A protein import receptor in pea chloroplasts, Toc86, is only a proteolytic fragment of a larger polypeptide

Structural basis of metabolite transport by the chloroplast outer envelope channel OEP21

Triose phosphates (TPs) are the primary products of photosynthetic CO₂ fixation in chloroplasts, which need to be exported into the cytosol across the chloroplast inner envelope (IE) and outer envelope (OE) membranes to sustain plant growth. While transport across the IE is well understood, the mode of action of the transporters in the OE remains unclear. Here we present the high-resolution nuclear magnetic resonance (NMR) structure of the outer envelope protein 21 (OEP21) from garden pea, the main exit pore for TPs in C₃ plants. OEP21 is a cone-shaped β-barrel pore with a highly positively charged interior that enables binding and translocation of negatively charged metabolites in a competitive manner, up to a size of ~1 kDa. ATP stabilizes the channel and keeps it in an open state. Despite the broad substrate selectivity of OEP21, these results suggest that control of metabolite transport across the OE might be possible.

A Protochlorophyllide (Pchlide) a Oxygenase for Plant Viability

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

Import of Soluble Proteins into Chloroplasts and Potential Regulatory Mechanisms

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

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

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

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.

Protein import into isolated pea root leucoplasts

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

Structure and function of POTRA domains of Omp85/TPS superfamily

The Omp85/TPS (outer-membrane protein of 85 kDa/two-partner secretion) superfamily is a ubiquitous and major class of β-barrel proteins. This superfamily is restricted to the outer membranes of gram-negative bacteria, mitochondria, and chloroplasts. The common architecture, with an N-terminus consisting of repeats of soluble polypeptide-transport-associated (POTRA) domains and a C-terminal β-barrel pore is highly conserved. The structures of multiple POTRA domains and one full-length TPS protein have been solved, yet discovering roles of individual POTRA domains has been difficult. This review focuses on similarities and differences between POTRA structures, emphasizing POTRA domains in autotrophic organisms including plants and cyanobacteria. Unique roles, specific for certain POTRA domains, are examined in the context of POTRA location with respect to their attachment to the β-barrel pore, and their degree of biological dispensability. Finally, because many POTRA domains may have the ability to interact with thousands of partner proteins, possible modes of these interactions are also explored.

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.

Border control: selectivity of chloroplast protein import and regulation at the TOC-complex

Plants have evolved complex and sophisticated molecular mechanisms to regulate their development and adapt to their surrounding environment. Particularly the development of their specific organelles, chloroplasts and other plastid-types, is finely tuned in accordance with the metabolic needs of the cell. The normal development and functioning of plastids require import of particular subsets of nuclear encoded proteins. Most preproteins contain a cleavable sequence at their N terminal (transit peptide) serving as a signal for targeting to the organelle and recognition by the translocation machinery TOC-TIC (translocon of outer membrane complex-translocon of inner membrane complex) spanning the dual membrane envelope. The plastid proteome needs constant remodeling in response to developmental and environmental factors. Therefore selective regulation of preprotein import plays a crucial role in plant development. In this review we describe the diversity of transit peptides and TOC receptor complexes, and summarize the current knowledge and potential directions for future research concerning regulation of the different Toc isoforms.

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.

Isolation of dimorphic chloroplasts from the single-cell C4 species Bienertia sinuspersici

Three terrestrial plants are known to perform C4 photosynthesis without the dual-cell system by partitioning two distinct types of chloroplasts in separate cytoplasmic compartments. We report herein a protocol for isolating the dimorphic chloroplasts from Bienertia sinuspersici. Hypo-osmotically lysed protoplasts under our defined conditions released intact compartments containing the central chloroplasts and intact vacuoles with adhering peripheral chloroplasts. Following Percoll step gradient purification both chloroplast preparations demonstrated high homogeneities as evaluated from the relative abundance of respective protein markers. This protocol will open novel research directions toward understanding the mechanism of single-cell C4 photosynthesis.

Chloroplast Omp85 proteins change orientation during evolution

The majority of outer membrane proteins (OMPs) from gram-negative bacteria and many of mitochondria and chloroplasts are β-barrels. Insertion and assembly of these proteins are catalyzed by the Omp85 protein family in a seemingly conserved process. All members of this family exhibit a characteristic N-terminal polypeptide-transport-associated (POTRA) and a C-terminal 16-stranded β-barrel domain. In plants, two phylogenetically distinct and essential Omp85's exist in the chloroplast outer membrane, namely Toc75-III and Toc75-V. Whereas Toc75-V, similar to the mitochondrial Sam50, is thought to possess the original bacterial function, its homolog, Toc75-III, evolved to the pore-forming unit of the TOC translocon for preprotein import. In all current models of OMP biogenesis and preprotein translocation, a topology of Omp85 with the POTRA domain in the periplasm or intermembrane space is assumed. Using self-assembly GFP-based in vivo experiments and in situ topology studies by electron cryotomography, we show that the POTRA domains of both Toc75-III and Toc75-V are exposed to the cytoplasm. This unexpected finding explains many experimental observations and requires a reevaluation of current models of OMP biogenesis and TOC complex function.

