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Title: The Sec and Tat Protein Translocation Pathways in Chloroplasts

Abstract

Most chloroplast proteins are encoded by nuclear genes, translated in the cytosol as precursor proteins, and posttranslationally imported into the chloroplast. A subset of imported proteins is further localized to the thylakoid membrane and lumen by mechanisms conserved from the cyanobacterial endosymbiont that evolved into the chloroplast. The Sec and Twin arginine translocation (Tat) pathways are the major systems for transporting proteins across the thylakoid membrane into the lumen. Both systems employ hydrophobic cleavable signal peptides for targeting, but Tat signal peptides also contain an essential twin arginine motif. Biochemical studies indicate that the thylakoid Sec system operates similarly to the Escherichia coli Sec system, that is a chloroplast SecA powers transport of unfolded protein substrates through a fixed cpSecYE channel. Indirect evidence also suggests that the thylakoid Sec system can integrate plastid-encoded multispanning membrane proteins cotranslationally. The Tat pathway is a newly discovered translocation system that can transport folded protein domains using the Δμ H+ as the sole energy source. Three membrane proteins, High chlorophyll fluorescence 106 (Hcf106), Thylakoid assembly 4 (Tha4), and cpTatC constitute the components of the Tat machinery. Precursor proteins bind to a large cpTatC-Hcf106 complex by contact of the signal peptide twin arginine region tomore » cpTatC and its hydrophobic core to Hcf106. This triggers recruitment of a Tha4 oligomer, setting the stage for transport. During the translocation step, the Tha4 oligomer undergoes a conformation shift that aligns its amphipathic helices and carboxyl tails, possibly in association with the bilayer interface. Furthermore, these results have been interpreted in a general model in which the Tha4 oligomer facilitates passage of the substrate across the lipid bilayer.« less

Authors:
 [1];  [2]
  1. Univ. of Florida, Gainesville, FL (United States)
  2. Univ. of California, Davis, CA (United States)
Publication Date:
Research Org.:
Univ. of California, Davis, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22). Chemical Sciences, Geosciences & Biosciences Division
OSTI Identifier:
1490442
Grant/Contract Number:  
FG02-03ER15405
Resource Type:
Accepted Manuscript
Journal Name:
Enzymes
Additional Journal Information:
Journal Volume: 25
Country of Publication:
United States
Language:
English
Subject:
59 BASIC BIOLOGICAL SCIENCES

Citation Formats

Cline, Kenneth C., and Theg, Steven M. The Sec and Tat Protein Translocation Pathways in Chloroplasts. United States: N. p., 2008. Web. doi:10.1016/S1874-6047(07)25018-8.
Cline, Kenneth C., & Theg, Steven M. The Sec and Tat Protein Translocation Pathways in Chloroplasts. United States. doi:10.1016/S1874-6047(07)25018-8.
Cline, Kenneth C., and Theg, Steven M. Fri . "The Sec and Tat Protein Translocation Pathways in Chloroplasts". United States. doi:10.1016/S1874-6047(07)25018-8. https://www.osti.gov/servlets/purl/1490442.
@article{osti_1490442,
title = {The Sec and Tat Protein Translocation Pathways in Chloroplasts},
author = {Cline, Kenneth C. and Theg, Steven M.},
abstractNote = {Most chloroplast proteins are encoded by nuclear genes, translated in the cytosol as precursor proteins, and posttranslationally imported into the chloroplast. A subset of imported proteins is further localized to the thylakoid membrane and lumen by mechanisms conserved from the cyanobacterial endosymbiont that evolved into the chloroplast. The Sec and Twin arginine translocation (Tat) pathways are the major systems for transporting proteins across the thylakoid membrane into the lumen. Both systems employ hydrophobic cleavable signal peptides for targeting, but Tat signal peptides also contain an essential twin arginine motif. Biochemical studies indicate that the thylakoid Sec system operates similarly to the Escherichia coli Sec system, that is a chloroplast SecA powers transport of unfolded protein substrates through a fixed cpSecYE channel. Indirect evidence also suggests that the thylakoid Sec system can integrate plastid-encoded multispanning membrane proteins cotranslationally. The Tat pathway is a newly discovered translocation system that can transport folded protein domains using the ΔμH+ as the sole energy source. Three membrane proteins, High chlorophyll fluorescence 106 (Hcf106), Thylakoid assembly 4 (Tha4), and cpTatC constitute the components of the Tat machinery. Precursor proteins bind to a large cpTatC-Hcf106 complex by contact of the signal peptide twin arginine region to cpTatC and its hydrophobic core to Hcf106. This triggers recruitment of a Tha4 oligomer, setting the stage for transport. During the translocation step, the Tha4 oligomer undergoes a conformation shift that aligns its amphipathic helices and carboxyl tails, possibly in association with the bilayer interface. Furthermore, these results have been interpreted in a general model in which the Tha4 oligomer facilitates passage of the substrate across the lipid bilayer.},
doi = {10.1016/S1874-6047(07)25018-8},
journal = {Enzymes},
number = ,
volume = 25,
place = {United States},
year = {2008},
month = {5}
}

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