Targeting and assembly of components of the TOC protein import complex at the chloroplast outer envelope membrane

The journey of preproteins across the chloroplast membrane systems

The photosynthetic capacity of chloroplasts is vital for autotrophic growth in algae and plants. The origin of the chloroplast has been explained by the endosymbiotic theory that proposes the engulfment of a cyanobacterium by an ancestral eukaryotic cell followed by the transfer of many cyanobacterial genes to the host nucleus. As a result of the gene transfer, the now nuclear-encoded proteins acquired chloroplast targeting peptides (known as transit peptides; transit peptide) and are translated as preproteins in the cytosol. Transit peptides contain specific motifs and domains initially recognized by cytosolic factors followed by the chloroplast import components at the outer and inner envelope of the chloroplast membrane. Once the preprotein emerges on the stromal side of the chloroplast protein import machinery, the transit peptide is cleaved by stromal processing peptidase. In the case of thylakoid-localized proteins, cleavage of the transit peptides may expose a second targeting signal guiding the protein to the thylakoid lumen or allow insertion into the thylakoid membrane by internal sequence information. This review summarizes the common features of targeting sequences and describes their role in routing preproteins to and across the chloroplast envelope as well as the thylakoid membrane and lumen.

Targeting BAM for Novel Therapeutics against Pathogenic Gram-Negative Bacteria

The growing emergence of multidrug resistance in bacterial pathogens is an immediate threat to human health worldwide. Unfortunately, there has not been a matching increase in the discovery of new antibiotics to combat this alarming trend. Novel contemporary approaches aimed at antibiotic discovery against Gram-negative bacterial pathogens have expanded focus to also include essential surface-exposed receptors and protein complexes, which have classically been targeted for vaccine development. One surface-exposed protein complex that has gained recent attention is the β-barrel assembly machinery (BAM), which is conserved and essential across all Gram-negative bacteria. BAM is responsible for the biogenesis of β-barrel outer membrane proteins (β-OMPs) into the outer membrane. These β-OMPs serve essential roles for the cell including nutrient uptake, signaling, and adhesion, but can also serve as virulence factors mediating pathogenesis. The mechanism for how BAM mediates β-OMP biogenesis is known to be dynamic and complex, offering multiple modes for inhibition by small molecules and targeting by larger biologics. In this review, we introduce BAM and establish why it is a promising and exciting new therapeutic target and present recent studies reporting novel compounds and vaccines targeting BAM across various bacteria. These reports have fueled ongoing and future research on BAM and have boosted interest in BAM for its therapeutic promise in combatting multidrug resistance in Gram-negative bacterial pathogens.

Research progress on maintaining chloroplast homeostasis under stress conditions: a review

On a global scale, drought, salinity, extreme temperature, and other abiotic stressors severely limit the quality and yield of crops. Therefore, it is crucial to clarify the adaptation strategies of plants to harsh environments. Chloroplasts are important environmental sensors in plant cells. For plants to thrive in different habitats, chloroplast homeostasis must be strictly regulated, which is necessary to maintain efficient plant photosynthesis and other metabolic reactions under stressful environments. To maintain normal chloroplast physiology, two important biological processes are needed: the import and degradation of chloroplast proteins. The orderly import of chloroplast proteins and the timely degradation of damaged chloroplast components play a key role in adapting plants to their environment. In this review, we briefly described the mechanism of chloroplast TOC-TIC protein transport. The importance and recent progress of chloroplast protein turnover, retrograde signaling, and chloroplast protein degradation under stress are summarized. Furthermore, the potential of targeted regulation of chloroplast homeostasis is emphasized to improve plant adaptation to environmental stresses.

