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Title: The Sec and Tat Protein Translocation Pathways in Chloroplasts

Journal Article · · The Enzymes
 [1];  [2]
  1. Univ. of Florida, Gainesville, FL (United States)
  2. Univ. of California, Davis, CA (United States)
+ Show Author Affiliations

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.

Research Organization:
Univ. of California, Davis, CA (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences, and Biosciences Division
Grant/Contract Number:
FG02-03ER15405
OSTI ID:
1490442
Journal Information:
The Enzymes, Vol. 25; ISSN 1874-6047
Publisher:
ElsevierCopyright Statement
Country of Publication:
United States
Language:
English
Citation Metrics:
Cited by: 32 works
Citation information provided by
Web of Science

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Cited By (7)

Identification and Roles of Photosystem II Assembly, Stability, and Repair Factors in Arabidopsis journal February 2016
Co-operation between different targeting pathways during integration of a membrane protein journal October 2012
FtsH2 and FtsH5: two homologous subunits use different integration mechanisms leading to the same thylakoid multimeric complex: FtsH integration pathways journal January 2011
The twin-arginine translocation (Tat) protein export pathway journal June 2012
Unprecedented Parallel Photosynthetic Losses in a Heterotrophic Orchid Genus journal May 2019
Twin-arginine-dependent translocation of folded proteins journal April 2012
Twin-arginine-dependent translocation of folded proteins journal August 2012

Figures / Tables (4)

Figure 1(p. 25)figure Figure 1
Figure 2(p. 26)figure Figure 2
Figure 3(p. 27)figure Figure 3
Figure 4(p. 28)figure Figure 4

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