Modifications at the A-domain of the chloroplast import receptor Toc159

Two families of GTPases, the Toc34 and Toc159 GTPase families, take on the task of preprotein recognition at the translocon at the outer membrane of chloroplasts (TOC translocon). The major Toc159 family members have highly acidic N-terminal domains (A-domains) that are non-essential and so far have escaped functional characterization. But recently, interest in the role of the A-domain has strongly increased. The new data of three independent studies provide evidence that the Toc159 A-domain I) participates in preprotein selectivity, II) has typical features of intrinsically unfolded proteins and III) is highly phosphorylated and possibly released from the rest of the protein by a proteolytic event. This hints to a complex regulation of A-domain function that is important for the maintenance of the preprotein selectivity at the TOC translocons.

Nucleotide binding and dimerization at the chloroplast pre-protein import receptor, atToc33, are not essential in vivo but do increase import efficiency

The atToc33 protein is one of several pre-protein import receptors in the outer envelope of Arabidopsis chloroplasts. It is a GTPase with motifs characteristic of such proteins, and its loss in the plastid protein import 1 (ppi1) mutant interferes with the import of photosynthesis-related pre-proteins, causing a chlorotic phenotype in mutant plants. To assess the significance of GTPase cycling by atToc33, we generated several atToc33 point mutants with predicted effects on GTP binding (K49R, S50N and S50N/S51N), GTP hydrolysis (G45R, G45V, Q68A and N101A), both binding and hydrolysis (G45R/K49N/S50R), and dimerization or the functional interaction between dimeric partners (R125A, R130A and R130K). First, a selection of these mutants was assessed in vitro, or in yeast, to confirm that the mutations have the desired effects: in relation to nucleotide binding and dimerization, the mutants behaved as expected. Then, activities of selected mutants were tested in vivo, by assessing for complementation of ppi1 in transgenic plants. Remarkably, all tested mutants mediated high levels of complementation: complemented plants were similar to the wild type in growth rate, chlorophyll accumulation, photosynthetic performance, and chloroplast ultrastructure. Protein import into mutant chloroplasts was also complemented to >50% of the wild-type level. Overall, the data indicate that neither nucleotide binding nor dimerization at atToc33 is essential for chloroplast import (in plants that continue to express the other TOC receptors in native form), although both processes do increase import efficiency. Absence of atToc33 GTPase activity might somehow be compensated for by that of the Toc159 receptors. However, overexpression of atToc33 (or its close relative, atToc34) in Toc159-deficient plants did not mediate complementation, indicating that the receptors do not share functional redundancy in the conventional sense.

The acidic A-domain of Arabidopsis TOC159 occurs as a hyperphosphorylated protein

The translocon at the outer membrane of the chloroplast assists the import of a large class of preproteins with amino-terminal transit sequences. The preprotein receptors Toc159 and Toc33 in Arabidopsis (Arabidopsis thaliana) are specific for the accumulation of abundant photosynthetic proteins. The receptors are homologous GTPases known to be regulated by phosphorylation within their GTP-binding domains. In addition to the central GTP-binding domain, Toc159 has an acidic N-terminal domain (A-domain) and a C-terminal membrane-anchoring domain (M-domain). The A-domain of Toc159 is dispensable for its in vivo activity in Arabidopsis and prone to degradation in pea (Pisum sativum). Therefore, it has been suggested to have a regulatory function. Here, we show that in Arabidopsis, the A-domain is not simply degraded but that it accumulates as a soluble, phosphorylated protein separated from Toc159. However, the physiological relevance of this process is unclear. The data show that the A-domain of Toc159 as well as those of its homologs Toc132 and Toc120 are targets of a casein kinase 2-like activity.

The TOC complex: preprotein gateway to the chloroplast

Photosynthetic eukaryotes strongly depend on chloroplast metabolic pathways. Most if not all involve nuclear encoded proteins. These are synthesized as cytosolic preproteins with N-terminal, cleavable targeting sequences (transit peptide). Preproteins are imported by a major pathway composed of two proteins complexes: TOC and TIC (Translocon of the Outer and Inner membranes of the Chloroplasts, respectively). These selectively recognize the preproteins and facilitate their transport across the chloroplast envelope. The TOC core complex consists of three types of components, each belonging to a small family: Toc34, Toc75 and Toc159. Toc34 and Toc159 isoforms represent a subfamily of the GTPase superfamily. The members of the Toc34 and Toc159 subfamily act as GTP-dependent receptors at the chloroplast surface and distinct members of each occur in defined, substrate-specific TOC complexes. Toc75, a member of the Omp85 family, is conserved from prokaryotes and functions as the unique protein-conducting channel at the outer membrane. In this review we will describe the current state of knowledge regarding the composition and function of the TOC complex.

