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Plant Biol (Stuttg) 2005; 7(1): 1-14
DOI: 10.1055/s-2004-830447
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Georg Thieme Verlag Stuttgart KG · New York

Let's Talk about … Chloroplast Import

T. Becker1 , S. Qbadou1 , M. Jelic1 , E. Schleiff1
  • 1Botanisches Institut, Ludwig-Maximilian-Universität München, Menzinger Straße 67, 80368 München, Germany
Further Information

Publication History

Received: March 18, 2004

Accepted: September 13, 2004

Publication Date:
21 January 2005 (online)

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An Easy Beginning - The Introduction

The double membrane surrounding chloroplasts and other plastids originated from the engulfment of an ancestral cyanobacterium by a eukaryotic host cell, in a process called endosymbiosis ([Weeden, 1981]). During evolution, the genes of the endosymbiont were transferred to the host nuclear genome ([Martin et al., 1998]; [Blanchard and Lynch, 2000]; [Timmis et al., 2004]). Thus, the semi-autonomous genome of modern plastids contains only information for about 150 genes, implying a drastic reduction in genetic information compared to the genome of a modern cyanobacterium, which encodes several thousands of genes ([Leister, 2003]). This raises questions regarding the ability of chloroplasts to maintain various biochemical functions, such as photosynthesis, amino acid, and lipid synthesis. The evolutionary solution is the post-translational import of nuclear encoded proteins into the chloroplasts to fulfill their catalytic functions. Therefore, a complex system of cytosolic targeting, translocation across the two envelope membranes, and a subsequent intraorganellar sorting had to be established. For that, a multi-component complex in the envelope membranes is facilitated (Fig. [1]). This review will present a detailed description of the cytosolic targeting, as well as the translocation of pre-proteins across both chloroplast membranes and will highlight the latest discoveries. Another focus will be to indicate unsolved questions and to propose mechanistic models for cytosolic targeting and translocation.

Fig. 1 A model of the currently known composition of the translocation machinery. The Toc complex consists of two receptor proteins, Toc34 and Toc159, the protein conducting channel Toc75-III, and the dynamically associated Toc64. Two Hsp70 homologues, either projecting to the cytosol or exposed to the intermembrane space, assists translocation across the outer envelope membrane. Toc75-V might form an additional complex with so far unidentified components. Furthermore, Oep16 in association with Toc34 were suggested to be components for an alternative import pathway. The Tic complex, comprising Tic110, Tic62, Tic55, Tic40, Tic22, and Tic20, catalyses transfer of pre-proteins across the inner envelope. The import is driven with the aid of stromal chaperones like Hsp100, Cpn60, and the stromal Hsp70. Finally, the transit peptide (tp) is removed by a stromal processing peptidase (SPP).

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E. Schleiff

Botanisches Institut
Ludwig-Maximilian-Universität

Menzinger Straße 67

80368 München

Germany

Email: schleiff@lrz.uni-muenchen.de

Editor: G. Thiel

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