Abstract
BackgroundFlexibility of plant metabolism is supported by redox regulation of enzymes via posttranslational modification of cysteine residues, especially in plastids. Here, the redox states of cysteine residues are partly coupled to the thioredoxin system and partly to the glutathione pool for reduction. Moreover, several plastid enzymes involved in reactive oxygen species (ROS) scavenging and damage repair draw electrons from glutathione. In addition, cysteine residues can be post-translationally modified by forming a mixed disulfide with glutathione (S-glutathionylation), which protects thiol groups from further oxidation and can influence protein activity. However, the evolution of the plastid glutathione-dependent redox network in land plants and the conservation of cysteine residues undergoing S-glutathionylation is largely unclear.ResultsWe analysed the genomes of nine representative model species from streptophyte algae to angiosperms and found that the antioxidant enzymes and redox proteins belonging to the plastid glutathione-dependent redox network are largely conserved, except for lambda- and the closely related iota-glutathione S-transferases. Focussing on glutathione-dependent redox modifications, we screened the literature for target thiols of S-glutathionylation, and found that 151 plastid proteins have been identified as glutathionylation targets, while the exact cysteine residue is only known for 17% (26 proteins), with one or multiple sites per protein, resulting in 37 known S-glutathionylation sites for plastids. However, 38% (14) of the known sites were completely conserved in model species from green algae to flowering plants, with 22% (8) on non-catalytic cysteines. Variable conservation of the remaining sites indicates independent gains and losses of cysteines at the same position during land plant evolution.ConclusionsWe conclude that the glutathione-dependent redox network in plastids is highly conserved in streptophytes with some variability in scavenging and damage repair enzymes. Our analysis of cysteine conservation suggests that S-glutathionylation in plastids plays an important and yet under-investigated role in redox regulation and stress response.
Highlights
Flexibility of plant metabolism is supported by redox regulation of enzymes via posttranslational modification of cysteine residues, especially in plastids
We included all known scavenging and damage repair enzyme families with members that were reported to (1) localise to plastids and (2) use GSH and produce glutathione disulfide (GSSG): dehydroascorbate reductase (DHAR), glutathione S-transferases lambda and iota (GSTL, GSTI), atypical methionine sulfoxide reductase B1 (MSRB1), peroxiredoxin IIE (PRXIIE), glutaredoxins (GRXs), as well as glutathione reductase (GR), which is responsible for reduction of GSSG
We found that plastid targeting of several GSSG-producing enzymes was already established before the transition to land and that no large expansions occurred in the gene families of DHAR, GSTL, GSTI, PRXIIE or class I and class II GRX during land plant evolution
Summary
Flexibility of plant metabolism is supported by redox regulation of enzymes via posttranslational modification of cysteine residues, especially in plastids. In order to balance the photosynthetic process, a tight regulation of electron flux is required to harmonise light capture with metabolic activities, as well as to prevent extensive energy or electron transfer to oxygen, generating reactive oxygen species (ROS) that can damage DNA, lipids or proteins [3, 4]. Cell compartments can contain metabolic enzymes with conditional or opposing activities, and regulation in time and space may avoid futile cycling of metabolites [9]. These complex regulatory processes are partly mediated by changes in the redox status of cysteine thiol groups in proteins enabling reversible modulation of protein function and structure that is influenced by the intracellular redox state
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