox sensor Tic62 in more detail. The experimental results provide evidence that Tic62 can act as a bona fide dehydrogenase in vitro, and that it changes its localization in the chloroplast dependent on the NADP+/NADPH ratio in the stroma. Moreover, the redox state influences the interactions of Tic62 with the translocon and the flavoenzyme ferredoxin-NADP+ oxidoreductase. Additionally, we give initial experimental insights into the Tic62 structure using circular dichroism measurements and demonstrate that the protein consists of two structurally different domains. Our results indicate that Tic62 possesses redox-dependent properties that would allow it to fulfill a role as redox sensor protein in the chloroplast.

Policing Tic 'n' Toc, the doorway to chloroplasts

The organization of eukaryotic cells into different membrane-enclosed compartments requires an ordered and regulated system for targeting and translocating proteins synthesized in the cytosol across organellar membranes. Protein translocation through integral membrane proteinaceous complexes shares common principles in different organelles, whereas molecular mechanisms and energy requirements are diverse. Translocation into mitochondria and plastids requires most proteins to cross two membranes, and translocation must be regulated to accommodate environmental or metabolic changes. In the last decade, the first ideas were formulated about the regulation of protein translocation into chloroplasts, thereby laying the foundation for this field. Here, we describe recent models for the regulation of translocation by precursor protein phosphorylation, receptor dimerization, redox sensing and calcium signaling. We suggest how these mechanisms might fit within the regulatory framework for the entry of proteins into chloroplasts.

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.

Toc64 - A Preprotein-receptor at the Outer Membrane with Bipartide Function

Protein translocation across membranes is assisted by translocation machineries present in the membrane targeted by the precursor proteins. Translocon subunits can be functionally divided into receptor proteins warranting the specificity of this machine and a translocation channel. At the outer envelope of chloroplasts two sets of receptor proteins regulate protein translocation facing the cytosol or acting in the intermembrane space. One, Toc64 is a receptor of the translocon at the outer envelope of chloroplasts (Toc complex) with dual function. Toc64 recognizes Hsp90 delivered precursor proteins via a cytosolic exposed domain containing three tetratrico-peptide repeat motifs and as demonstrated in here, Toc64 functions also as a major component of a complex facing the intermembrane space. The latter complex is composed of an Hsp70 localized in the intermembrane space, its interaction partner Toc12, a J-domain containing protein and the intermembrane space protein Tic22. We analyzed the intermembrane space domain of Toc64. This domain is involved in preprotein recognition and association with the Toc-complex independent of the cytosolic domain of the Toc64 receptor. Therefore, Toc64 is involved in preprotein translocation across the outer envelope at both sites of the membrane.

A protein kinase family in Arabidopsis phosphorylates chloroplast precursor proteins

A serine/threonine protein kinase that is able to phosphorylate chloroplast-destined precursor proteins was purified from leaf extract of Arabidopsis thaliana and was identified by mass spectrometry. The protein kinase, encoded by AT2G17700, belongs to a small protein family comprising in addition AT4G35780 and AT4G38470. All three proteins were expressed heterologously in Escherichia coli and characterized with regard to their properties in precursor protein phosphorylation. They were able to phosphorylate several chloroplast-destined precursor proteins within their cleavable presequences. In contrast, a mitochondria-destined precursor protein was not a substrate for these kinases. For all three enzymes, the phosphorylation reaction was specific for ATP with apparent K(m) values between 14 and 67 microM. They did not utilize other NTPs nor were those able to compete for ATP in the reaction. An excess of ADP was able to inhibit ATP-dependent phosphorylation. Furthermore, all three kinases exhibited autophosphorylation. The protein kinases described here could represent subunits of a regulatory network involved in the cytosolic events of chloroplast protein import.

Calcium signaling in plant cell organelles delimited by a double membrane

Increases in the concentration of free calcium in the cytosol are one of the general events that relay an external stimulus to the internal cellular machinery and allow eukaryotic organisms, including plants, to mount a specific biological response. Different lines of evidence have shown that other intracellular organelles contribute to the regulation of free calcium homeostasis in the cytosol. The vacuoles, the endoplasmic reticulum and the cell wall constitute storage compartments for mobilizable calcium. In contrast, the role of organelles surrounded by a double membrane (e.g. mitochondria, chloroplasts and nuclei) is more complex. Here, we review experimental data showing that these organelles harbor calcium-dependent biological processes. Mitochondria, chloroplasts as well as nuclei are equipped to generate calcium signal on their own. Changes in free calcium in a given organelle may also favor the relocalization of proteins and regulatory components and therefore have a profound influence on the integrated functioning of the cell. Studying, in time and space, the dynamics of different components of calcium signaling pathway will certainly give clues to understand the extraordinary flexibility of plants to respond to stimuli and mount adaptive responses. The availability of technical and biological resources should allow breaking new grounds by unveiling the contribution of signaling networks in integrative plant biology.

Calcium regulation of chloroplast protein translocation is mediated by calmodulin binding to Tic32

The import of nuclear-encoded proteins into chloroplasts is tightly controlled on both sides of the envelope membranes. Regulatory circuits include redox-control as well as calcium-regulation, with calmodulin being the likely mediator of the latter. Using affinity-chromatography on calmodulin-agarose, we could identify the inner envelope translocon component Tic32 as the predominant calmodulin-binding protein of this membrane. Calmodulin-binding assays corroborate the interaction for heterologously expressed as well as native Tic32. The interaction is calcium-dependent and is mediated by a calmodulin-binding domain between Leu-296 and Leu-314 close to the C-proximal end of the pea Tic32. We furthermore could establish Tic32 as a bona fide NADPH-dependent dehydrogenase. NADPH but not NADH or NADP(+) affects the interaction of Tic110 with Tic32 as well as Tic62. At the same time, dehydrogenase activity of Tic32 is affected by calmodulin. In particular, binding of NADPH and calmodulin to Tic32 appear to be mutually exclusive. These results suggest that redox modulation and calcium regulation of chloroplast protein import convene at the Tic translocon and that both could be mediated by Tic32.