Mild proteasomal stress improves photosynthetic performance in Arabidopsis chloroplasts

Abstract

The proteasome is an essential protein-degradation machinery in eukaryotic cells that controls protein turnover and thereby the biogenesis and function of cell organelles. Chloroplasts import thousands of nuclear-encoded precursor proteins from the cytosol, suggesting that the bulk of plastid proteins is transiently exposed to the cytosolic proteasome complex. Therefore, there is a cytosolic equilibrium between chloroplast precursor protein import and proteasomal degradation. We show here that a shift in this equilibrium, induced by mild genetic proteasome impairment, results in elevated precursor protein abundance in the cytosol and significantly increased accumulation of functional photosynthetic complexes in protein import-deficient chloroplasts. Importantly, a proteasome lid mutant shows improved photosynthetic performance, even in the absence of an import defect, signifying that functional precursors are continuously degraded. Hence, turnover of plastid precursors in the cytosol represents a mechanism to constrain thylakoid membrane assembly and photosynthetic electron transport.

RETRACTED ARTICLE:The Arabidopsis NOT4A E3 ligase promotes PGR3 expression and regulates chloroplast translation

“…CHLORADtargeted TOC subunits are ubiquitinated by the integral chloroplast outer membrane ubiquitin-ligase SP1, promoting removal and delivery to the 26 S proteasome via the mp85-type beta-barrel channel SP2 and AAA + chaperone CDC48 [12][13][14] . In addition, the 26 S proteasome was recently shown to counter-balance protein import into chloroplasts, with proteasome inhibition leading to enhanced import, and partial rescue of the TOC19a import mutant (ppi2) through the introduction of mutations within proteasome subunits 15 . However, despite the evolutionary expansion and significance of the UPS in plants (accounting for more than 5% of the genome in Arabidopsis 16 ) the full extent to which the UPS can influence chloroplast function remains to be determined.…”

Abundance of metalloprotease FtsH12 modulates chloroplast development in Arabidopsis thaliana

“…Eleven of these did not contain a Lys that would allow quantification in our assay, while the remaining precursor of an uncharacterized protein encoded by gene At4g28440 showed no significant difference in accumulation. This was not unexpected, as unprocessed precursor proteins accumulating in the cytosol are rapidly degraded by the ubiquitin/proteasome system ( Lee et al , 2009 ; Grimmer et al , 2020 ).…”

Chromoplast differentiation in bell pepper (Capsicum annuum) fruits

“…The following proteins/enzymes are presented: ferredoxin (A0A1U8GXA1, A0A1U8H397, A0A2G2YH86, A0A1U8DVL9, B1PDK3, and A0A2G2XUS9), Fd-dependent glutamate synthase (Fd-GS; A0A2G3ANJ0 and A0A2G2ZX64), Fd-dependent NADP reductase (FNR; A0A1U8FJG3 and A0A1U8FMQ7), acetyl CoA-carboxylase (ACCD; A0A2G2ZQ00 and A0A2G2Z7T4), plastid 70S ribosome (ribosome; see Table S2), phytoene synthase (PSY; A0A1U8FLW3 and A0A1U8GGI5), phytoene desaturase (PDS; A0A1U8FU73), ORANGE (OR; A0A1U8FXL9 and A0A2G2ZY11), fibrillin (Q42493), pheophorbide a oxygenase (PaO; A0A2G2YHJ4), cytochrome b 6 f complex (see Table S2), and plastid terminal oxidase (PTOX; A0A2G2Y4F7). Significant differences with the GL sample (as chloroplast reference) are highlighted by one ( (Bischof et al, 2011;Dutta et al, 2014;Grimmer et al, 2020), the importance of Toc159 during de-etiolation and seed germination was clearly demonstrated (Ling et al, 2012;Shanmugabalaji et al, 2018). The bell pepper Toc159 homolog was identified at low concentrations in only one to two replicates in the GL and GU samples, thus hampering its quantification.…”