The molecular basis for distinct pathways for protein import into Arabidopsis chloroplasts

The translocons at the outer envelope membrane of chloroplasts (TOCs) initiate the import of thousands of nucleus-encoded proteins into the organelle. The identification of structurally and functionally distinct TOC complexes has led to the hypothesis that the translocons constitute different import pathways that are required to coordinate the import of sets of proteins whose expression varies in response to organelle biogenesis and physiological adaptation. To test this hypothesis, we examined the molecular basis for distinct TOC pathways by analyzing the functional diversification among the Toc159 family of TOC receptors. We demonstrate that the N-terminal A-domains of the Toc159 receptors regulate their selectivity for preprotein binding. Furthermore, the in vivo function of the two major Toc159 family members (atToc159 and atToc132) can be largely switched by swapping their A-domains in transgenic Arabidopsis thaliana. On the basis of these results, we propose that the A-domains of the Toc159 receptors are major determinants of distinct pathways for protein import into chloroplasts.

The acidic domains of the Toc159 chloroplast preprotein receptor family are intrinsically disordered protein domains

Background

The Toc159 family of proteins serve as receptors for chloroplast-destined preproteins. They directly bind to transit peptides, and exhibit preprotein substrate selectivity conferred by an unknown mechanism. The Toc159 receptors each include three domains: C-terminal membrane, central GTPase, and N-terminal acidic (A-) domains. Although the function(s) of the A-domain remains largely unknown, the amino acid sequences are most variable within these domains, suggesting they may contribute to the functional specificity of the receptors.

Results

The physicochemical properties of the A-domains are characteristic of intrinsically disordered proteins (IDPs). Using CD spectroscopy we show that the A-domains of two Arabidopsis Toc159 family members (atToc132 and atToc159) are disordered at physiological pH and temperature and undergo conformational changes at temperature and pH extremes that are characteristic of IDPs.

Conclusions

Identification of the A-domains as IDPs will be important for determining their precise function(s), and suggests a role in protein-protein interactions, which may explain how these proteins serve as receptors for such a wide variety of preprotein substrates.

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

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

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.

Recent overview of the Mg branch of the tetrapyrrole biosynthesis leading to chlorophylls

In plants, chlorophylls (chlorophyll a and chlorophyll b) are the most abundant tetrapyrrole molecules and are essential for photosynthesis. The first committed step of chlorophyll biosynthesis is the insertion of Mg(2+) into protoporphyrin IX, and thus subsequent steps of the biosynthesis are called the Mg branch. As the Mg branch in higher plants is complex, it was not until the last decade--after many years of intensive research--that most of the genes encoding the enzymes for the pathway were identified. Biochemical and molecular genetic analyses have certainly modified the classic metabolic map of tetrapyrrole biosynthesis, and only recently have the molecular mechanisms of regulatory pathways governing chlorophyll metabolism been elucidated. As a result, novel functions of tetrapyrroles and biosynthetic enzymes have been proposed. In this review, I summarize the recent findings on enzymes involved in the Mg branch, mainly in higher plants.

Toc159- and Toc75-independent import of a transit sequence-less precursor into the inner envelope of chloroplasts

Chloroplast envelope quinone oxidoreductase (ceQORH) is an inner plastid envelope protein that is synthesized without cleavable chloroplast transit sequence for import. In the present work, we studied the in vitro-import characteristics of Arabidopsis ceQORH. We demonstrate that ceQORH import requires ATP and is dependent on proteinaceous receptor components exposed at the outer plastid surface. Competition experiments using small subunit precursor of ribulose-bisphosphate carboxylase/oxygenase and precursor of ferredoxin, as well as antibody blocking experiments, revealed that ceQORH import does not involve the main receptor and translocation channel proteins Toc159 and Toc75, respectively, which operate in import of proteins into the chloroplast. Molecular dissection of the ceQORH amino acid sequence by site-directed mutagenesis and subsequent import experiments in planta and in vitro highlighted that ceQORH consists of different domains that act concertedly in regulating import. Collectively, our results provide unprecedented evidence for the existence of a specific import pathway for transit sequence-less inner plastid envelope membrane proteins into chloroplasts.

