Trends in Cell Biology
Volume 18, Issue 1, January 2008, Pages 19-27
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Policing Tic ‘n’ Toc, the doorway to chloroplasts

https://doi.org/10.1016/j.tcb.2007.10.002Get rights and content

The organization of eukaryotic cells into different membrane-enclosed compartments requires an ordered and regulated system for targeting and translocating proteins synthesized in the cytosol across organellar membranes. Protein translocation through integral membrane proteinaceous complexes shares common principles in different organelles, whereas molecular mechanisms and energy requirements are diverse. Translocation into mitochondria and plastids requires most proteins to cross two membranes, and translocation must be regulated to accommodate environmental or metabolic changes. In the last decade, the first ideas were formulated about the regulation of protein translocation into chloroplasts, thereby laying the foundation for this field. Here, we describe recent models for the regulation of translocation by precursor protein phosphorylation, receptor dimerization, redox sensing and calcium signaling. We suggest how these mechanisms might fit within the regulatory framework for the entry of proteins into chloroplasts.

Section snippets

The gospel of protein import

Subdivision of cells into different compartments paralleled the development of eukaryotes [1], but at the same time necessitated exchange of solutes and proteins between the cellular compartments. Thus, the development of specific mechanisms was required for protein transport in the cytosol and across membranes. Despite the structural and functional diversity of the proteinaceous translocation complexes, some basic principles apply to the recognition of proteins by the organellar membranes and

Targeting of precursor proteins to plastids

Most proteins required for plastid function are encoded in the nucleus and translated on cytosolic ribosomes as precursors with an N-terminal transit peptide 5, 6. The transit peptide bears the information for targeting to the organelle and is cleaved after the precursor is imported into the organelle. Some proteins are not synthesized with an N-terminal transit peptide and are either directly targeted to the chloroplast outer membrane (for example, see Ref. [7]) or transported by alternative

The Toc translocon at the outer envelope membrane

Chloroplast membranes contain two complexes termed Toc and Tic, which reside on the outer and inner chloroplast membrane, respectively (for example, see Refs 5, 6). The Toc complex consists of five proteins: Toc159, Toc75, Toc64, Toc34 and Toc12, named according to their approximate molecular weights (Figure 1, Box 1). Toc34 and Toc159 are associated in the Toc core complex with the β-barrel-shaped channel Toc75 16, 17, 18. Both, Toc34 and Toc159 belong to the class of

Triggering Toc – dimerization as a GAP ersatz?

Recent reports point to a connection between homo- and/or hetero-dimerization of the G domains of Toc34 and Toc159 and their function 23, 24, 27, 29, 30, 31, 32, 33. It is thought that GTPase interaction is required for the de novo assembly of the Toc translocon [27]. Furthermore, concentration-dependent homodimerization of atToc33 [24] – the Toc34 isoform found in A. thaliana with similar properties to psToc34 – and of psToc159 [33] has a moderate stimulatory effect on GTPase activity.

Reaching the Tic translocon

After targeting to the Toc translocon, the precursor protein is transferred through the translocation pore Toc75 and then emerges into the intermembrane space (IMS). To proceed into the plastid stroma, the precursor protein must next be localized to the Tic complex. Contact sites between the envelopes [40] were initially discussed to allow for the interaction of Toc and Tic complexes, enabling a direct precursor transfer (for example, see Ref. [41]). Recent findings compromise this idea,

The Tic translocon

The structure and function of the Tic complex are not well understood, and even the identity of the translocation pore is controversial as at least two candidates have been proposed for this function – the inner membrane proteins Tic20 and Tic110 42, 46. The newly discovered protein named Tic21 was discussed as another candidate [47]. However, this protein was also simultaneously described as an iron-transporting permease, designated as PIC1 [48]. Hence, the link of Tic21 to protein

Tic32 – a global player for regulation of the Tic

The interaction of Tic32 with Tic110 could be a mechanism for regulating the activity of the Tic complex [54]. The gene encoding Tic32 in A. thaliana is equally expressed in green and non-green tissues [55], and this implies that Tic32 has a function in all plastid types. Its function seems to be modulated by calmodulin and NADPH binding [56]. The interaction of Tic32 with calmodulin is compromised by the depletion of calcium or by binding of NADPH to the receptor [56]. In line with a

Tic55 and Tic62 – coupling protein translocation and photosynthesis

Tic55 and Tic62 were found to be associated with Tic110 52, 53, 54, 56. The association of Tic55 is either weak or involves only a subpopulation of Tic110 because Tic110 was detected by immunoprecipitation with Tic55 antibodies [53] but not vice versa [42]. An indication that Tic55 and Tic62 might modulate translocation efficiency of the Tic complex came from the following observations: First, precursor import was inhibited by addition of diethylpyrocarbonate, which modifies (ethoxyformylates)

Concluding remarks

Protein translocation across chloroplast membranes is not yet understood at the molecular level. Our knowledge of important interactions and their sequence is incomplete. In addition, elucidation of the energetic requirements for discernable translocation steps is required for a full understanding of recognition and translocation. So far, we know that the GTPases regulate complex assembly, precursor protein recognition and transfer (for example, see Refs 18, 19, 20, 21, 26), possibly involving

Acknowledgement

Special thanks to Patrick Koenig and Anja Höfle for critical discussions. We also thank B.B. Buchanan for critical editing of the manuscript. Financial support from the Deutsche Forschungsgemeinschaft (DFG, SFB594) and the Volkswagenstiftung to E.S. is acknowledged.

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