Abstract
One strategy for enhancing photosynthesis in crop plants is to improve their ability to repair photosystem II (PSII) in response to irreversible damage by light. Despite the pivotal role of thylakoid-embedded FtsH protease complexes in the selective degradation of PSII subunits during repair, little is known about the factors involved in regulating FtsH expression. Here we show using the cyanobacterium Synechocystis sp. PCC 6803 that the Psb29 subunit, originally identified as a minor component of His-tagged PSII preparations, physically interacts with FtsH complexes in vivo and is required for normal accumulation of the FtsH2/FtsH3 hetero-oligomeric complex involved in PSII repair. We show using X-ray crystallography that Psb29 from Thermosynechococcus elongatus has a unique fold consisting of a helical bundle and an extended C-terminal helix and contains a highly conserved region that might be involved in binding to FtsH. A similar interaction is likely to occur in Arabidopsis chloroplasts between the Psb29 homologue, termed THF1, and the FTSH2/FTSH5 complex. The direct involvement of Psb29/THF1 in FtsH accumulation helps explain why THF1 is a target during the hypersensitive response in plants induced by pathogen infection. Downregulating FtsH function and the PSII repair cycle via THF1 would contribute to the production of reactive oxygen species, the loss of chloroplast function and cell death.This article is part of the themed issue ‘Enhancing photosynthesis in crop plants: targets for improvement’.
Highlights
Plants exposed to excessive light suffer from impaired photosynthetic activity termed chronic photoinhibition [1,2]
One of the main targets of damage is the oxygen-evolving photosystem II (PSII) complex embedded in the thylakoid membrane system, which uses light energy to extract electrons from water to feed into the photosynthetic electron transport chain to produce the ATP and NADPH required for CO2 fixation [3]
Previous work in Arabidopsis has shown that the absence of THF1 leads to a 40–80% decrease in the amount of the type A and type B FTSH subunits involved in PSII repair as judged by immunoblotting [23]
Summary
Plants exposed to excessive light suffer from impaired photosynthetic activity termed chronic photoinhibition [1,2]. We show that Psb in Synechocystis 6803, like THF1 in Arabidopsis, is important for normal accumulation of the FtsH heterocomplex involved in PSII repair. For purification of protein complexes, the FtsH2-FLAG strain was grown as described above in 500 ml of medium using 2 l conical flasks. For purification of Psb29-FLAG protein complexes, 4 l of Psb29-FLAG strain were grown in a 10 l flask in BG11 medium supplemented with 1 mM glucose, agitated with magnetic stirrer and bubbled with air. In both cases, surface irradiance was increased to 100 mmol photons m22 s21 of light to compensate for the longer path length of the flasks. The above MAFFT alignment was trimmed of columns containing gaps of over 90%; columns corresponding to the chloroplast transit peptide domain of Arabidopsis thaliana THF1, predicted by ChloroP 1.1 Server [52], were removed
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