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Abstract
Although the chloroplasts in plants are characterized by an inherent genome, the chloroplast proteome is composed of proteins encoded by not only the chloroplast genome but also the nuclear genome. Nuclear-encoded chloroplast proteins are synthesized on cytosolic ribosomes and post-translationally targeted to the chloroplasts. In the latter process, an N-terminal cleavable transit peptide serves as a targeting signal required for the import of nuclear-encoded chloroplast interior proteins. This import process is mediated via an interaction between the sequence motifs in transit peptides and the components of the TOC/TIC (translocon at the outer/inner envelope of chloroplasts) translocons. Despite a considerable diversity in primary structures, several common features have been identified among transit peptides, including N-terminal moderate hydrophobicity, multiple proline residues dispersed throughout the transit peptide, preferential usage of basic residues over acidic residues, and an absence of N-terminal arginine residues. In this review, we will recapitulate and discuss recent progress in our current understanding of the functional organization of sequence elements commonly present in diverse transit peptides, which are essential for the multi-step import of chloroplast proteins.
Highlights
The chloroplast, a type of plastid, is an organelle uniquely present in plants and is believed to have been derived via the endosymbiosis of ancient cyanobacteria (Dyall et al, 2004; Zimorski et al, 2014; Lee and Hwang, 2021)
Proteins localized to the outer membrane of chloroplasts are primarily inserted directly into the outer membrane from the cytosol, with a few exceptions (Lee et al, 2014, 2017; Day et al, Sequence Motifs in Transit Peptides 2019; Gross et al, 2021)
Extensive mutagenesis analysis of the Arabidopsis Rubisco small subunit (RbcS) transit peptide revealed the presence of distinct sequence motifs, each of which proved to be essential for the correct cytosolic navigation, chloroplast binding, or translocation of chloroplast preproteins across the envelope membranes (Lee et al, 2006; Figure 1)
Summary
The chloroplast, a type of plastid, is an organelle uniquely present in plants and is believed to have been derived via the endosymbiosis of ancient cyanobacteria (Dyall et al, 2004; Zimorski et al, 2014; Lee and Hwang, 2021). It has been established that different transit peptides contain distinct sequence motifs that play vital roles during the multi-step import process (Lee et al, 2006, 2008; Li and Teng, 2013; Lee and Hwang, 2018).
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