Structural analysis of photosystem II in far-red-light-adapted thylakoid membranes. New crystal forms provide evidence for a dynamic reorganization of light-harvesting antennae subunits.

We studied two-dimensional crystals of the major pigment-protein complex, photosystem II, in far-red-light-adapted thylakoid membranes of the viridis-zb63 mutant of barley. Significantly larger grana membranes were produced with an increased synthesis of the entire photosystem II complex. These red-light-adapted membranes also contained two-dimensional crystals with a high frequency. Three different crystal forms of photosystem II were observed, providing the following data which further our understanding of the architecture of the native complex. (a) The oligomeric form of photosystem II in the membrane was monomeric in all crystal forms, but with a clear non-crystallographic pseudo-twofold symmetry. This was more apparent on the lumenal face of the complex. (b) The variability of unit cell contacts in different crystal forms implied that the peripheral light-harvesting antenna complex and the core of the complex were loosely connected. These peripheral subunits were predicted to rearrange so that they can either encircle the core complex or associate in parallel channels separated by lines of core complexes. (c) Grana membranes were found to retain a double-layered inside-out character, with a stromal face-to-stromal face packing. However, the presence of a crystal in one membrane did not necessarily impose crystallinity on its pair.     

Diffusion of light-harvesting complex II in the thylakoid membranes

The light-harvesting complex II (LHCII) is the main energy absorber for photosynthesis in green plants, and its translocation between photosystems I and II is the primary means of energy redistribution between them. Using single-particle tracking, we performed the first measurement of the mobility of LHCII in the photosynthetic membranes in both the nonphosphorylated and the phosphorylated (P-LHCII) conformations. These are part of an important, reversible, energy re-equilibration process called the state transition. We found that the population of P-LHCII in unappressed membranes is more mobile than the population of non-P-LHCII from the same regions.     

Assembly of light-harvesting complex II and biogenesis of thylakoid membranes in chloroplasts

A critical review of studies on import of Lhcb (apoproteins of LHC II) by chloroplasts uncovered a mechanism for initiation of assembly of light-harvesting complexes. Manipulation of in vivo systems and mutagenesis of specific residues in the protein showed that accumulation of physiological amounts of Lhcb by the plastid requires interaction of the protein with Chl within the inner membrane of the chloroplast envelope. Retention motifs, commonly -EXXHXR- in the first membrane-spanning region (helix-1) and -EXXNXR- in the third membrane-spanning region (helix-3), occur in the primary sequence of the protein. Mutations in these sequences prevent accumulation of Lhcb by isolated chloroplasts. We propose that the His or Asn sidechain and a transient intrahelix ion-pair with the Glu and Arg residues provide ligands for two molecules of Chl in each motif, which serve as a sensing mechanism for the availability of Chl. Interaction of two Chl molecules with both motifs is required for stable insertion of the protein into the membrane. Chl(ide) is possibly quenched by interaction with xanthophylls immediately after synthesis, and Chl-lutein pairs may initiate folding of Lhcb. Lhcb that does not immediately interact with sufficient Chl molecules is not retained by the organelle and, in vivo, is retracted into the cytosol or shunted to vacuoles for degradation rather than imported into the plastid stroma. The ubiquitous existence of retention motifs from small Lhcb-like polypeptides in cyanobacteria to all nuclear-encoded Chl-binding proteins (the Lhcb and Lhca families and related proteins) testify to the importance of these sequences in assembly of Chl-protein complexes.     

Functional domain size in aggregates of light-harvesting complex II and thylakoid membranes

