STATE TRANSITION7-Dependent Phosphorylation Is Modulated by Changing Environmental Conditions, and Its Absence Triggers Remodeling of Photosynthetic Protein Complexes

Abstract

In plants and algae, the serine/threonine kinase STN7/STT7, orthologous protein kinases in Chlamydomonas reinhardtii and Arabidopsis (Arabidopsis thaliana), respectively, is an important regulator in acclimation to changing light environments. In this work, we assessed STT7-dependent protein phosphorylation under high light in C. reinhardtii, known to fully induce the expression of LIGHT-HARVESTING COMPLEX STRESS-RELATED PROTEIN3 (LHCSR3) and a nonphotochemical quenching mechanism, in relationship to anoxia where the activity of cyclic electron flow is stimulated. Our quantitative proteomics data revealed numerous unique STT7 protein substrates and STT7-dependent protein phosphorylation variations that were reliant on the environmental condition. These results indicate that STT7-dependent phosphorylation is modulated by the environment and point to an intricate chloroplast phosphorylation network responding in a highly sensitive and dynamic manner to environmental cues and alterations in kinase function. Functionally, the absence of the STT7 kinase triggered changes in protein expression and photoinhibition of photosystem I (PSI) and resulted in the remodeling of photosynthetic complexes. This remodeling initiated a pronounced association of LHCSR3 with PSI-LIGHT HARVESTING COMPLEX I (LHCI)-ferredoxin-NADPH oxidoreductase supercomplexes. Lack of STT7 kinase strongly diminished PSII-LHCII supercomplexes, while PSII core complex phosphorylation and accumulation were significantly enhanced. In conclusion, our study provides strong evidence that the regulation of protein phosphorylation is critical for driving successful acclimation to high light and anoxic growth environments and gives new insights into acclimation strategies to these environmental conditions. Oxygenic photosynthesis converts solar energy into chemical energy. This energy is utilized for carbon dioxide assimilation, allowing the formation of complex organic material. Plant photosynthesis is performed by a series of reactions in and at the thylakoid membrane, resulting in light-dependent water oxidation, NADP reduction, and ATP formation (Whatley et al., 1963). These light reactions are catalyzed by two photosystems (PSI and PSII). A third multiprotein complex, also embedded in the thylakoid membrane, is the cytochrome b 6 f (cyt b 6 f) complex that links photosynthetic electron transfer processes between the two photosystems and functions in proton translocation. The ATP synthase takes advantage of the proton-motive force that is generated by the light reactions (Mitchell, 1961) to produce ATP. ATP and NADPH, generated through linear electron flow from PSII to PSI, drive the CalvinBenson-Bassham cycle (Bassham et al., 1950) to fix CO 2 . Alternatively, cyclic electron flow (CEF) between PSI and the cyt b 6 f complex solely produces ATP (Arnon, 1959).Under normal growth conditions, CEF provides additionally required ATP for CO 2 fixation (Lucker and Kramer, 2013), counteracts overreduction of the PSI acceptor side under stressful environmental cues,...

The Antarctic Psychrophile Chlamydomonas sp. UWO 241 Preferentially Phosphorylates a Photosystem I-Cytochrome b6/f Supercomplex

“…In addition, Lhcbm4, Lhcbm6, Lhcbm9, Lhcbm11, and CP26 (Lhcb5) have also been shown to undergo phosphorylation during high-light exposure in C. reinhardtii (Turkina et al, 2006;Vener, 2007). However, in addition to the regulation of state transitions, thylakoid protein phosphorylation has been shown to be crucial for the acclimation to high light and anoxia in C. reinhardtii (Bergner et al, 2015) as well as to low light and fluctuating irradiance in Arabidopsis Tikkanen et al, 2006;Grieco et al, 2012). Although the phosphorylation of PSI polypeptides appears to be quite rare (Vener, 2006(Vener, , 2007, PsaD, an extrinsic stroma-exposed protein, was reported as the first phosphorylated PSI subunit in Arabidopsis (Hansson and Vener, 2003).…”

Novel insights into the function of LHCSR3 in Chlamydomonas reinhardtii

“…New data indicated that LHCSR3 also interacts with PSI-LHCI-FNR supercomplexes 26 and that the absence of LHCSR3 accelerates photoinhibition of PSI under conditions where PSI in prone to degradation due to deletion of PGRL1 26 . Now, what determines the interaction of LHCSR3 with other photosynthetic proteins and protein complexes?…”

“…In line, all LHCSR protein sequences shown (Figure 2A) possess S and T residues in their N-terminal parts. According to quantitative analyses 60 % of the phosphorylatable residues S 26 , S 28 , T 32 and T 33 are indeed phosphorylated under high light conditions 26 . Notably these N-terminal residues are phosphorylated in an STT7-dependent manner 26 .…”

Chlamydomonas reinhardtii PsbS Protein Is Functional and Accumulates Rapidly and Transiently under High Light

“…S1; Bonente et al, 2011) increased during the light stress. Phosphorylation of LhcSR3 could be involved in the localization of the protein (Allorent et al, 2013;Bergner et al, 2015). Minor and major LHCII antenna can be phosphorylated in Chlamydomonas to move from PSII to PSI during state transition (Iwai et al, 2008;Rochaix, 2014), and this could also be the case for LhcSR3.…”

“…It has been suggested that LhcSR3 itself is the energy quencher due to its pigment binding ability, the unusual fluorescence properties that show very short lifetime components, and its pH sensitivity (Bonente et al, 2011;Liguori et al, 2013;Tokutsu and Minagawa, 2013). A recent investigation suggests the LhcSR is localized both in grana membranes and stroma lamellae in the moss Physcomitrella patens (Pinnola et al, 2015), and investigations on Chlamydomonas suggest that LhcSR3 can bind both to PSI or PSII depending on illumination conditions (Allorent et al, 2013;Bergner et al, 2015). In contrast to psbS, lhcSR genes are widespread in a many photosynthetic organisms, including green algae, brown algae, diatoms, nonvascular plants, and even some vascular plants (Buchel, 2015).…”