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Targeting of nucleus‐encoded proteins to chloroplasts in plants
Author(s) -
Jarvis Paul
Publication year - 2008
Publication title -
new phytologist
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.742
H-Index - 244
eISSN - 1469-8137
pISSN - 0028-646X
DOI - 10.1111/j.1469-8137.2008.02452.x
Subject(s) - transit peptide , protein targeting , endoplasmic reticulum , biology , chloroplast , microbiology and biotechnology , arabidopsis , golgi apparatus , biochemistry , transport protein , proteome , membrane protein , plastid , gene , mutant , membrane
Summary Most chloroplast proteins are encoded in the nucleus and synthesized on free, cytosolic ribosomes in precursor form. Each precursor has an amino‐terminal extension called a transit peptide, which directs the protein through a post‐translational targeting pathway and is removed upon arrival inside the organelle. This ‘protein import’ process is mediated by the coordinate action of two multiprotein complexes, one in each of the envelope membranes: the TOC and TIC ( T ranslocon at the O uter/ I nner envelope membrane of C hloroplasts) machines. Many components of these complexes have been identified biochemically in pea; these include transit peptide receptors, channel proteins, and molecular chaperones. Intriguingly, the Arabidopsis genome encodes multiple, homologous genes for receptor components of the TOC complex. Careful analysis indicated that the different receptor isoforms operate in different import pathways with distinct precursor recognition specificities. These ‘substrate‐specific’ import pathways might play a role in the differentiation of different plastid types, and/or act to prevent deleterious competition effects between abundant and nonabundant precursors. Until recently, all proteins destined for internal chloroplast compartments were thought to possess a cleavable transit peptide, and to engage the TOC/TIC machinery. New studies using proteomics and other approaches have revealed that this is far from true. Remarkably, a significant number of chloroplast proteins are transported via a pathway that involves the endoplasmic reticulum and Golgi apparatus. Other recent reports have elucidated an intriguing array of protein targeting routes leading to the envelope membranes themselves.ContentsSummary 257 I. Introduction 258 II. Targeting signals 260 III. Envelope translocation by the TOC/TIC pathway 262 IV. Early stages of the TOC/TIC pathway 263 V. Later stages of the TOC/TIC pathway 272 VI. Targeting to the envelope system 274 VII. Alternative or ‘noncanonical’ import pathways 276 VIII. Conclusion and future perspectives 278Acknowledgements 278References 278