A substrate-independent, 14:3:3 protein-mediated plastid import pathway of NADPH:protochlorophyllide oxidoreductase A

Plastids are semiautonomous organelles that contain only limited coding information in their own DNA. Because most of their genome was transferred to the nucleus after their endosymbiotic origin, plastids must import the major part of their protein constituents from the cytosol. The exact role of cytosolic targeting factors in the regulation of plastid protein import has not been determined. Here, we report that the nucleus-encoded NADPH:protochlorophyllide (Pchlide) oxidoreductase A plastid precursor (pPORA) can use two different plastid import pathways that differ by the requirements for cytosolic 14:3:3 proteins and Hsp70. pPORA synthesized in a wheat germ lysate segregated into different precursor fractions. While import of free pPORA and only Hsp70-complexed pPORA was Pchlide-dependent and involved the previously identified Pchlide-dependent translocon, 14:3:3 protein- and Hsp70-complexed pPORA was transported into Pchlide-free chloroplasts through the Toc75-containing standard translocon at the outer chloroplast membrane/translocon at the inner chloroplast membrane machinery. A 14:3:3 protein binding site was identified in the mature region of the (35)S-pPORA, which governed 14:3:3 protein- and Hsp70-mediated, Pchlide-independent plastid import. Collectively, our results reveal that the import of pPORA into the plastids is tightly regulated and involves different cytosolic targeting factors and plastid envelope translocon complexes.

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

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

Recognition and envelope translocation of chloroplast preproteins

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

A role of Toc33 in the protochlorophyllide-dependent plastid import pathway of NADPH:protochlorophyllide oxidoreductase (POR) A

NADPH:protochlorophyllide oxidoreductase (POR) A is a key enzyme of chlorophyll biosynthesis in angiosperms. It is nucleus-encoded, synthesized as a larger precursor in the cytosol and imported into the plastids in a substrate-dependent manner. Plastid envelope membrane proteins, called protochlorophyllide-dependent translocon proteins, Ptcs, have been identified that interact with pPORA during import. Among them are a 16-kDa ortholog of the previously characterized outer envelope protein Oep16 (named Ptc16) and a 33-kDa protein (Ptc33) related to the GTP-binding proteins Toc33 and Toc34 of Arabidopsis. In the present work, we studied the interactions and roles of Ptc16 and Ptc33 during pPORA import. Radiolabeled Ptc16/Oep16 was synthesized from a corresponding cDNA and imported into isolated Arabidopsis plastids. Crosslinking experiments revealed that import of 35S-Oep16/Ptc16 is stimulated by GTP. 35S-Oep16/Ptc16 forms larger complexes with Toc33 but not Toc34. Plastids of the ppi1 mutant of Arabidopsis lacking Toc33, were unable to import pPORA in darkness but imported the small subunit precursor of ribulose-1,5-bisphosphate carboxylase/oxygenase (pSSU), precursor ferredoxin (pFd) as well as pPORB which is a close relative of pPORA. In white light, partial suppressions of pSSU, pFd and pPORB import were observed. Our results unveil a hitherto unrecognized role of Toc33 in pPORA import and suggest photooxidative membrane damage, induced by excess Pchlide accumulating in ppi1 chloroplasts because of the lack of pPORA import, to be the cause of the general drop of protein import.

Characterization of a T-DNA insertion mutant for the protein import receptor atToc33 from chloroplasts

In Arabidopsis thaliana, the Toc34 receptor component of the chloroplast import machinery is encoded by two independent but highly homologous genes, atToc33 and atToc34. We have isolated a T-DNA insertion mutant of atToc33 which is characterized by a pale phenotype, due to reductions in the levels of photosynthetic pigments, and alterations in protein composition. The latter involve not only chloroplast proteins but also some cytosolic polypeptides, including 14-3-3 proteins which, among other functions, have been proposed to be cytosolic targeting factors for nucleus-encoded chloroplast proteins. Within the chloroplast, many, though not all, proteins of the photosynthetic apparatus, as well as proteins not directly involved in photosynthesis, are found in significantly reduced amounts in the mutant. However, the accumulation of other chloroplast proteins is unaffected. This suggests that the atToc33 receptor is responsible for the import of a specific subset of nucleus-encoded chloroplast proteins. Supporting evidence for this conclusion was obtained by antisense repression of the atToc34 gene in the atToc33 mutant, which results in an exacerbation of the phenotype.

Functional specialization amongst the Arabidopsis Toc159 family of chloroplast protein import receptors