The functional domain size for efficient excited singlet state quenching was studied in artificial aggregates of the main light-harvesting complex II (LHCIIb) from spinach and in native thylakoid membranes by picosecond time-resolved fluorescence spectroscopy and quantum yield measurements. The domain size was estimated from the efficiency of added exogenous singlet excitation quenchers-phenyl-p-benzoquinone (PPQ) and dinitrobenzene (DNB). The mean fluorescence lifetimes τ(av) were quantified for a range of quencher concentrations. Applying the Stern-Volmer formalism, apparent quenching rate constants k(q) were determined from the dependencies on quencher concentration of the ratio τ(0)(av)/τ(av), where τ(0)(av) is the average fluorescence lifetime of the sample without addition of an exogenous quencher. The functional domain size was gathered from the ratio k(q)'/k(q), i.e., the apparent quenching rate constants determined in aggregates (or membranes), k(q)', and in detergent-solubilised LHCII trimers, k(q), respectively. In LHCII macroaggregates, the resulting values for the domain size were 15-30 LHCII trimers. In native thylakoid membranes the domain size was equivalent to 12-24 LHCII trimers, corresponding to 500-1000 chlorophylls. Virtually the same results were obtained when membranes were suspended in buffers promoting either membrane stacking or destacking. These domain sizes are orders of magnitude smaller than the number of physically connected pigment-protein complexes. Therefore our results imply that the physical size of an antenna system beyond the numbers of a functional domain size has little or no effect on improving the light-harvesting efficiency.     

Heptameric association of light-harvesting complex II trimers in partially solubilized photosystem II membranes.

We report a structural characterization by electron microscopy and image analysis of a supramolecular complex consisting of seven trimeric light-harvesting complex II proteins. The complex was readily observed in partially-solubilized Tris-washed photosystem II membranes from spinach but was also found to occur, with a low frequency, in oxygen-evolving photosystem II membranes. The structure reveals six peripheral trimers with the same rotational orientation and a central trimer with the opposite orientation. We conclude that the heptamer represents a naturally occurring aggregation state of part of the light-harvesting complex II trimers in the thylakoid membranes.     

Fluorescence emission spectra of photosystem I, photosystem II and the light-harvesting chlorophyll a/b complex of higher plants

Fluorescence emission spectra excited at 514 and 633 nm were measured at -196 degrees C on dark-grown bean leaves which had been partially greened by a repetitive series of brief xenon flashes. Excitation at 514 nm resulted in a greater relative enrichment of the 730 nm emission band of Photosystem I than was obtained with 633 nm excitation. The difference spectrum between the 514 nm excited fluorescence and the 633 nm excited fluorescence was taken to be representative of a pure Photosystem I emission spectrum at -196 degrees C. It was estimated from an extrapolation of low temperature emission spectra taken from a series of flashed leaves of different chlorophyll content that the emission from Photosystem II at 730 nm was 12% of the peak emission at 694 nm. Using this estimate, the pure Photosystem I emission spectrum was subtracted from the measured emission spectrum of a flashed leaf to give an emission spectrum representative of pure Photosystem II fluorescence at -196 degrees C. Emission spectra were also measured on flashed leaves which had been illuminated for several hours in continuous light. Appreciable amounts of the light-harvesting chlorophyll a/b protein, which has a low temperature fluorescence emission maximum at 682 nm, accumulate during greening in continuous light. The emission spectra of Photosystem I and Photosystem II were subtracted from the measured emission spectrum of such a leaf to obtain the emission spectrum of the light-harvesting chlorophyll a/b protein at -196 degrees C.     

Investigations of the role of the main light-harvesting chlorophyll- protein complex in thylakoid membranes. Reconstitution of depleted membranes from intermittent-light-grown plants with the isolated complex

The functions of the light-harvesting complex of photosystem II (LHC- II) have been studied using thylakoids from intermittent-light-grown (IML) plants, which are deficient in this complex. These chloroplasts have no grana stacks and only limited lamellar appression in situ. In vitro the thylakoids showed limited but significant Mg2+-induced membrane appression and a clear segregation of membrane particles into such regions. This observation, together with the immunological detection of small quantities of LHC-II apoproteins, suggests that the molecular mechanism of appression may be similar to the more extensive thylakoid stacking seen in normal chloroplasts and involve LHC-II polypeptides directly. To study LHC-II function directly, a sonication- freeze-thaw procedure was developed for controlled insertion of purified LHC-II into IML membranes. Incorporation was demonstrated by density gradient centrifugation, antibody agglutination tests, and freeze-fracture electron microscopy. The reconstituted membranes, unlike the parent IML membranes, exhibited both extensive membrane appression and increased room temperature fluorescence in the presence of cations, and a decreased photosystem I activity at low light intensity. These membranes thus mimic normal chloroplasts in this regard, suggesting that the incorporated LHC-II interacts with photosystem II centers in IML membranes and exerts a direct role in the regulation of excitation energy distribution between the two photosystems.     

