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Protein import into chloroplasts

Nature Reviews Molecular Cell Biology volume 5, pages 198–208 (2004)Cite this article

Key Points

  • Protein import into chloroplasts occurs post-translationally across two membranes. The molecular machines that facilitate this movement are called the TOC and TIC translocons, and they are located in the outer- and inner-chloroplast-envelope membranes, respectively.

  • The TOC core complex consists of three functionally distinct subunits — a GTP-dependent receptor (TOC34), a β-barrel-type translocation channel (TOC75) and a GTP-dependent molecular motor (TOC159).

  • Translocation across the TIC complex is driven by ATP hydrolysis, which is probably required for the action of molecular chaperones, and import across the TIC complex might be regulated by redox control.

  • Chloroplasts are endosymbiotic organelles with an ancestral cyanobacterium origin. The TOC75 import channel is clearly of bacterial origin and could have been the initial import site.

  • Many TOC and TIC translocon subunits are coded for by several genes, which seem to be differentially expressed. The model plant Arabidopsis thaliana can be used for genetic approaches that can help us elucidate the function of individual isoforms.

Abstract

Chloroplasts are organelles of endosymbiotic origin, and they transferred most of their genetic information to the host nucleus during this process. They therefore have to import more than 95% of their protein complement post-translationally from the cytosol. In vivo results from the model plant Arabidopsis thaliana — together with biochemical, biophysical and structural data from other plants — now allow us to outline the mechanistic details of the molecular machines that facilitate this translocation. It has become clear that chloroplasts evolved a unique translocation system, which is inherited, in part, from their bacterial ancestors.

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Figure 1: Protein-import pathways into chloroplasts.
Figure 2: The chloroplast translocon complexes.
Figure 3: Model of the Arabidopsis thaliana TOC33 and TOC34 receptor cycles.
Figure 4: Working hypothesis for the action of TOC159.

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Acknowledgements

We would like to thank colleagues who made manuscripts available to us before their publication. The work described from our laboratory was supported by grants from the Deutsche Forschungsgemeinschaft, Fonds der Chemischen Industrie and the Volkswagenstiftung.

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  1. Department für Biologie I, Botanik, Ludwig-Maximilians-Universität München, Menzingerstraße 67, München, D-80638, Germany

    Jürgen Soll & Enrico Schleiff

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Correspondence to Jürgen Soll.

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DATABASES

TAIR

HSP70

TOC34

TOC75

TOC132

TOC159

Glossary

PLASTID

A plant-specific family of organelles, the differentiation of which is dependent on the plant organ and on plant development.

APICOPLAST

A non-photosynthetic plastid that is surrounded by four membranes and that still synthesizes essential metabolites for the host.

HSP70 FAMILY

Heat shock proteins that have a molecular weight of 70 kDa and that function as molecular chaperones.

14-3-3 PROTEINS

A family of ubiquitous regulatory molecules that function through protein–protein interactions.

T-DNA

(transfer DNA). The part of the Agrobacterium tumefaciens tumour-inducing (Ti) plasmid that is incorporated into the genome of infected plant cells. These conjugative plasmids can be used as tools to insert foreign DNA into plant cells.

PHOTOAUTOTROPHIC

Organisms — for example, plants or cyanobacteria — that use light as an energy source to convert inorganic material into organic matter.

SEED ABORTION

The development of a seed — that is, an embryo — is stopped because of a fatal error in differentiation.

MERISTEMATIC

Actively dividing plant issue.

COTYLEDON STAGE

This stage corresponds to the appearance of the first 'leaves' that emerge during germination of the embryo.

TETRATRICOPEPTIDE MOTIF

A loosely conserved domain of 30–40 amino acids that is involved in protein–protein interactions.

IRON–SULPHUR CENTRE

Iron ions that are complexed by inorganic sulphur and by the amino-acid cysteine function as redox elements in electron-transfer reactions.

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Soll, J., Schleiff, E. Protein import into chloroplasts. Nat Rev Mol Cell Biol 5, 198–208 (2004). https://doi.org/10.1038/nrm1333

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