The chloroplast protein import system: from algae to trees

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

The translocon at the outer envelope membrane of chloroplasts (TOC) initiates the import of thousands of nuclear encoded preproteins required for chloroplast biogenesis and function. The multimeric TOC complex contains two GTP-regulated receptors, Toc34 and Toc159, which recognize the transit peptides of preproteins and initiate protein import through a β-barrel membrane channel, Toc75. Different isoforms of Toc34 and Toc159 assemble with Toc75 to form structurally and functionally diverse translocons, and the composition and levels of TOC translocons is required for the import of specific subsets of coordinately expressed proteins during plant growth and development. Consequently, the proper assembly of the TOC complexes is key to ensuring organelle homeostasis. This review will focus on our current knowledge of the targeting and assembly of TOC components to form functional translocons at the outer membrane. Our analyses reveal that the targeting of TOC components involves elements common to the targeting of other outer membrane proteins, but also include unique features that appear to have evolved to specifically facilitate assembly of the import apparatus.

Import determinants of organelle-specific and dual targeting peptides of mitochondria and chloroplasts in Arabidopsis thaliana

Most of the mitochondrial and chloroplastic proteins are synthesized in the cytosol as precursor proteins carrying an N-terminal targeting peptide (TP) directing them specifically to a correct organelle. However, there is a group of proteins that are dually targeted to mitochondria and chloroplasts using an ambiguous N-terminal dual targeting peptide (dTP). Here, we have investigated pattern properties of import determinants of organelle-specific TPs and dTPs combining mathematical multivariate data analysis (MVDA) with in vitro organellar import studies. We have used large datasets of mitochondrial and chloroplastic proteins found in organellar proteomes as well as manually selected data sets of experimentally confirmed organelle-specific TPs and dTPs from Arabidopsis thaliana. Two classes of organelle-specific TPs could be distinguished by MVDA and potential patterns or periodicity in the amino acid sequence contributing to the separation were revealed. dTPs were found to have intermediate sequence features between the organelle-specific TPs. Interestingly, introducing positively charged residues to the dTPs showed clustering towards the mitochondrial TPs in silico and resulted in inhibition of chloroplast, but not mitochondrial import in in vitro organellar import studies. These findings suggest that positive charges in the N-terminal region of TPs may function as an 'avoidance signal' for the chloroplast import.

Plastid-bearing sea slugs fix CO2 in the light but do not require photosynthesis to survive

Several sacoglossan sea slugs (Plakobranchoidea) feed upon plastids of large unicellular algae. Four species--called long-term retention (LtR) species--are known to sequester ingested plastids within specialized cells of the digestive gland. There, the stolen plastids (kleptoplasts) remain photosynthetically active for several months, during which time LtR species can survive without additional food uptake. Kleptoplast longevity has long been puzzling, because the slugs do not sequester algal nuclei that could support photosystem maintenance. It is widely assumed that the slugs survive starvation by means of kleptoplast photosynthesis, yet direct evidence to support that view is lacking. We show that two LtR plakobranchids, Elysia timida and Plakobranchus ocellatus, incorporate (14)CO2 into acid-stable products 60- and 64-fold more rapidly in the light than in the dark, respectively. Despite this light-dependent CO2 fixation ability, light is, surprisingly, not essential for the slugs to survive starvation. LtR animals survived several months of starvation (i) in complete darkness and (ii) in the light in the presence of the photosynthesis inhibitor monolinuron, all while not losing weight faster than the control animals. Contrary to current views, sacoglossan kleptoplasts seem to be slowly digested food reserves, not a source of solar power.

Three proteins mediate import of transit sequence-less precursors into the inner envelope of chloroplasts in Arabidopsis thaliana

A family of 17 putative preprotein and amino acid transporters designated PRAT has been identified in Arabidopsis thaliana, comprising PRAT proteins in mitochondria and chloroplasts. Although some PRAT proteins, such as the translocon of the mitochondrial inner membrane (TIM) proteins TIM22 and TIM23, play decisive roles for the translocation and import of mitochondrial inner membrane proteins, little is known about the role of the different PRAT members in chloroplasts. Here we report the identification of three distinct PRAT proteins as part of a unique protein import site. One of the identified PRAT proteins is identical with a previously characterized hypothetical protein (HP) of 20 kDa designated HP20 of the outer plastid envelope membrane. The second PRAT component is represented by HP30, and the third is identical to HP30-2, a close relative of HP30. Both HP30 and HP30-2 are inner plastid envelope membrane proteins of chloroplasts. Using biochemical, cell biological, and genetic approaches we demonstrate that all three PRAT proteins cooperate during import of transit sequence-less proteins, such as the quinone oxidoreductase homolog ceQORH used as model, into the inner chloroplast envelope membrane. Our data are reminiscent of findings reported for the TIM22 translocase, which is involved in the import of carrier proteins and other, hydrophobic membrane proteins lacking cleavable transit sequences into the inner mitochondrial membrane. Together our results establish the PRAT family as a widely used system of protein translocases in different membranes of endosymbiotic origin.