Split-Ubiquitin Two-Hybrid Screen for Proteins Interacting with slToc159-1 and slToc159-2, Two Chloroplast Preprotein Import Receptors in Tomato (Solanum lycopersicum)

The post-translational import of nuclear-encoded chloroplast preproteins is critical for chloroplast biogenesis, and the Toc159 family of proteins is the receptor for this process. Our previous work identified and analyzed the Toc GTPase in tomato; however, the tomato-specific transport substrate for Toc159 is still unknown, which limits the study of the function of the TOC receptor in tomato. In this study, we expand the number of preprotein substrates of slToc159 receptor family members using slToc159-1 and slToc159-2 as bait via a split-ubiquitin yeast two-hybrid membrane system. Forty-one specific substrates were identified in tomato for the first time. Using slToc159-1GM and slToc159-2GM as bait, we compared the affinity of the two bait proteins, with and without the A domain, to the precursor protein, which suggested that the A domain endowed the proproteins with subclass specificity. The presence of the A domain enhanced the interaction intensity of slToc159-1 with the photosynthetic preprotein but decreased the interaction intensity of slToc159-2 with the photosynthetic preprotein. Similarly, the presence of the A domain also altered the affinity of slToc159 to non-photosynthetic preproteins. Bimolecular fluorescence complementation (BiFC) analysis showed that A domain had the ability to recognize the preprotein, and the interaction occurred in the chloroplast. Further, the localization of the A domain in Arabidopsis protoplasts showed that the A domain did not contain chloroplast membrane targeting signals. Our data demonstrate the importance of a highly non-conserved A domain, which endows the slToc159 receptor with specificity for different protein types. However, the domain containing the information on targeting the chloroplast needs further study.

New Insights into the Chloroplast Outer Membrane Proteome and Associated Targeting Pathways

Plastids are a dynamic class of organelle in plant cells that arose from an ancient cyanobacterial endosymbiont. Over the course of evolution, most genes encoding plastid proteins were transferred to the nuclear genome. In parallel, eukaryotic cells evolved a series of targeting pathways and complex proteinaceous machinery at the plastid surface to direct these proteins back to their target organelle. Chloroplasts are the most well-characterized plastids, responsible for photosynthesis and other important metabolic functions. The biogenesis and function of chloroplasts rely heavily on the fidelity of intracellular protein trafficking pathways. Therefore, understanding these pathways and their regulation is essential. Furthermore, the chloroplast outer membrane proteome remains relatively uncharted territory in our understanding of protein targeting. Many key players in the cytosol, receptors at the organelle surface, and insertases that facilitate insertion into the chloroplast outer membrane remain elusive for this group of proteins. In this review, we summarize recent advances in the understanding of well-characterized chloroplast outer membrane protein targeting pathways as well as provide new insights into novel targeting signals and pathways more recently identified using a bioinformatic approach. As a result of our analyses, we expand the known number of chloroplast outer membrane proteins from 117 to 138.

Liquid-Liquid Phase Separation Phenomenon on Protein Sorting Within Chloroplasts

In higher plants, chloroplasts are vital organelles possessing highly complex compartmentalization. As most chloroplast-located proteins are encoded in the nucleus and synthesized in the cytosol, the correct sorting of these proteins to appropriate compartments is critical for the proper functions of chloroplasts as well as plant survival. Nuclear-encoded chloroplast proteins are imported into stroma and further sorted to distinct compartments via different pathways. The proteins predicted to be sorted to the thylakoid lumen by the chloroplast twin arginine transport (cpTAT) pathway are shown to be facilitated by STT1/2 driven liquid-liquid phase separation (LLPS). Liquid-liquid phase separation is a novel mechanism to facilitate the formation of membrane-less sub-cellular compartments and accelerate biochemical reactions temporally and spatially. In this review, we introduce the sorting mechanisms within chloroplasts, and briefly summarize the properties and significance of LLPS, with an emphasis on the novel function of LLPS in the sorting of cpTAT substrate proteins. We conclude with perspectives for the future research on chloroplast protein sorting and targeting mechanisms.

