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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    • "In addition to the targeting signals and protein receptors, it has been proposed that cytosolic chaperone proteins may also play a role in maintaining precursor proteins in an importcompetent state and may play a role in determining targeting specificity. However, in plants, the role of cytosolic chaperones has only been characterized to some extent for protein import into chloroplasts (Jarvis, 2008; Fellerer et al., 2011; Flores-Pérez and Jarvis, 2013; Lee et al., 2013; Schweiger et al., 2013) but not into mitochondria. "
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    ABSTRACT: The nuclear-encoded mitochondrial-targeted proteins, multiple organellar RNA editing factors (MORF3, MORF5, MORF6) interact with AtPAP2 (Purple acid phosphatase 2) located on the chloroplast and mitochondrial outer membranes in a presequence dependent manner. Phosphorylation of the presequence of the precursor MORF3 (pMORF3) by endogenous kinases in wheat germ translation lysate, leaf extracts, or STY kinases, but not in rabbit reticulocyte translation lysate, resulted in the inhibition of protein import into mitochondria. This inhibition of import could be overcome by altering threonine/serine residues to alanine on the presequence, thus preventing phosphorylation. Phosphorylated pMORF3, but not the phosphorylation deficient pMORF3, can form a complex with 14-3-3 proteins and HSP70. The phosphorylation deficient mutant of pMORF3 also displayed faster rates of import when translated in wheat germ lysates. Mitochondria isolated from plants with altered amounts of AtPAP2 displayed altered protein import kinetics. The import rate of pMORF3 synthesized in wheat germ translation lysate into pap2 mitochondria was slower than that into wild-type mitochondria, and this rate disparity was not seen for pMORF3 synthesized in rabbit reticulocyte translation lysate, the latter translation lysate largely deficient in kinase activity. Taken together, these results support a role for the phosphorylation and dephosphorylation of pMORF3 during the import into plant mitochondria. These results suggest that kinases, possibly STY kinases, and AtPAP2 are involved in the import of protein into both mitochondria and chloroplasts, and provides a mechanism by which the import of proteins into both organelles may be coordinated. Copyright © 2015, Plant Physiology.
    Plant physiology 08/2015; 169(2). DOI:10.1104/pp.15.01115 · 6.84 Impact Factor
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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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