Trends in Cell Biology
OpinionPolicing Tic ‘n’ Toc, the doorway to 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.
References (76)
- et al.
Light, redox state, thylakoid-protein phosphorylation and signaling gene expression
Trends Biochem. Sci.
(2003) - et al.
Chloroplast protein import: solve the GTPase riddle for entry
Trends Cell Biol.
(2004) - et al.
Without a little help from ‘my’ friends: direct insertion of proteins into chloroplast membranes?
Biochim. Biophys. Acta
(2001) - et al.
Evidence for an ER to Golgi to chloroplast protein transport pathway
Trends Cell Biol.
(2006) Classification and evolution of P-loop GTPases and related ATPases
J. Mol. Biol.
(2002)The M domain of atToc159 plays an essential role in the import of proteins into chloroplasts and chloroplast biogenesis
J. Biol. Chem.
(2003)In vitro comparative kinetic analysis of the chloroplast toc GTPases
J. Biol. Chem.
(2007)The roles of toc34 and toc75 in targeting the toc159 preprotein receptor to chloroplasts
J. Biol. Chem.
(2003)Dimerization of Toc-GTPases at the chloroplast protein import machinery
J. Biol. Chem.
(2003)Dimerization is important for the GTPase activity of chloroplast translocon components atToc33 and psToc159
J. Biol. Chem.
(2007)