Stt7-dependent Phosphorylation during State Transitions in the Green Alga Chlamydomonas reinhardtii

Abstract

Photosynthetic organisms are able to adapt to changes in light conditions by balancing the light excitation energy between the light-harvesting systems of photosystem (PS) II and photosystem I to optimize the photosynthetic yield. A key component in this process, called state transitions, is the chloroplast protein kinase Stt7/STN7, which senses the redox state of the plastoquinone pool. Upon preferential excitation of photosystem II, this kinase is activated through the cytochrome b 6 f complex and required for the phosphorylation of the light-harvesting system of photosystem II, a portion of which migrates to photosystem I (state 2). Preferential excitation of photosystem I leads to the inactivation of the kinase and to dephosphorylation of light-harvesting complex (LHC) II and its return to photosystem II (state 1). Here we compared the thylakoid phosphoproteome of the wild-type strain and the stt7 mutant of Chlamydomonas under state 1 and state 2 conditions. This analysis revealed that under state 2 conditions several Stt7-dependent phosphorylations of specific Thr residues occur in Lhcbm1/Lhcbm10, Lhcbm4/Lhcbm6/Lhcbm8/Lhcbm9, Lhcbm3, Lhcbm5, and CP29 located at the interface between PSII and its light-harvesting system. Among the two phosphorylation sites detected specifically in CP29 under state 2, one is Stt7-dependent. This phosphorylation may play a crucial role in the dissociation of CP29 from PSII and/or in its association to PSI where it serves as a docking site for LHCII in state 2. Moreover, Stt7 was required for the phosphorylation of the thylakoid protein kinase Stl1 under state 2 conditions, suggesting the existence of a thylakoid protein kinase cascade. Stt7 itself is phosphorylated at Ser 533 in state 2, but analysis of mutants with a S533A/D change indicated that this phosphorylation is not required forstatetransitions.Moreover,wealsoidentifiedphosphorylation sites that are redox (state 2)-dependent but independent of Stt7 and additional phosphorylation sites that are redox-independent. Molecular & Cellular Proteomics 9:1281-1295, 2010.The primary photochemical reactions of photosynthesis are catalyzed by the pigment-protein complexes photosystem II (PSII) 1 and PSI (PSI), which are linked in series through the plastoquinone pool, the cytochrome b 6 f complex, and plastocyanin in the thylakoid membranes. Upon light absorption by the antenna systems of PSII and PSI, charge separations occur across the membrane that lead to the oxidation of water by PSII and electron flow to PSI and ultimately to the reduction of NADP ϩ . Because the antenna systems of PSII and PSI have different pigment composition, they are differentially sensitized upon changes in light quality and quantity. However, photosynthetic organisms have the ability to adapt to changes in light. They balance energy input and consumption in the short term through dissipation of excess absorbed light energy into heat through non-photochemical quenching and regulate absorption of excitation energy between PSII and PSI through state tra...

Protein kinases and phosphatases involved in the acclimation of the photosynthetic apparatus to a changing light environment

“…Comparison of the target phosphorylation sites of Stt7 revealed a consensus motif in which the phosphorylated Ser is flanked by basic amino acids [36]. This motif was used for a bioinformatics search for new potential substrates of Stt7.…”

“…This motif was used for a bioinformatics search for new potential substrates of Stt7. Interestingly, this search revealed several nucleus-encoded factors that are involved in post-transcriptional steps of chloroplast gene expression, including Raa3 (a protein involved in trans-splicing of the psaA RNA), Tab2 (required 36,41] for translation of the psaB mRNA) and Nac2 (involved in the stability/processing of the psbD RNA). These data raise the possibility that the Stt7/STN7 kinase has a wider role and is also involved in chloroplast gene expression, although the phosphorylation of these proteins needs to be confirmed by phosphoproteomics.…”

“…To identify the corresponding phosphorylated residues, thylakoid membranes from the wild-type and the stt7 mutant were isolated and digested with trypsin to release the LHCII phosphopeptides that are located on the stromal side of the membrane [36]. These peptides were then enriched by metal affinity chromatography and subjected to mass-spectrometric analysis.…”

Changes in the Phosphoproteome and Metabolome Link Early Signaling Events to Rearrangement of Photosynthesis and Central Metabolism in Salinity and Oxidative Stress Response in Arabidopsis

“…We found that phosphorylation at Thr 537 and Thr 541 of State Transition Protein Kinase7 (STN7) was affected by stress. In Arabidopsis, STN7 functions in the phosphorylation of the light-harvesting system of PSII and for the state transition, a process that balances the light excitation energy between PSII and PSI and optimizes the photosynthetic yield upon changes in light quality and quantity (Lemeille et al, 2010). Changes in phosphorylation at the known C-terminal STN7 sites Thr 537, Thr 539, and Thr 541 were also detected, and previous studies reported that phosphorylation at the C terminus of STN7 kinase is required for its stability under state 2 conditions (Willig et al, 2011).…”

Natural variation in phosphorylation of photosystem II proteins in Arabidopsis thaliana : is it caused by genetic variation in the STN kinases?

“…No interdependence seems to exist between the STN kinases at protein levels [25]. Nevertheless, it has been proposed that the activity of STN8 may be regulated by STN7 in Arabidopsis, because in the green alga Chlamydomonas reinhardtii the activity of STN8 homologue Stl1 is regulated by its phosphorylation in an Stt7-dependent manner [27].…”