The initial stages of preprotein import into chloroplasts are mediated by the receptor GTPase Toc159. In Arabidopsis thaliana, Toc159 is encoded by a small gene family: atTOC159, atTOC132, atTOC120, and atTOC90. Phylogenetic analysis suggested that at least two distinct Toc159 subtypes, characterized by atToc159 and atToc132/atToc120, exist in plants. atTOC159 was strongly expressed in young, photosynthetic tissues, whereas atTOC132 and atTOC120 were expressed at a uniformly low level and so were relatively prominent in nonphotosynthetic tissues. Based on the albino phenotype of its knockout mutant, atToc159 was previously proposed to be a receptor with specificity for photosynthetic preproteins. To elucidate the roles of the other isoforms, we characterized Arabidopsis knockout mutants for each one. None of the single mutants had strong visible phenotypes, but toc132 toc120 double homozygotes appeared similar to toc159, indicating redundancy between atToc132 and atToc120. Transgenic complementation studies confirmed this redundancy but revealed little functional overlap between atToc132/atToc120 and atToc159 or atToc90. Unlike toc159, toc132 toc120 caused structural abnormalities in root plastids. Furthermore, when proteomics and transcriptomics were used to compare toc132 with ppi1 (a receptor mutant that is specifically defective in the expression, import, and accumulation of photosynthetic proteins), major differences were observed, suggesting that atToc132 (and atToc120) has specificity for nonphotosynthetic proteins. When both atToc159 and the major isoform of the other subtype, atToc132, were absent, an embryo-lethal phenotype resulted, demonstrating the essential role of Toc159 in the import mechanism.

Identification of plastid envelope proteins required for import of protochlorophyllide oxidoreductase A into the chloroplast of barley

Chloroplasts synthesize an abundance of different tetrapyrrole compounds. Among them are chlorophyll and its precursor protochlorophyllide (Pchlide), which accumulate in light- and dark-grown plants, respectively. Pchlide is converted to chlorophyllide by virtue of the NADPH:Pchlide oxidoreductase (POR), which, in angiosperms, is the only known light-dependent enzyme of the chlorophyll biosynthetic pathway. In etiolated barley plants, two closely related POR proteins exist termed PORA and PORB, which are nuclear gene products. Here we identified plastid envelope proteins that interact with the cytosolic PORA precursor (pPORA) during its posttranslational chloroplast import. We demonstrate that pPORA interacts with several previously unreported components. Among them is a Pchlide a oxygenase, which provides Pchlide b as import substrate for pPORA, and a tyrosine aminotransferase thought to be involved in the synthesis of photoprotective vitamin E. Two other constituents were found to be orthologs of the GTP-binding proteins Toc33/34 and of the outer plastid envelope protein Oep16.

The Arabidopsis ppi1 mutant is specifically defective in the expression, chloroplast import, and accumulation of photosynthetic proteins

The import of nucleus-encoded proteins into chloroplasts is mediated by translocon complexes in the envelope membranes. A component of the translocon in the outer envelope membrane, Toc34, is encoded in Arabidopsis by two homologous genes, atTOC33 and atTOC34. Whereas atTOC34 displays relatively uniform expression throughout development, atTOC33 is strongly upregulated in rapidly growing, photosynthetic tissues. To understand the reason for the existence of these two related genes, we characterized the atTOC33 knockout mutant ppi1. Immunoblotting and proteomics revealed that components of the photosynthetic apparatus are deficient in ppi1 chloroplasts and that nonphotosynthetic chloroplast proteins are unchanged or enriched slightly. Furthermore, DNA array analysis of 3292 transcripts revealed that photosynthetic genes are moderately, but specifically, downregulated in ppi1. Proteome differences in ppi1 could be correlated with protein import rates: ppi1 chloroplasts imported the ribulose-1,5-bisphosphate carboxylase/oxygenase small subunit and 33-kD oxygen-evolving complex precursors at significantly reduced rates, but the import of a 50S ribosomal subunit precursor was largely unaffected. The ppi1 import defect occurred at the level of preprotein binding, which is consistent with a role for atToc33 during preprotein recognition. The data suggest that atToc33 is involved preferentially in the import of photosynthetic proteins and, by extension, that atToc34 is involved in the import of nonphotosynthetic chloroplast proteins.

Proteomics of the chloroplast envelope membranes from Arabidopsis thaliana

The development of chloroplasts and the integration of their function within a plant cell rely on the presence of a complex biochemical machinery located within their limiting envelope membranes. To provide the most exhaustive view of the protein repertoire of chloroplast envelope membranes, we analyzed this membrane system using proteomics. To this purpose, we first developed a procedure to prepare highly purified envelope membranes from Arabidopsis chloroplasts. We then extracted envelope proteins using different methods, i.e. chloroform/methanol extraction and alkaline or saline treatments, in order to retrieve as many proteins as possible, from the most to least hydrophobic ones. Liquid chromatography tandem mass spectrometry analyses were then performed on each envelope membrane subfraction, leading to the identification of more than 100 proteins. About 80% of the identified proteins are known to be, or are very likely, located in the chloroplast envelope. The validation of localization in the envelope of two phosphate transporters exemplifies the need for a combination of strategies to perform the most exhaustive identification of genuine chloroplast envelope proteins. Interestingly, some of the identified proteins are found to be Nalpha-acetylated, which indicates the accurate location of the N terminus of the corresponding mature protein. With regard to function, more than 50% of the identified proteins have functions known or very likely to be associated with the chloroplast envelope. These proteins are a) involved in ion and metabolite transport, b) components of the protein import machinery, and c) involved in chloroplast lipid metabolism. Some soluble proteins, like proteases, proteins involved in carbon metabolism, or proteins involved in responses to oxidative stress, were associated with envelope membranes. Almost one-third of the proteins we identified have no known function. The present work helps understanding chloroplast envelope metabolism at the molecular level and provides a new overview of the biochemical machinery of the chloroplast envelope membranes.

