Interference with plastome gene expression and Clp protease activity in Arabidopsis triggers a chloroplast unfolded protein response to restore protein homeostasis
Ernesto Llamas, Pablo Pulido, Manuel Rodriguez-Concepcion
Published: September 22, 2017 https://doi.org/10.1371/journal.pgen.1007022
Abstract
Disruption of protein homeostasis in chloroplasts impairs the correct functioning of essential metabolic pathways, including the methylerythritol 4-phosphate (MEP) pathway for the production of plastidial isoprenoids involved in photosynthesis and growth. We previously found that misfolded and aggregated forms of the first enzyme of the MEP pathway are degraded by the Clp protease with the involvement of Hsp70 and Hsp100/ClpC1 chaperones in Arabidopsis thaliana. By contrast, the combined unfolding and disaggregating actions of Hsp70 and Hsp100/ClpB3 chaperones allow solubilization and hence reactivation of the enzyme. The repair pathway is promoted when the levels of ClpB3 proteins increase upon reduction of Clp protease activity in mutants or wild-type plants treated with the chloroplast protein synthesis inhibitor lincomycin (LIN). Here we show that LIN treatment rapidly increases the levels of aggregated proteins in the chloroplast, unleashing a specific retrograde signaling pathway that up-regulates expression of ClpB3 and other nuclear genes encoding plastidial chaperones. As a consequence, folding capacity is increased to restore protein homeostasis. This sort of chloroplast unfolded protein response (cpUPR) mechanism appears to be mediated by the heat shock transcription factor HsfA2. Expression of HsfA2 and cpUPR-related target genes is independent of GUN1, a central integrator of retrograde signaling pathways. However, double mutants defective in both GUN1 and plastome gene expression (or Clp protease activity) are seedling lethal, confirming that the GUN1 protein is essential for protein homeostasis in chloroplasts.
Author summary
Chloroplasts are central metabolic factories for plant cells. Yet, they are constantly challenged by stress episodes that alter protein homeostasis and disrupt normal chloroplast functions. To deal with this problem, protein quality control pathways involving particular chaperones and proteases promote correct protein folding and remove irreversibly damaged proteins. In the case of DXS, the main regulatory enzyme of the isoprenoid pathway, misfolded and aggregated forms of the enzyme are refolded back to its active form by stromal chaperones of the Hsp70 and Hsp100/ClpB families, hence preventing their degradation by the Clp protease complex. In this paper we report that saturated or defective Clp protease activity triggers a chloroplast unfolded protein response that results in the up-regulation of nuclear genes encoding chloroplast chaperones. Higher levels of these chaperones (particularly the disaggregase ClpB3) eventually restore the activity of DXS and other chloroplast proteins that accumulate in a non-functional form when Clp protease activity and chloroplast functions are compromised.
Citation: Llamas E, Pulido P, Rodriguez-Concepcion M (2017) Interference with plastome gene expression and Clp protease activity in Arabidopsis triggers a chloroplast unfolded protein response to restore protein homeostasis. PLoS Genet13(9): e1007022. https://doi.org/10.1371/journal.pgen.1007022
Editor: Cole Haynes, University of Massachusetts Medical School, UNITED STATES
Received: May 26, 2017; Accepted: September 15, 2017; Published: September 22, 2017
Copyright: © 2017 Llamas et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: All relevant data are within the paper and its Supporting Information files.
Funding: This work was funded by grants from the Spanish Ministry of Economy and Competitiveness (BIO2015-71703-REDT and BIO2014-59092-P) and Generalitat de Catalunya (2014SGR-1434). We also acknowledge the financial support from the Severo Ochoa Programme for Centres of Excellence in R&D 2016-2019 (SEV‐2015‐0533) and the CERCA Programme of the Generalitat de Catalunya. EL was supported by the Mexican CoNaCyT (PhD fellowships 421688 and “beca complemento”). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
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