PRAT Proteins Operate in Organellar Protein Import and Export in Arabidopsis thaliana

Chloroplasts need to import preproteins and amino acids from the cytosol during their light-induced differentiation. Similarly, chloroplasts have to export organic matter including proteins and amino acids during leaf senescence. Members of the PRAT (preprotein and amino acid transporter) family are candidate transporters for both processes. Here, we defined the role of two small PRAT gene families, At4g26670 and At5g55510 (HP20 subfamily) versus At3g49560 and At5g24650 (HP30 subfamily) during greening of etiolated plants and during leaf senescence. Using a combination of reverse genetics, protein biochemistry and physiological tools, evidence was obtained for a role of chloroplast HP20, HP30 and HP30-2 in protein, but not amino acid, import into chloroplasts. HP20, HP30 and HP30-2 form larger complexes involved in the uptake of transit sequence-less cytosolic precursors. In addition, we identified a fraction of HP30-2 in mitochondria where it served a similar function as found for chloroplasts and operated in the uptake of transit sequence-less cytosolic precursor proteins. By contrast, HP22 was found to act in the export of proteins from chloroplasts during leaf senescence, and thus its role is entirely different from that of its orthologue, HP20. HP22 is part of a unique protein complex in the envelope of senescing chloroplasts that comprises at least 11 proteins and contains with HP65b (At5g55220) a protein that is related to the bacterial trigger factor chaperone. An ortholog of HP65b exists in the cyanobacterium Synechocystis and has previously been implicated in protein secretion. Whereas plants depleted of either HP22 or HP65b or even both were increasingly delayed in leaf senescence and retained much longer stromal chloroplast constituents than wild-type plants, HP22 overexpressors showed premature leaf senescence that was associated with accelerated losses of stromal chloroplast proteins. Together, our results identify the PRAT protein family as a unique system for importing and exporting proteins from chloroplasts.

TtOmp85, a single Omp85 member protein functions as a β-barrel protein insertase and an autotransporter translocase without any accessory proteins

The biogenesis of outer membrane proteins requires the function of β-barrel assembly machinery (BAM), whose function is highly conserved while its composition is variable. The Escherichia coli BAM is composed of five subunits, while Thermus thermophilus seems to contain a single BAM protein, named TtOmp85. To search for the primitive form of a functional BAM, we investigated and compared the function of TtOmp85 and E. coli BAM by use of a reconstitution assay that examines the integration of OmpA and BamA from E. coli and TtoA from T. thermophilus, as well as the translocation of the E. coli Ag43. Our results show that a single TtOmp85 protein can substitute for the collective function of the five subunits constituting E. coli BAM.

Targeted delivery of nanomaterials with chemical cargoes in plants enabled by a biorecognition motif

Current approaches for nanomaterial delivery in plants are unable to target specific subcellular compartments with high precision, limiting our ability to engineer plant function. We demonstrate a nanoscale platform that targets and delivers nanomaterials with biochemicals to plant photosynthetic organelles (chloroplasts) using a guiding peptide recognition motif. Quantum dot (QD) fluorescence emission in a low background window allows confocal microscopy imaging and quantitative detection by elemental analysis in plant cells and organelles. QD functionalization with β-cyclodextrin molecular baskets enables loading and delivery of diverse chemicals, and nanoparticle coating with a rationally designed and conserved guiding peptide targets their delivery to chloroplasts. Peptide biorecognition provides high delivery efficiency and specificity of QD with chemical cargoes to chloroplasts in plant cells in vivo (74.6 ± 10.8%) and more specific tunable changes of chloroplast redox function than chemicals alone. Targeted delivery of nanomaterials with chemical cargoes guided by biorecognition motifs has a broad range of nanotechnology applications in plant biology and bioengineering, nanoparticle-plant interactions, and nano-enabled agriculture.

Chloroplast Outer Membrane β-Barrel Proteins Use Components of the General Import Apparatus

Chloroplasts evolved from a cyanobacterial endosymbiont that resided within a eukaryotic cell. Due to their prokaryotic heritage, chloroplast outer membranes contain transmembrane β-barrel proteins. While most chloroplast proteins use N-terminal transit peptides to enter the chloroplasts through the translocons at the outer and inner chloroplast envelope membranes (TOC/TIC), only one β-barrel protein, Toc75, has been shown to use this pathway. The route other β-barrel proteins use has remained unresolved. Here we use in vitro pea (Pisum sativum) chloroplast import assays and transient expression in Nicotiana benthamiana to address this. We show that a paralog of Toc75, outer envelope protein 80 kD (OEP80), also uses a transit peptide but has a distinct envelope sorting signal. Our results additionally indicate that β-barrels that do not use transit peptides also enter the chloroplast using components of the general import pathway.

