Molecular chaperone involvement in chloroplast protein import

Department of Biology, University of Leicester, Leicester LE1 7RH, UK
Biochimica et Biophysica Acta (Impact Factor: 4.66). 04/2012; 1833(2). DOI: 10.1016/j.bbamcr.2012.03.019
Source: PubMed


Chloroplasts are organelles of endosymbiotic origin that perform essential functions in plants. They contain about 3000 different proteins, the vast majority of which are nucleus-encoded, synthesized in precursor form in the cytosol, and transported into the chloroplasts post-translationally. These preproteins are generally imported via envelope complexes termed TOC and TIC (Translocon at the Outer/Inner envelope membrane of Chloroplasts). They must navigate different cellular and organellar compartments (e.g., the cytosol, the outer and inner envelope membranes, the intermembrane space, and the stroma) before arriving at their final destination. It is generally considered that preproteins are imported in a largely unfolded state, and the whole process is energy-dependent. Several chaperones and cochaperones have been found to mediate different stages of chloroplast import, in similar fashion to chaperone involvement in mitochondrial import. Cytosolic factors such as Hsp90, Hsp70 and 14-3-3 may assist preproteins to reach the TOC complex at the chloroplast surface, preventing their aggregation or degradation. Chaperone involvement in the intermembrane space has also been proposed, but remains uncertain. Preprotein translocation is completed at the trans side of the inner membrane by ATP-driven motor complexes. A stromal Hsp100-type chaperone, Hsp93, cooperates with Tic110 and Tic40 in one such motor complex, while stromal Hsp70 is proposed to act in a second, parallel complex. Upon arrival in the stroma, chaperones (e.g., Hsp70, Cpn60, cpSRP43) also contribute to the folding, assembly or onward intraorganellar guidance of the proteins. In this review, we focus on chaperone involvement during preprotein translocation at the chloroplast envelope. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.

