Kessler, F. & Schnell, D. J. Chloroplast biogenesis: diversity and regulation of the protein import apparatus. Curr. Opin. Cell Biol. 21, 494-500

Laboratoire de Physiologie Végétale, Université de Neuchâtel, Neuchâtel, Switzerland.
Current opinion in cell biology (Impact Factor: 8.47). 05/2009; 21(4):494-500. DOI: 10.1016/
Source: PubMed


The biogenesis of chloroplasts is dependent on the coordinate expression of genes encoded in both nuclear and plastid genomes. The chloroplast protein import machinery plays key roles in organelle biogenesis by mediating the import and assembly of thousands of nuclear-encoded proteins into the organelle. It is now apparent that multiple levels of control exist to integrate the activities of the protein import apparatus into the network of chloroplast-nuclear communication that is essential to maintain organelle homeostasis. The import apparatus has diversified into small, functionally specialized gene families to coordinate the import of distinct classes of differentially expressed proteins. Protein targeting to chloroplasts also has evolved regulatory mechanisms that respond to cellular developmental and physiological changes, including redox sensing, phosphorylation, and dual targeting. Recent studies also have revealed new components that could represent additional levels of control on the import process.

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    • "They include mitochondrial TOM and TIM [10] [11] and peroxisomal PEX complexes [12] [13]. Chloroplasts require such translocons, namely TOC and TIC, in their double envelope membranes to import thousands of nucleus-encoded proteins synthesized in the cytosol [14] [15] [16] [17] [18]. These translocons differ entirely in their protein composition from each other, and therefore, elucidation of their detailed molecular architectures and underlying mechanisms is of fundamental importance in cell biology. "
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    ABSTRACT: Chloroplasts must import thousands of nuclear-encoded preproteins synthesized in the cytosol through two successive protein translocons at the outer and inner envelope membranes, termed TOC and TIC, respectively, to fulfill their complex physiological roles. The molecular identity of the TIC translocon had long remained controversial; two proteins, namely Tic20 and Tic110, had been proposed to be central to protein translocation across the inner envelope membrane. Tic40 also had long been considered to be another central player in this process. However, recently, a novel 1-megadalton complex consisting of Tic20, Tic56, Tic100, and Tic214 was identified at the chloroplast inner membrane of Arabidopsis and was demonstrated to constitute a general TIC translocon which functions in concert with the well-characterized TOC translocon. On the other hand, direct interaction between this novel TIC transport system and Tic110 or Tic40 was hardly observed. Consequently, the molecular model for protein translocation across the inner envelope membrane of chloroplasts might need to be extensively revised. In this review article, I intend to propose such alternative view regarding the TIC transport system in contradistinction to the classical view. I also would emphasize importance of reevaluation of previous works in terms of with what methods these classical Tic proteins such as Tic110 or Tic40 were picked up as TIC constituents at the very beginning as well as what actual evidence there were to support their direct and specific involvement in chloroplast protein import. Copyright © 2015. Published by Elsevier B.V.
    Biochimica et Biophysica Acta (BBA) - Bioenergetics 02/2015; 1847(9). DOI:10.1016/j.bbabio.2015.02.011 · 5.35 Impact Factor
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    • "In Arabidopsis, atToc159 and atToc132/120 preferentially assemble with atToc33 and atToc34, the two Toc34 isoforms, respectively [111]. This has led to the hypothesis that the Toc159 and Toc34 family members assemble in a combinatorial manner to generate translocons with distinct binding specificities [20]. Mutants deficient in atToc33 (ppi1) or atToc34 (ppi3) exhibit different phenotypes and some differential effects on the import of specific preproteins [48] [112] [114] [115]. "
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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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    • "Each subcompartment plays roles in synthesizing, storing, catabolizing, and transporting metabolites. Hence, the biogenesis and function of each compartment rely on the import and proper suborganelle targeting of nuclear-encoded proteins (Kessler and Schnell, 2009). "
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    ABSTRACT: The inner envelope membrane (IEM) of the chloroplast plays crucial roles in forming an osmotic barrier and controlling metabolite exchange between the organelle and the cytosol. The IEM therefore harbours a number of membrane proteins and requires the import and integration of these nuclear-encoded proteins for its biogenesis. Recent studies have demonstrated that the transmembrane segment of single-spanning IEM proteins plays key roles in determining their IEM localization. However, few studies have focused on the molecular mechanisms by which polytopic membrane proteins are targeted to the IEM. In this study, we investigated the targeting mechanism of polytopic IEM proteins using the protein Cor413im1 as a model substrate. Cor413im1 does not utilize a soluble intermediate for its targeting to the IEM. Furthermore, we show that the putative fifth transmembrane segment of Cor413im1 is necessary for its targeting to the IEM. The C-terminal portion containing this transmembrane segment is also able to deliver Cor413im1 protein to the IEM. However, the fifth transmembrane segment of Cor413im1 itself is insufficient to target a fusion protein to the IEM. These data suggest that the targeting of polytopic membrane proteins to the chloroplast IEM in vivo involves multiple transmembrane segments and that chloroplasts have evolved a unique mechanism for the integration of polytopic proteins to the IEM.
    Journal of Experimental Botany 07/2014; 65(18). DOI:10.1093/jxb/eru290 · 5.53 Impact Factor
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