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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    • "However, Hsp70 chaperones also facilitate[36,45,46], co-immunoprecipitation experiments showed that both chaperones can be found together in the chloroplast envelope[54,55]. It is therefore possible that Hsp70 and ClpC might interact either directly (using unidentified chaperone binding motifs) or indirectly (via third partners) to participate in PQC events at the stromal side of the inner envelope membrane[1,42,56,57]. Because in Arabidopsis the two plastidial isoforms of Hsp70 (Hsp70.1 and Hsp70.2) and ClpC (ClpC1 and ClpC2) are also found in the stroma[42,58], we reasoned that Hsp70 and ClpC proteins might collaborate to deliver DXS to the Clp protease using J20 as an adaptor. "
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    ABSTRACT: The lifespan and activity of proteins depend on protein quality control systems formed by chaperones and proteases that ensure correct protein folding and prevent the formation of toxic aggregates. We previously found that the Arabidopsis thaliana J-protein J20 delivers inactive (misfolded) forms of the plastidial enzyme deoxyxylulose 5-phosphate synthase (DXS) to the Hsp70 chaperone for either proper folding or degradation. Here we show that the fate of Hsp70-bound DXS depends on pathways involving specific Hsp100 chaperones. Analysis of individual mutants for the four Hsp100 chaperones present in Arabidopsis chloroplasts showed increased levels of DXS proteins (but not transcripts) only in those defective in ClpC1 or ClpB3. However, the accumulated enzyme was active in the clpc1 mutant but inactive in clpb3 plants. Genetic evidence indicated that ClpC chaperones might be required for the unfolding of J20-delivered DXS protein coupled to degradation by the Clp protease. By contrast, biochemical and genetic approaches confirmed that Hsp70 and ClpB3 chaperones interact to collaborate in the refolding and activation of DXS. We conclude that specific J-proteins and Hsp100 chaperones act together with Hsp70 to recognize and deliver DXS to either reactivation (via ClpB3) or removal (via ClpC1) depending on the physiological status of the plastid.
    Full-text · Article · Jan 2016 · PLoS Genetics
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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.
    Full-text · Article · Aug 2015 · Plant physiology
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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.
    Full-text · Article · Aug 2014 · Journal of Molecular Biology
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