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    • "The energetics of protein import into plastids is reminiscent of the energetics of the Hsp70-driven motors that function in post-translational translocation into the ER [159] or mitochondria [160]. However, a significant difference between plastids and other organelles is the apparent complexity of the importassociated chaperone system [24] [161]. To date, four molecular chaperones have been shown to be associated with Tic110 and the TIC machinery: cpHsp70, Hsp90C, Hsp93/ClpC and Cpn60 (Fig. 5 "
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    ABSTRACT: The translocons at the outer (TOC) and inner (TIC) envelope membranes of chloroplasts mediate the targeting and import of several thousand nuclear encoded preproteins that are required for organelle biogenesis and homeostasis. The cytosolic events in preprotein targeting remain largely unknown, although cytoplasmic chaperones have been proposed to facilitate delivery to the TOC complex. Preprotein recognition is mediated by the TOC GTPase receptors, Toc159 and Toc34. The receptors constitute a GTP-regulated switch, which initiates membrane translocation via Toc75, a member of the OMP85 (Outer Membrane Protein 85)/TpsB (two partner secretion system B) family of bacterial, plastid and mitochondrial β-barrel outer membrane proteins. The TOC receptor systems have diversified to recognize distinct sets of preproteins, thereby maximizing the efficiency of targeting in response to changes in gene expression during developmental and physiological events that impact organelle function. The TOC complex interacts with the TIC translocon to allow simultaneous translocation of preproteins across the envelope. Two inner membrane complexes, the Tic110 and 1 MDa complexes, have both been implicated as constituents of the TIC translocon, and it remains to be determined how they interact to form the TIC channel and assemble the import-associated chaperone network in the stroma that drives import across the envelope membranes. This review will focus on recent developments in our understanding of the mechanisms and diversity of the TOC-TIC systems. Our goal is to incorporate these recent studies with previous work and present updated or revised models for the function of TOC-TIC in protein import.
    Journal of Molecular Biology 08/2014; 427(5). DOI:10.1016/j.jmb.2014.08.016 · 4.33 Impact Factor
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    • "Additionally, AKR2A displays chaperone activity and prevents non-specific aggregation of its client proteins by binding to the hydrophobic TMD. Chaperone activity should be an integral part of cytosolic targeting factors for the post-translational targeting of membrane proteins because these factors can use this activity to keep their clients in an insertion-competent form in the cytosol by preventing non-specific aggregate formation, proteolytic degradation, or unproductive interactions with other proteins before organellar membrane proteins are delivered to the target membranes (Flores-Pérez and Jarvis, 2013; Kim and Hwang, 2013). In addition, AKR2 binds to chloroplasts through its C-terminal ankyrin-repeat domain (ARD) and facilitates insertion of its client proteins into the chloroplast outer membrane, where Toc75 assists with their insertion (Tu et al., 2004). "
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    ABSTRACT: Chloroplasts and mitochondria are endosymbiotic organelles thought to be derived from endosymbiotic bacteria. In present-day eukaryotic cells, these two organelles play pivotal roles in photosynthesis and ATP production. In addition to these major activities, numerous reactions, and cellular processes that are crucial for normal cellular functions occur in chloroplasts and mitochondria. To function properly, these organelles constantly communicate with the surrounding cellular compartments. This communication includes the import of proteins, the exchange of metabolites and ions, and interactions with other organelles, all of which heavily depend on membrane proteins localized to the outer envelope membranes. Therefore, correct and efficient targeting of these membrane proteins, which are encoded by the nuclear genome and translated in the cytosol, is critically important for organellar function. In this review, we summarize the current knowledge of the mechanisms of protein targeting to the outer membranes of mitochondria and chloroplasts in two different directions, as well as targeting signals and cytosolic factors.
    Frontiers in Plant Science 04/2014; 5:173. DOI:10.3389/fpls.2014.00173 · 3.95 Impact Factor
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    • "At present, Tic110, Tic20 and Tic21 are discussed as channel forming proteins for preprotein translocation [9], while Tic62, Tic55 and Tic32 are assumed to regulate the translocon activity [77]. The energy for the translocation is fuelled by stromal chaperones Hsp70 and Hsp93 and the co-chaperone Tic40 [78]. The recently described Sec system is composed of SecY2 and SecA2 [11] and might be involved in integration or translocation of either stromal targeted or plastome encoded inner envelope proteins. "
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    ABSTRACT: Background Protein translocation across membranes is a central process in all cells. In the past decades the molecular composition of the translocation systems in the membranes of the endoplasmic reticulum, peroxisomes, mitochondria and chloroplasts have been established based on the analysis of model organisms. Today, these results have to be transferred to other plant species. We bioinformatically determined the inventory of putative translocation factors in tomato (Solanum lycopersicum) by orthologue search and domain architecture analyses. In addition, we investigated the diversity of such systems by comparing our findings to the model organisms Saccharomyces cerevisiae, Arabidopsis thaliana and 12 other plant species. Results The literature search end up in a total of 130 translocation components in yeast and A. thaliana, which are either experimentally confirmed or homologous to experimentally confirmed factors. From our bioinformatic analysis (PGAP and OrthoMCL), we identified (co-)orthologues in plants, which in combination yielded 148 and 143 orthologues in A. thaliana and S. lycopersicum, respectively. Interestingly, we traced 82% overlap in findings from both approaches though we did not find any orthologues for 27% of the factors by either procedure. In turn, 29% of the factors displayed the presence of more than one (co-)orthologue in tomato. Moreover, our analysis revealed that the genomic composition of the translocation machineries in the bryophyte Physcomitrella patens resemble more to higher plants than to single celled green algae. The monocots (Z. mays and O. sativa) follow more or less a similar conservation pattern for encoding the translocon components. In contrast, a diverse pattern was observed in different eudicots. Conclusions The orthologue search shows in most cases a clear conservation of components of the translocation pathways/machineries. Only the Get-dependent integration of tail-anchored proteins seems to be distinct. Further, the complexity of the translocation pathway in terms of existing orthologues seems to vary among plant species. This might be the consequence of palaeoploidisation during evolution in plants; lineage specific whole genome duplications in Arabidopsis thaliana and triplications in Solanum lycopersicum.
    BMC Genomics 03/2013; 14(1):189. DOI:10.1186/1471-2164-14-189 · 3.99 Impact Factor
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