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.

An RK/ST C-Terminal Motif is Required for Targeting of OEP7.2 and a Subset of Other Arabidopsis Tail-Anchored Proteins to the Plastid Outer Envelope Membrane

Tail-anchored (TA) proteins are a unique class of integral membrane proteins that possess a single C-terminal transmembrane domain and target post-translationally to the specific organelles at which they function. While significant advances have been made in recent years in elucidating the mechanisms and molecular targeting signals involved in the proper sorting of TA proteins, particularly to the endoplasmic reticulum and mitochondria, relatively little is known about the targeting of TA proteins to the plastid outer envelope. Here we show that several known or predicted plastid TA outer envelope proteins (OEPs) in Arabidopsis possess a C-terminal RK/ST sequence motif that serves as a conserved element of their plastid targeting signal. Evidence for this conclusion comes primarily from experiments with OEP7.2, which is a member of the Arabidopsis 7 kDa OEP family. We confirmed that OEP7.2 is localized to the plastid outer envelope and possesses a TA topology, and its C-terminal sequence (CTS), which includes the RK/ST motif, is essential for proper targeting to plastids. The CTS of OEP7.2 is functionally interchangeable with the CTSs of other TA OEPs that possess similar RK/ST motifs, but not with those that lack the motif. Further, a bioinformatics search based on a consensus sequence led to the identification of several new OEP TA proteins. Collectively, this study provides new insight into the mechanisms of TA protein sorting in plant cells, defines a new targeting signal element for a subset of TA OEPs and expands the number and repertoire of TA proteins at the plastid outer envelope.

Molecular Topology of the Transit Peptide during Chloroplast Protein Import

Chloroplast protein import is directed by the interaction of the targeting signal (transit peptide) of nucleus-encoded preproteins with translocons at the outer (TOC) and inner (TIC) chloroplast envelope membranes. Studies of the energetics and determinants of transit peptide binding have led to the hypothesis that import occurs through sequential recognition of transit peptides by components of TOC and TIC during protein import. To test this hypothesis, we employed a site-specific cross-linking approach to map transit peptide topology in relation to TOC-TIC components at specific stages of import in Arabidopsis thaliana and pea (Pisum sativum). We demonstrate that the transit peptide is in contact with Tic20 at the inner envelope in addition to TOC complex components at the earliest stages of chloroplast binding. Low levels of ATP hydrolysis catalyze the commitment of the preprotein to import by promoting further penetration across the envelope membranes and stabilizing the association of the preprotein with TOC-TIC. GTP hydrolysis at the TOC receptors serves as a checkpoint to regulate the ATP-dependent commitment of the preprotein to import and is not essential to drive preprotein import. Our results demonstrate the close cooperativity of the TOC and TIC machinery at each stage of transit peptide recognition and membrane translocation during protein import.

Genetics and Physiology of the Nuclearly Inherited Yellow Foliar Mutants in Soybean

Plant photosynthetic pigments are important in harvesting the light energy and transfer of energy during photosynthesis. There are several yellow foliar mutants discovered in soybean and chromosomal locations for about half of them have been deduced. Viable-yellow mutants are capable of surviving with decreased photosynthesis, while lethal-yellow mutants die shortly after germination. In addition to the decreased chlorophyll content, other features associated with yellow mutants include altered Chl a and Chl b ratio, reduction in chloroplast size and number, lower levels of other photosynthetic pigments, inability of thylakoids to stack into granum, lack of lamellae to interconnect granum and reduced size of the light harvesting complex. For some yellow mutants, temperature and/or light play a critical role in the manifestation of phenotype. Although yellow foliar mutants are viewed as undesirable for crop production, there is the possibility of these mutants to create a positive impact by reducing the total amount of chlorophyll and diverting resources toward increased biochemical photosynthetic capacity leading to increased yield. Recent advances in model plants led to the isolation and characterization of various genes associated with yellow foliar phenotype. Knowledge gained from the model plants can be applied using homology based cloning approach to isolate genes in soybean and understanding the modes of actions of the involved proteins. Identifying and characterizing yellow foliar mutants will not only aid in understanding the biosynthetic pathways involved in the photosynthetic machinery, but may also provide ways to increase soybean productivity.