Characterization of photosystem II in stroma thylakoid membranes

The functional state of the PS II population localized in the stroma exposed non-appressed thylakoid region was investigated by direct analysis of the PS II content of isolated stroma thylakoid vesicles. This PS II population, possessing an antenna size typical for PS II, was found to have a fully functional oxygen evolving capacity in the presence of an added quinone electron acceptor such as phenyl-p-benzoquinone. The sensitivity to DCMU for this PS II population was the same as for PS II in control thylakoids. However, under more physiological conditions, in the absence of an added quinone acceptor, no oxygen was evolved from stroma thylakoid vesicles and their PS II centers were found to be incapable to pass electrons to PS I and to yield NADPH. By comparison of the effect of a variety of added quinone acceptors with different midpoint potentials, it is concluded that the inability of PS II in the stroma thylakoid membranes to contribute to NADPH formation probably is due to that Q_A of this population is not able to reduce PQ, although it can reduce some artificial acceptors like phenyl-p-benzoquinone. These data give further support to the notion of a discrete PS II population in the non-appressed stroma thylakoid region, PS II, having a higher midpoint potential of Q_A than the PS II population in the appressed thylakoid region, PS II. The physiological significance of a PS II population that does not produce any NADPH is discussed.Abbreviations pBQ p-benzoquinone - Chl chlorophyll - DCBQ 2,6-dichloro-p-benzoquinone - DCIP 2,6-dichloroindophenol - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DMBQ 2,5-dimethyl-p-benzoquinone - DQ duroquinone(tetramethyl-p-benzoquinone) - FeCN ferricyanide (potassium hexacyanoferrat) - MV methylviologen - NADPH,NADP⁺ reduced or oxidized form of nicotinamide adenine dinucleotide phosphate respectively - PpBQ phenyl-p-benzoquinone - PQ plastoquinone - PS II photosystem II - PS I photosystem I - Q_A primary quinone acceptor of PS II - Q_B secondary quinone acceptor of PS II - E microEinstein     

Carotenoid-binding sites of the major light-harvesting complex II of higher plants.

Recombinant light-harvesting complex II (LHCII) proteins with modified carotenoid composition have been obtained by in vitro reconstitution of the Lhcb1 protein overexpressed in bacteria. The monomeric protein possesses three xanthophyll-binding sites. The L1 and L2 sites, localized by electron crystallography in the helix A/helix B cross, have the highest affinity for lutein, but also bind violaxanthin and zeaxanthin with lower affinity. The latter xanthophyll causes disruption of excitation energy transfer. The occupancy of at least one of these sites, probably L1, is essential for protein folding. Neoxanthin is bound to a distinct site (N1) that is highly selective for this species and whose occupancy is not essential for protein folding. Whereas xanthophylls in the L1 and L2 sites interact mainly with chlorophyll a, neoxanthin shows strong interaction with chlorophyll b, inducing the hyperchromic effect of the 652 nm absorption band. This observation explains the recent results of energy transfer from carotenoids to chlorophyll b obtained by femtosecond absorption spectroscopy. Whereas xanthophylls in the L1 and L2 sites are active in photoprotection through chlorophyll-triplet quenching, neoxanthin seems to act mainly in (1)O(2)(*) scavenging.     

Supramolecular organization of photosystem II and its light-harvesting antenna in partially solubilized photosystem II membranes.

We present an extended analysis of the organization of green plant photosystem II and its associated light-harvesting antenna using electron microscopy and image analysis. The analysis is based on a large dataset of 16 600 projections of negatively stained PSII-LHCII supercomplexes and megacomplexes prepared by means of three different pretreatments. In addition to our previous work on this system [Boekema, E.J., van Roon, H., Calkoen, F., Bassi, R. and Dekker, J.P. (1999) Biochemistry 38, 2233-2239], the following results were obtained. The rotational orientation of trimeric LHCII at the S, M and L binding positions was determined. It was found that compared to the S trimer, the M and L trimers are rotationally shifted by about -20 degrees and -50 degrees, respectively. The number of projections with empty CP29, CP26 and CP24 binding sites was found to be about 0, 18 and 4%, respectively. We suggest that CP26 and CP24 are not required for the binding of trimeric LHCII at any of the three binding positions. A new type of megacomplex was observed with a characteristic windmill-like shape. This type III megacomplex consists of two C2S2 supercomplexes connected at their CP26 tips. Structural variation in the region of the central dimeric photosystem II complex was found to occur at one specific position near the periphery of the complex. We attribute this variation to the partial absence of an extrinsic polypeptide or one or more small intrinsic membrane proteins.     

