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Review
, 41 (3), 161-167

Evolution and Design Principles of the Diverse Chloroplast Transit Peptides

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Review

Evolution and Design Principles of the Diverse Chloroplast Transit Peptides

Dong Wook Lee et al. Mol Cells.

Abstract

Chloroplasts are present in organisms belonging to the kingdom Plantae. These organelles are thought to have originated from photosynthetic cyanobacteria through endosymbiosis. During endosymbiosis, most cyanobacterial genes were transferred to the host nucleus. Therefore, most chloroplast proteins became encoded in the nuclear genome and must return to the chloroplast after translation. The N-terminal cleavable transit peptide (TP) is necessary and sufficient for the import of nucleus-encoded interior chloroplast proteins. Over the past decade, extensive research on the TP has revealed many important characteristic features of TPs. These studies have also shed light on the question of how the many diverse TPs could have evolved to target specific proteins to the chloroplast. In this review, we summarize the characteristic features of TPs. We also highlight recent advances in our understanding of TP evolution and provide future perspectives about this important research area.

Keywords: chloroplast evolution; endosymbiosis; protein import into chloroplasts; transit peptide.

Figures

Fig. 1
Fig. 1. The multiple steps of preprotein import into chloroplasts
The transit peptide (TP) is a signal sequence in chloroplast interior proteins. In the cytosol, the TP is recognized by cytosolic chaperones such as Hsp70, Hsp90, or factors yet to be identified, which leads to the targeting of preproteins to the chloroplast. At the outer envelope membrane of the chloroplast, the TP interacts with the GTPase receptors Toc159 and Toc34 of the Toc complex, which initiates the translocation of the preprotein through the import channel at the outer envelope membrane. The TP of the preprotein is released from the Toc complex and recognized by the POTRA domains of the Toc75 channel, which coordinate the transfer of the preprotein to the Tic complex. The TP is released from the Tic complex and interacts with stromal chaperone Hsp70 or Hsp93, which has ATPase activity, allowing them to pull the preprotein into the chloroplast. The TP is cleaved off by a stromal processing peptidase during or after the import process.
Fig. 2
Fig. 2. Design principle of the diverse transit peptides
During chloroplast evolution, the diverse transit peptides (TPs) might have arisen via selective assembly of a large pool of potential motifs responsible for the interaction with import factors that function at various steps in the import process in the cytosol and at the chloroplast. We propose that one or multiple molecular factors are involved in each step of protein import and that each of these factors is able to interact with multiple motifs.

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