POR - import and membrane association of a key element in chloroplast development

The development of proplastids or etioplasts to chloroplast is visualized by the accumulation of chlorophyll in leaves of higher plants. The biosynthesis of chlorophyll includes a light-dependent reduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide). This light-dependent step is catalysed by the nucleus-encoded NADPH:Pchlide oxidoreductase (POR, EC 1.6.99.1). POR is active within plastids and therefore has to be translocated over the plastid envelope membranes. The import of chloroplast proteins seems to follow a general import pathway using translocons at the outer and inner envelope membrane. POR cross-linking to Toc75, one of the major translocon components at the outer envelope membrane, indicates its use of the general import pathway. However, since variations exist within the so-called general import pathway one has to consider previous data suggesting a novel totally Pchlide-dependent import pathway of one POR isoform, PORA. The suggested Pchlide dependency of POR import is discussed since recent observations contradict this idea. In the stroma the POR transit peptide is cleaved off and the mature POR protein is targeted to the plastid inner membranes. The correct and stable association of POR to the membrane requires the cofactor NADPH. Functional activity of POR calls for formation of an NADPH-Pchlide-POR complex, a formation that probably takes place after the membrane association and is dependent on a phosphorylation reaction.

Characterization of the translocon of the outer envelope of chloroplasts

The protein translocon of the outer envelope of chloroplasts (Toc) consists of the core subunits Toc159, Toc75, and Toc34. To investigate the molecular structure, the core complex was purified. This core complex has an apparent molecular mass of approximately 500 kD and a molecular stoichiometry of 1:4:4-5 between Toc159, Toc75, and Toc34. The isolated translocon recognizes both transit sequences and precursor proteins in a GTP-dependent manner, suggesting its functional integrity. The complex is embedded by the lipids phosphatidylcholine and digalactosyldiacylglyceride. Two-dimensional structural analysis by EM revealed roughly circular particles consistent with the formation of a stable core complex. The particles show a diameter of approximately 130 A with a solid ring and a less dense interior structure. A three-dimensional map obtained by random conical tilt reconstruction of electron micrographs suggests that a "finger"-like central region separates four curved translocation channels within one complex.

In vitro analysis of chloroplast protein import

This unit describes protocols for isolating chloroplasts from pea (Pisum sativum) and Arabidopsis thaliana for the study of nuclear-encoded plastid precursor proteins. Chloroplasts from both preparations are competent for the in vitro import of recombinant preproteins synthesized using in vitro translation systems derived from reticulocyte or wheat germ lysates. These assays can be used to test whether a particular protein is targeted to chloroplasts, for analyzing the suborganellar location of newly imported preproteins, or to study the mechanism of import itself.

Essential role of the G-domain in targeting of the protein import receptor atToc159 to the chloroplast outer membrane

Two homologous GTP-binding proteins, atToc33 and atToc159, control access of cytosolic precursor proteins to the chloroplast. atToc33 is a constitutive outer chloroplast membrane protein, whereas the precursor receptor atToc159 also exists in a soluble, cytosolic form. This suggests that atToc159 may be able to switch between a soluble and an integral membrane form. By transient expression of GFP fusion proteins, mutant analysis, and biochemical experimentation, we demonstrate that the GTP-binding domain regulates the targeting of cytosolic atToc159 to the chloroplast and mediates the switch between cytosolic and integral membrane forms. Mutant atToc159, unable to bind GTP, does not reinstate a green phenotype in an albino mutant (ppi2) lacking endogenous atToc159, remaining trapped in the cytosol. Thus, the function of atToc159 in chloroplast biogenesis is dependent on an intrinsic GTP-regulated switch that controls localization of the receptor to the chloroplast envelope.

