Cooperative regulation of light-harvesting complex II phosphorylation via the plastoquinol and ferredoxin-thioredoxin system in chloroplasts

Abstract

Light induces phosphorylation of photosystem II (PSII) proteins in chloroplasts by activating the protein kinase(s) via reduction of plastoquinone and the cytochrome b6f complex. The recent finding of high-light-induced inactivation of the phosphorylation of chlorophyll a͞b-binding proteins (LHCII) of the PSII antenna in floated leaf discs, but not in vitro, disclosed a second regulatory mechanism for LHCII phosphorylation. Here we show that this regulation of LHCII phosphorylation is likely to be mediated by the chloroplast ferredoxin-thioredoxin system. We present a cooperative model for the function of the two regulation mechanisms that determine the phosphorylation level of the LHCII proteins in vivo, based on the following results: (i) Chloroplast thioredoxins f and m efficiently inhibit LHCII phosphorylation. (ii) A disulfide bond in the LHCII kinase, rather than in its substrate, may be a target component regulated by thioredoxin. (iii) The target disulfide bond in inactive LHCII kinase from dark-adapted leaves is exposed and easily reduced by external thiol mediators, whereas in the activated LHCII kinase the regulatory disulfide bond is hidden. This finding suggests that the activation of the kinase induces a conformational change in the enzyme. The active state of LHCII kinase prevails in chloroplasts under low-light conditions, inducing maximal phosphorylation of LHCII proteins in vivo. (iv) Upon high-light illumination of leaves, the target disulfide bond becomes exposed and thus is made available for reduction by thioredoxin, resulting in a stable inactivation of LHCII kinase. R eversible protein phosphorylation is a unique regulatory mechanism for modification of the structure and function of proteins in both prokaryotic and eukaryotic organisms. It plays a fundamental role in signal transduction pathways that relay information from outside of the cell to the gene level. Bennett (1, 2) discovered the reversible, light-dependent phosphorylation of proteins in thylakoid membranes of plant chloroplasts. All thylakoid phosphoproteins identified so far are closely associated with photosystem II (PSII), a light-driven waterplastoquinone-oxidoreductase enzyme. Four core proteins of PSII, including the D1 and D2 reaction center proteins, the 43-kDa chlorophyll a-binding protein (CP43 protein), and the psbH gene product (PsbH protein) are prone to redox-regulated reversible phosphorylation. Three of six chlorophyll a͞b-binding proteins of the PSII antenna, Lhcb1 and Lhcb2 (designated LHCII) as well as Lhcb4 proteins, also undergo light-dependent phosphorylation. The protein kinase(s) involved in phosphorylation of the PSII proteins is associated with thylakoid membranes (2) and is activated by light via reduction of electron transfer components, plastoquinone and cytochrome b 6 ͞f complex (2-6).A large majority of the experimental data support the existence of two different kinases for PSII phosphoproteins, one for LHCII and another for PSII core proteins with distinct redox regulation systems (3, 6-10). Th...

Regulation of the photosynthetic apparatus under fluctuating growth light

“…The traditional use of different qualities of low intensity light to study state transitions contradict the natural redox regulation of the thylakoid protein kinases and phosphatases in plant chloroplasts, leading to a full dephosphorylation (state I) or a full phosphorylation (state II) of both the PSII and LHCII phosphoproteins. LHCII phosphorylation and dephosphorylation by different intensities of white light, however, does not lead to such state transitions, apparently owing to opposite behaviours of the PSII core and LHCII protein phosphorylation under these light conditions [41,79]. We have provided evidence that LHCII phosphorylation establishes a common antenna bed for both PSII and PSI in the so-called thylakoid megacomplexes [80], which ensure proper distribution of excitation energy to PSII and PSI.…”

Activation of the Stt7/STN7 Kinase through Dynamic Interactions with the Cytochrome b6f Complex

“…It was proposed that inactivation of the kinase is mediated by the ferredoxin-thioredoxin system and that a disulfide bond in the kinase rather than in the substrate may be the target site of thioredoxin (Rintamäki et al, 1997(Rintamäki et al, , 2000. Analysis of the Stt7/STN7 protein sequences indeed reveals the presence of two conserved Cys residues close to the N-terminal end of this kinase, which are conserved in all species examined and both are essential for kinase activity although they are located outside of the kinase catalytic domain ( Fig.…”

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

“…This process requires the STN7 kinase, which is activated by a reduced state of the PQ pool under these light conditions [5,22]. Upon exposure to HL, the kinase is inactivated by a thioredoxin-mediated reduction of disulfide bonds [22].…”