The novel chloroplast outer membrane kinase KOC1 is a required component of the plastid protein import machinery

The biogenesis and maintenance of cell organelles such as mitochondria and chloroplasts require the import of many proteins from the cytosol, a process that is controlled by phosphorylation. In the case of chloroplasts, the import of hundreds of different proteins depends on translocons at the outer and inner chloroplast membrane (TOC and TIC, respectively) complexes. The essential protein TOC159 functions thereby as an import receptor. It has an N-terminal acidic (A-) domain that extends into the cytosol, controls receptor specificity, and is highly phosphorylated in vivo However, kinases that phosphorylate the TOC159 A-domain to enable protein import have remained elusive. Here, using co-purification with TOC159 from Arabidopsis, we discovered a novel component of the chloroplast import machinery, the regulatory kinase at the outer chloroplast membrane 1 (KOC1). We found that KOC1 is an integral membrane protein facing the cytosol and stably associates with TOC. Moreover, KOC1 phosphorylated the A-domain of TOC159 in vitro, and in mutant koc1 chloroplasts, preprotein import efficiency was diminished. koc1 Arabidopsis seedlings had reduced survival rates after transfer from the dark to the light in which protein import into plastids is required to rapidly complete chloroplast biogenesis. In summary, our data indicate that KOC1 is a functional component of the TOC machinery that phosphorylates import receptors, supports preprotein import, and contributes to efficient chloroplast biogenesis.

Deletion of FtsH11 protease has impact on chloroplast structure and function in Arabidopsis thaliana when grown under continuous light

The membrane-integrated metalloprotease FtsH11 of Arabidopsis thaliana is proposed to be dual-targeted to mitochondria and chloroplasts. A bleached phenotype was observed in ftsh11 grown at long days or continuous light, pointing to disturbances in the chloroplast. Within the chloroplast, FtsH11 was found to be located exclusively in the envelope. Two chloroplast-located proteins of unknown function (Tic22-like protein and YGGT-A) showed significantly higher abundance in envelope membranes and intact chloroplasts of ftsh11 and therefore qualify as potential substrates for the FtsH11 protease. No proteomic changes were observed in the mitochondria of 6-week-old ftsh11 compared with wild type, and FtsH11 was not immunodetected in these organelles. The abundance of plastidic proteins, especially of photosynthetic proteins, was altered even during standard growth conditions in total leaves of ftsh11. At continuous light, the amount of photosystem I decreased relative to photosystem II, accompanied by a drastic change of the chloroplast morphology and a drop of non-photochemical quenching. FtsH11 is crucial for chloroplast structure and function during growth in prolonged photoperiod.

New Insights Into Roles of Ubiquitin Modification in Regulating Plastids and Other Endosymbiotic Organelles

Recent findings have revealed important and diverse roles for the ubiquitin modification of proteins in the regulation of endosymbiotic organelles, which include the primary plastids of plants as well as complex plastids: the secondary endosymbiotic organelles of cryptophytes, alveolates, stramenopiles, and haptophytes. Ubiquitin modifications have a variety of potential consequences, both to the modified protein itself and to cellular regulation. The ubiquitin-proteasome system (UPS) can target individual proteins for selective degradation by the cytosolic 26S proteasome. Ubiquitin modifications can also signal the removal of whole endosymbiotic organelles, for example, via autophagy as has been well characterized in mitochondria. As plastids must import over 90% of their proteins from the cytosol, the observation that the UPS selectively targets the plastid protein import machinery is particularly significant. In this way, the UPS may influence the development and interconversions of different plastid types, as well as plastid responses to stress, by reconfiguring the organellar proteome. In complex plastids, the Symbiont-derived ERAD-Like Machinery (SELMA) has coopted the protein transport capabilities of the ER-Associated Degradation (ERAD) system, whereby misfolded proteins are retrotranslocated from ER for proteasomal degradation, uncoupling them from proteolysis: SELMA components have been retargeted to the second outermost plastid membrane to mediate protein import. In spite of this wealth of new information, there still remain a large number of unanswered questions and a need to define the roles of ubiquitin modification further in the regulation of plastids.