Single molecule spectroscopy on the light-harvesting complex II of higher plants.

Spectroscopic and polarization properties of single light-harvesting complexes of higher plants (LHC-II) were studied at both room temperature and T < 5 K. Monomeric complexes emit roughly linearly polarized fluorescence light thus indicating the existence of only one emitting state. Most probably this observation is explained by efficient triplet quenching restricted to one chlorophyll a (Chl a) molecule or by rather irreversible energy transfer within the pool of Chl a molecules. LHC-II complexes in the trimeric (native) arrangement bleach in a number of steps, suggesting localization of excitations within the monomeric subunits. Interpretation of the fluorescence polarization properties of trimers requires the assumption of transition dipole moments tilted out of the symmetry plane of the complex. Low-temperature fluorescence emission of trimers is characterized by several narrow spectral lines. Even at lowest excitation intensities, we observed considerable spectral diffusion most probably due to low temperature protein dynamics. These results also indicate weak interaction between Chls belonging to different monomeric subunits within the trimer thus leading to a localization of excitations within the monomer. The experimental results demonstrate the feasibility of polarization sensitive studies on single LHC-II complexes and suggest an application for determination of the Chl transition-dipole moment orientations, a key issue in understanding the structure-function relationships.     

Chlorophyll a and carotenoid triplet states in light-harvesting complex II of higher plants.

Laser-flash-induced transient absorption measurements were performed on trimeric light-harvesting complex II to study carotenoid (Car) and chlorophyll (Chl) triplet states as a function of temperature. In these complexes efficient transfer of triplets from Chl to Car occurs as a protection mechanism against singlet oxygen formation. It appears that at room temperature all triplets are being transferred from Chl to Car; at lower temperatures (77 K and below) the transfer is less efficient and chlorophyll triplets can be observed. In the presence of oxygen at room temperature the Car triplets are partly quenched by oxygen and two different Car triplet spectral species can be distinguished because of a difference in quenching rate. One of these spectral species is replaced by another one upon cooling to 4 Ki demonstrating that at least three carotenoids are in close contact with chlorophylls. The triplet minus singlet absorption (T-S) spectra show maxima at 504-506 nm and 517-523 nm, respectively. In the Chl Qy region absorption changes can be observed that are caused by Car triplets. The T-S spectra in the Chl region show an interesting temperature dependence which indicates that various Car's are in contact with different Chl a molecules. The results are discussed in terms of the crystal structure of light-harvesting complex II.     

Characterization of thylakoid lipid membranes from cyanobacteria and higher plants by molecular dynamics simulations

The thylakoid membrane is mainly composed of non-common lipids, so called galactolipids. Despite the importance of these lipids for the function of the photosynthetic reaction centers, the molecular organization of these membranes is largely unexplored. Here we use multiscale molecular dynamics simulations to characterize the thylakoid membrane of both cyanobacteria and higher plants. We consider mixtures of up to five different galactolipids plus phosphatidylglycerol to represent these complex membranes. We find that the different lipids generally mix well, although nanoscale heterogeneities are observed especially in case of the plant membrane. The fluidity of the cyanobacterial membrane is markedly reduced compared to the plant membrane, even considering elevated temperatures at which thermophilic cyanobacteria are found. We also find that the plant membrane more readily undergoes a phase transformation to an inverted hexagonal phase. We furthermore characterized the conformation and dynamics of the cofactors plastoquinone and plastoquinol, revealing of the fast flip-flop rates for the non-reduced form. Together, our results provide a molecular view on the dynamical organization of the thylakoid membrane.     