The targeting of the atToc159 preprotein receptor to the chloroplast outer membrane is mediated by its GTPase domain and is regulated by GTP

The multimeric translocon at the outer envelope membrane of chloroplasts (Toc) initiates the recognition and import of nuclear-encoded preproteins into chloroplasts. Two Toc GTPases, Toc159 and Toc33/34, mediate preprotein recognition and regulate preprotein translocation. Although these two proteins account for the requirement of GTP hydrolysis for import, the functional significance of GTP binding and hydrolysis by either GTPase has not been defined. A recent study indicates that Toc159 is equally distributed between a soluble cytoplasmic form and a membrane-inserted form, raising the possibility that it might cycle between the cytoplasm and chloroplast as a soluble preprotein receptor. In the present study, we examined the mechanism of targeting and insertion of the Arabidopsis thaliana orthologue of Toc159, atToc159, to chloroplasts. Targeting of atToc159 to the outer envelope membrane is strictly dependent only on guanine nucleotides. Although GTP is not required for initial binding, the productive insertion and assembly of atToc159 into the Toc complex requires its intrinsic GTPase activity. Targeting is mediated by direct binding between the GTPase domain of atToc159 and the homologous GTPase domain of atToc33, the Arabidopsis Toc33/34 orthologue. Our findings demonstrate a role for the coordinate action of the Toc GTPases in assembly of the functional Toc complex at the chloroplast outer envelope membrane.

The chloroplast import receptor Toc34 functions as preprotein-regulated GTPase

Toc34 is a protein of the chloroplast outer envelope membrane that acts as receptor for preproteins containing a transit sequence. The recognition of preproteins by Toc34 is regulated by GTP binding and phosphorylation. The phosphorylation site of Toc34 is located at serine 113, close to the postulated triphosphate binding site. This can explain the down-regulation of Toc34 by phosphorylation, resulting in the loss of GTP binding. Vice versa, GTP but not GDP binding of Toc34 influences the phosphorylation. The nucleotide specificity of Toc34 is not only determined by the classical nucleotide binding domains but by a non-typical region at the N-terminus of the protein. As a result, the GTP binding properties are unusual, since the triphosphate moiety of GTP is bound with higher affinity than the purine base. Purified Toc34 hydrolyses GTP at a low rate, which could regulate the receptor function. The rate of hydrolysis is greatly stimulated by a precursor protein.

Chloroplast quest: a journey from the cytosol into the chloroplast and beyond

Chloroplasts are characteristic organelles of plants and algae and the site of oxygenic photosynthesis. They are surrounded by a double membrane and possess an internal membrane system, the thylakoids, on which the photosynthetic machinery is located. They originated more than 1.2 billion years ago from an endosymbiotic event between an already photosynthetic ancestor of present day cyanobacteria and a mitochondriate host cell. During the transformation of the internalized cyanobacterium into a cell organelle most of the genetic information of the endosymbiot got lost or was transferred into the nucleus of the host. Chloroplast proteins encoded by nuclear genes are synthesized on cytoplasmic ribosomes and have to be relocated into the organelle. This is achieved by a proteinaceous import machinery in the outer and inner envelope of the chloroplasts. Proteins destined for the thylakoid membrane and the thylakoid lumen are further translocated by several different pathways into or across this membrane. The subject of this review is the quest of nuclear encoded chloroplast proteins into the organelle and to their final suborganellar location.

The chloroplast protein import channel Toc75: pore properties and interaction with transit peptides

The channel properties of Toc75 (the protein import pore of the outer chloroplastic membrane) were further characterized by electrophysiological measurements in planar lipid bilayers. After improvement of the Toc75 reconstitution procedure the voltage dependence of the channel open probability resembled those observed for other beta-barrel pores. Studies concerning the pore size of the reconstituted Toc75 indicate the presence of a narrow restriction zone corresponding to the selectivity filter and a wider pore vestibule with diameters of approximately 14 A and 26 A, respectively. Interactions between Toc75 and different peptides (a genuine chloroplastic transit peptide, a synthetic peptide resembling a transit peptide, and a mitochondrial presequence) show that Toc75 itself is able to differentiate between these peptides and the recognition is based on both conformational and electrostatic interactions.

Toc, tic, and chloroplast protein import

The vast majority of chloroplast proteins are synthesized in precursor form on cytosolic ribosomes. Chloroplast precursor proteins have cleavable, N-terminal targeting signals called transit peptides. Transit peptides direct precursor proteins to the chloroplast in an organelle-specific way. They can be phosphorylated by a cytosolic protein kinase, and this leads to the formation of a cytosolic guidance complex. The guidance complex--comprising precursor, hsp70 and 14-3-3 proteins, as well as several unidentified components--docks at the outer envelope membrane. Translocation of precursor proteins across the envelope is achieved by the joint action of molecular machines called Toc (translocon at the outer envelope membrane of chloroplasts) and Tic (translocon at the inner envelope membrane of chloroplasts), respectively. The action of the Toc/Tic apparatus requires the hydrolysis of ATP and GTP at different levels, indicating energetic requirements and regulatory properties of the import process. The main subunits of the Toc and Tic complexes have been identified and characterized in vivo, in organello and in vitro. Phylogenetic evidence suggests that several translocon subunits are of cyanobacterial origin, indicating that today's import machinery was built around a prokaryotic core.