Investigating the interaction between peptides of the amphipathic helix of Hcf106 and the phospholipid bilayer by solid-state NMR spectroscopy

The chloroplast twin arginine translocation (cpTat) system transports highly folded precursor proteins into the thylakoid lumen using the protonmotive force as its only energy source. Hcf106, as one of the core components of the cpTat system, is part of the precursor receptor complex and functions in the initial precursor-binding step. Hcf106 is predicted to contain a single amino terminal transmembrane domain followed by a Pro-Gly hinge, a predicted amphipathic α-helix (APH), and a loosely structured carboxy terminus. Hcf106 has been shown biochemically to insert spontaneously into thylakoid membranes. To better understand the membrane active capabilities of Hcf106, we used solid-state NMR spectroscopy to investigate those properties of the APH. In this study, synthesized peptides of the predicted Hcf106 APH (amino acids 28-65) were incorporated at increasing mol.% into 1-palmitoyl-2-oleoyl-sn-glycero-phosphocholine (POPC) and POPC/MGDG (monogalactosyldiacylglycerol; mole ratio 85:15) multilamellar vesicles (MLVs) to probe the peptide-lipid interaction. Solid-state (31)P NMR and (2)H NMR spectroscopic experiments revealed that the peptide perturbs the headgroup and the acyl chain regions of phospholipids as indicated by changes in spectral lineshape, chemical shift anisotropy (CSA) line width, and (2)H order SCD parameters. In addition, the comparison between POPC MLVs and POPC/MGDG MLVs indicated that the lipid bilayer composition affected peptide perturbation of the lipids, and such perturbation appeared to be more intense in a system more closely mimicking a thylakoid membrane.

Structural characterizations of the chloroplast translocon protein Tic110

Tic110 is a major component of the chloroplast protein import translocon. Two functions with mutually exclusive structures have been proposed for Tic110: a protein-conducting channel with six transmembrane domains and a scaffold with two N-terminal transmembrane domains followed by a large soluble domain for binding transit peptides and other stromal translocon components. To investigate the structure of Tic110, Tic110 from Cyanidioschyzon merolae (CmTic110) was characterized. We constructed three fragments, CmTic110A , CmTic110B and CmTic110C , with increasing N-terminal truncations, to perform small-angle X-ray scattering (SAXS) and X-ray crystallography analyses and Dali structural comparison. Here we report the molecular envelope of CmTic110B and CmTic110C determined by SAXS, and the crystal structure of CmTic110C at 4.2 Å. Our data indicate that the C-terminal half of CmTic110 possesses a rod-shaped helix-repeat structure that is too flattened and elongated to be a channel. The structure is most similar to the HEAT-repeat motif that functions as scaffolds for protein-protein interactions.

How protein targeting to primary plastids via the endomembrane system could have evolved? A new hypothesis based on phylogenetic studies

Background

It is commonly assumed that a heterotrophic ancestor of the supergroup Archaeplastida/Plantae engulfed a cyanobacterium that was transformed into a primary plastid; however, it is still unclear how nuclear-encoded proteins initially were imported into the new organelle. Most proteins targeted to primary plastids carry a transit peptide and are transported post-translationally using Toc and Tic translocons. There are, however, several proteins with N-terminal signal peptides that are directed to higher plant plastids in vesicles derived from the endomembrane system (ES). The existence of these proteins inspired a hypothesis that all nuclear-encoded, plastid-targeted proteins initially carried signal peptides and were targeted to the ancestral primary plastid via the host ES.