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

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

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

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

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

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

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

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

Analytical ultracentrifugation and preliminary X-ray studies of the chloroplast envelope quinone oxidoreductase homologue from Arabidopsis thaliana

Quinone oxidoreductases reduce a broad range of quinones and are widely distributed among living organisms. The chloroplast envelope quinone oxidoreductase homologue (ceQORH) from Arabidopsis thaliana binds NADPH, lacks a classical N-terminal and cleavable chloroplast transit peptide, and is transported through the chloroplast envelope membrane by an unknown alternative pathway without cleavage of its internal chloroplast targeting sequence. To unravel the fold of this targeting sequence and its substrate specificity, ceQORH from A. thaliana was overexpressed in Escherichia coli, purified and crystallized. Crystals of apo ceQORH were obtained and a complete data set was collected at 2.34 Å resolution. The crystals belonged to space group C222₁, with two molecules in the asymmetric unit.

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.

The C-terminus of Bienertia sinuspersici Toc159 contains essential elements for its targeting and anchorage to the chloroplast outer membrane

Most nucleus-encoded chloroplast proteins rely on an N-terminal transit peptide (TP) as a post-translational sorting signal for directing them to the organelle. Although Toc159 is known to be a receptor for specific preprotein TPs at the chloroplast surface, the mechanism for its own targeting and integration into the chloroplast outer membrane is not completely understood. In a previous study, we identified a novel TP-like sorting signal at the C-terminus (CT) of a Toc159 homolog from the single-cell C4 species, Bienertia sinuspersici. In the current study, we have extended our understanding of the sorting signal using transient expression of fluorescently-tagged fusion proteins of variable-length, and with truncated and swapped versions of the CT. As was shown in the earlier study, the 56 residues of the CT contain crucial sorting information for reversible interaction of the receptor with the chloroplast envelope. Extension of this region to 100 residues in the current study stabilized the interaction via membrane integration, as demonstrated by more prominent plastid-associated signals and resistance of the fusion protein to alkaline extraction. Despite a high degree of sequence similarity, the plastid localization signals of the equivalent CT regions of Arabidopsis thaliana Toc159 homologs were not as strong as that of the B. sinuspersici counterparts. Together with computational and circular dichroism analyses of the CT domain structures, our data provide insights into the critical elements of the CT for the efficient targeting and anchorage of Toc159 receptors to the dimorphic chloroplasts in the single-cell C4 species.

Targeting and biogenesis of transporters and channels in chloroplast envelope membranes: Unsolved questions

Chloroplasts produce carbohydrates, hormones, vitamins, amino acids, pigments, nucleotides, ATP, and secondary metabolites. Channels and transporters are required for the movement of molecules across the two chloroplast envelope membranes. These transporters and channel proteins are grouped into two different types, including β-barrel proteins and transmembrane-domain (TMD) containing proteins. Most β-barrel proteins are localized at the outer chloroplast membrane, and TMD-containing proteins are localized at the inner chloroplast membrane. Many of these transporters and channels are encoded by nuclear genes; therefore, they have to be imported into chloroplasts after translation on cytosolic ribosomes. These proteins should have specific targeting signals for their final destination in the chloroplast membrane and for assembly into specific complexes. In this review, we summarize recent progress in the identification, functional characterization, and biogenesis of transporters and channels at the chloroplast envelope membranes, and discuss outstanding questions regarding transporter and channel protein biogenesis.