Protein translocon at the Arabidopsis outer chloroplast membrane

Chloroplasts are organelles essential for the photoautotrophic growth of plants. Their biogenesis from undifferentiated proplastids is triggered by light and requires the import of hundreds of different precursor proteins from the cytoplasm. Cleavable N-terminal transit sequences target the precursors to the chloroplast where translocon complexes at the outer (Toc complex) and inner (Tic complex) envelope membranes enable their import. In pea, the Toc complex is trimeric consisting of two surface-exposed GTP-binding proteins (Toc159 and Toc34) involved in precursor recognition and Toc75 forming an aequeous protein-conducting channel. Completion of the Arabidopsis genome has revealed an unexpected complexity of predicted components of the Toc complex in this plant model organism: four genes encode homologs of Toc159, two encode homologs of Toc34, but only one encodes a likely functional homolog of Toc75. The availability of the genomic sequence data and powerful molecular genetic techniques in Arabidopsis set the stage to unravel the mechanisms of chloroplast protein import in unprecedented depth.

The chloroplast protein import receptors Toc34 and Toc159 are phosphorylated by distinct protein kinases

The molecular composition of chloroplast outer and inner envelope translocons is fairly well established, but little is known about mechanisms and elements involved in import regulation. After synthesis in the cytosol, chloroplast targeted precursor proteins are recognized by outer envelope receptors Toc34 and Toc159. Phosphorylation plays an important role in regulation of Toc34 activity and preprotein binding. Using kinase renaturation assays, we have identified an ATP-dependent 98-kDa outer envelope kinase which is able to selectively phosphorylate Toc34 at a specific site. A 70-kDa outer envelope polypeptide phosphorylating Toc159 was identified by the same strategy. Antiserum against the 98-kDa kinase inhibits phosphorylation of Toc34, whereas labeling of Toc159 remains unaffected. Both kinases do not autophosphorylate in vitro and are unable to utilize myelin basic protein as substrate. We propose that distinct kinases are involved in regulation of chloroplast import via desensitization of preprotein receptors.

Toc, Tic, and chloroplast protein import

The vast majority of chloroplast proteins are synthesized in precursor form on cytosolic ribosomes. Chloroplast precursor proteins have cleavable, N-terminal targeting signals called transit peptides. Transit peptides direct precursor proteins to the chloroplast in an organelle-specific way. They can be phosphorylated by a cytosolic protein kinase, and this leads to the formation of a cytosolic guidance complex. The guidance complex--comprising precursor, hsp70 and 14-3-3 proteins, as well as several unidentified components--docks at the outer envelope membrane. Translocation of precursor proteins across the envelope is achieved by the joint action of molecular machines called Toc (translocon at the outer envelope membrane of chloroplasts) and Tic (translocon at the inner envelope membrane of chloroplasts), respectively. The action of the Toc/Tic apparatus requires the hydrolysis of ATP and GTP at different levels, indicating energetic requirements and regulatory properties of the import process. The main subunits of the Toc and Tic complexes have been identified and characterized in vivo, in organello and in vitro. Phylogenetic evidence suggests that several translocon subunits are of cyanobacterial origin, indicating that today's import machinery was built around a prokaryotic core.

Translocation of proteins across the multiple membranes of complex plastids

Secondary endosymbiosis describes the origin of plastids in several major algal groups such as dinoflagellates, euglenoids, heterokonts, haptophytes, cryptomonads, chlorarachniophytes and parasites such as apicomplexa. An integral part of secondary endosymbiosis has been the transfer of genes for plastid proteins from the endosymbiont to the host nucleus. Targeting of the encoded proteins back to the plastid from their new site of synthesis in the host involves targeting across the multiple membranes surrounding these complex plastids. Although this process shows many overall similarities in the different algal groups, it is emerging that differences exist in the mechanisms adopted.

The paradox of plastid transit peptides: conservation of function despite divergence in primary structure

Transit peptides are N-terminal extensions that facilitate the targeting and translocation of cytosolically synthesized precursors into plastids via a post-translational mechanism. With the complete Arabidopsis genome in hand, it is now evident that transit peptides direct more than 3500 different proteins into the plastid during the life of a typical plant. Deciphering a common mechanism for how this multitude of targeting sequences function has been hampered by the realization that at a primary sequence level, transit peptides are highly divergent in length, composition, and organization. This review addresses recent findings on several of the diverse functions that transit peptides must perform, including direct interaction with envelope lipids, association with a cis-acting guidance complex, recognition by envelope receptors, insertion into the Toc/Tic translocon, interaction with molecular motors, and finally, recognition/cleavage by the stromal processing peptidase. In addition to higher plants, transit peptides also direct the import of proteins into complex plastids derived from secondary endosymbiosis. An emerging concept suggests that transit peptides contain multiple domains that provide either distinct or possibly overlapping functions. Although still poorly characterized, evolutionary processes could yield transit peptides with alternative domain organizations.