Results

We present the first phylogenetic analyses of Arabidopsis thaliana α-carbonic anhydrase (CAH1), Oryza sativa nucleotide pyrophosphatase/phosphodiesterase (NPP1), and two O. sativa α-amylases (αAmy3, αAmy7), proteins that are directed to higher plant primary plastids via the ES. We also investigated protein disulfide isomerase (RB60) from the green alga Chlamydomonas reinhardtii because of its peculiar dual post- and co-translational targeting to both the plastid and ES. Our analyses show that these proteins all are of eukaryotic rather than cyanobacterial origin, and that their non-plastid homologs are equipped with signal peptides responsible for co-translational import into the host ES. Our results indicate that vesicular trafficking of proteins to primary plastids evolved long after the cyanobacterial endosymbiosis (possibly only in higher plants) to permit their glycosylation and/or transport to more than one cellular compartment.

Conclusions

The proteins we analyzed are not relics of ES-mediated protein targeting to the ancestral primary plastid. Available data indicate that Toc- and Tic-based translocation dominated protein import into primary plastids from the beginning. Only a handful of host proteins, which already were targeted through the ES, later were adapted to reach the plastid via the vesicular trafficking. They represent a derived class of higher plant plastid-targeted proteins with an unusual evolutionary history.

Reviewers

This article was reviewed by Prof. William Martin, Dr. Philippe Deschamps (nominated by Dr. Purificacion Lopez-Garcia) and Dr Simonetta Gribaldo.

The algal past and parasite present of the apicoplast

Plasmodium and Toxoplasma are genera of apicomplexan parasites that infect millions of people each year. The former causes malaria, and the latter causes neurotropic infections associated with a weakened or developing immune system. These parasites harbor a peculiar organelle, the apicoplast. The apicoplast is the product of an ancient endosymbiosis between a heterotrophic and a photosynthetic protist. We explore the cellular and molecular mechanisms that enabled a stable union of two previously independent organisms. These include the exchange of metabolites, transfer of genes, transport of proteins, and overall coordination of biogenesis and proliferation. These mechanisms are still active today and can be exploited to treat parasite infection. They were shaped by the dramatic changes that occurred in the evolution of the phylum Apicomplexa--including the gain and loss of photosynthesis, adaptation to symbiosis and parasitism, and the explosion of animal diversity-that ultimately provided an aquatic alga access to every biotope on this planet.

Evolutionary, molecular and genetic analyses of Tic22 homologues in Arabidopsis thaliana chloroplasts

The Tic22 protein was previously identified in pea as a putative component of the chloroplast protein import apparatus. It is a peripheral protein of the inner envelope membrane, residing in the intermembrane space. In Arabidopsis, there are two Tic22 homologues, termed atTic22-III and atTic22-IV, both of which are predicted to localize in chloroplasts. These two proteins defined clades that are conserved in all land plants, which appear to have evolved at a similar rates since their separation >400 million years ago, suggesting functional conservation. The atTIC22-IV gene was expressed several-fold more highly than atTIC22-III, but the genes exhibited similar expression profiles and were expressed throughout development. Knockout mutants lacking atTic22-IV were visibly normal, whereas those lacking atTic22-III exhibited moderate chlorosis. Double mutants lacking both isoforms were more strongly chlorotic, particularly during early development, but were viable and fertile. Double-mutant chloroplasts were small and under-developed relative to those in wild type, and displayed inefficient import of precursor proteins. The data indicate that the two Tic22 isoforms act redundantly in chloroplast protein import, and that their function is non-essential but nonetheless required for normal chloroplast biogenesis, particularly during early plant development.

Protein import into the photosynthetic organelles of Paulinella chromatophora and its implications for primary plastid endosymbiosis

The rhizarian amoeba Paulinella chromatophora harbors two photosynthetically active organelles of cyanobacterial origin that have been acquired independently of classic primary plastids. Because their acquisition did take place relatively recently, they are expected to provide new insight into the ancient cyanobacterial primary endosymbiosis. During the process of Paulinella endosymbiont-to-organelle transformation, more than 30 genes have been transferred from the organelle to the host nuclear genome via endosymbiotic gene transfer (EGT). The article discusses step-by-step protein import of EGT-derived proteins into Paulinella photosynthetic organelles with the emphasis on the nature of their targeting signals and the final passage of proteins through the inner organelle membrane. The latter most probably involves a simplified Tic translocon composed of Tic21- and Tic32-like proteins as well as a Hsp70-based motor responsible for pulling of imported proteins into the organelle matrix. Our results indicate that although protein translocation across the inner membrane of Paulinella photosynthetic organelles seems to resemble the one in classic primary plastids, the transport through the outer membrane does not. The differences could result from distinct integration pathways of Paulinella photosynthetic organelles and primary plastids with their respective host cells.