Excitation-energy transfer dynamics of higher plant photosystem I light-harvesting complexes

Photosystem I (PSI) plays a major role in the light reactions of photosynthesis. In higher plants, PSI is composed of a core complex and four outer antennas that are assembled as two dimers, Lhca1/4 and Lhca2/3. Time-resolved fluorescence measurements on the isolated dimers show very similar kinetics. The intermonomer transfer processes are resolved using target analysis. They occur at rates similar to those observed in transfer to the PSI core, suggesting competition between the two transfer pathways. It appears that each dimer is adopting various conformations that correspond to different lifetimes and emission spectra. A special feature of the Lhca complexes is the presence of an absorption band at low energy, originating from an excitonic state of a chlorophyll dimer, mixed with a charge-transfer state. These low-energy bands have high oscillator strengths and they are superradiant in both Lhca1/4 and Lhca2/3. This challenges the view that the low-energy charge-transfer state always functions as a quencher in plant Lhc's and it also challenges previous interpretations of PSI kinetics. The very similar properties of the low-energy states of both dimers indicate that the organization of the involved chlorophylls should also be similar, in disagreement with the available structural data.     

Structural variability of plant photosystem II megacomplexes in thylakoid membranes

Plant photosystem II (PSII) is organized into large supercomplexes with variable levels of membrane‐bound light‐harvesting proteins (LHCIIs). The largest stable form of the PSII supercomplex involves four LHCII trimers, which are specifically connected to the PSII core dimer via monomeric antenna proteins. The PSII supercomplexes can further interact in the thylakoid membrane, forming PSII megacomplexes. So far, only megacomplexes consisting of two PSII supercomplexes associated in parallel have been observed. Here we show that the forms of PSII megacomplexes can be much more variable. We performed single particle electron microscopy (EM) analysis of PSII megacomplexes isolated from Arabidopsis thaliana using clear‐native polyacrylamide gel electrophoresis. Extensive image analysis of a large data set revealed that besides the known PSII megacomplexes, there are distinct groups of megacomplexes with non‐parallel association of supercomplexes. In some of them, we have found additional LHCII trimers, which appear to stabilize the non‐parallel assemblies. We also performed EM analysis of the PSII supercomplexes on the level of whole grana membranes and successfully identified several types of megacomplexes, including those with non‐parallel supercomplexes, which strongly supports their natural origin. Our data demonstrate a remarkable ability of plant PSII to form various larger assemblies, which may control photochemical usage of absorbed light energy in plants in a changing environment.     

The PsbS protein controls the organization of the photosystem II antenna in higher plant thylakoid membranes

The PsbS subunit of photosystem II (PSII) plays a key role in nonphotochemical quenching (NPQ), the major photoprotective regulatory mechanism in higher plant thylakoid membranes, but its mechanism of action is unknown. Here we describe direct evidence that PsbS controls the organization of PSII and its light harvesting system (LHCII). The changes in chlorophyll fluorescence amplitude associated with the Mg(2+)-dependent restacking of thylakoid membranes were measured in thylakoids prepared from wild-type plants, a PsbS-deficient mutant and a PsbS overexpresser. The Mg(2+) requirement and sigmoidicity of the titration curves for the fluorescence rise were negatively correlated with the level of PsbS. Using a range of PsbS mutants, this effect of PsbS was shown not to depend upon its efficacy in controlling NPQ, but to be related only to protein concentration. Electron microscopy and fluorescence spectroscopy showed that this effect was because of enhancement of the Mg(2+)-dependent re-association of PSII and LHCII by PsbS, rather than an effect on stacking per se. In the presence of PsbS the LHCII.PSII complex was also more readily removed from thylakoid membranes by detergent, and the level of PsbS protein correlated with the amplitude of the psi-type CD signal originating from features of LHCII.PSII organization. It is proposed that PsbS regulates the interaction between LHCII and PSII in the grana membranes, explaining how it acts as a pH-dependent trigger of the conformational changes within the PSII light harvesting system that result in NPQ.     

Freezing of isolated thylakoid membranes in complex media

Chloroplast thylakoid membranes isolated from spinach leaves (Spinacia oleracea L. cv. Monatol) were subjected to a freeze-thaw treatment in a buffered medium containing 70 mM KCl, 30 mM NaNO₃ and 20 mM K₂SO₄ in different combinations. In the presence of the three predominant inorganic electrolytes, inactivation of photophosphorylation was mainly caused by a decrease in the capacity of the photosynthetic electron transport; release of proteins from the membranes was not manifest and light-induced H⁺ gradient and proton permeability were largely unaffected. Omission of nitrate from the medium had little effect. When either sulfate or chloride or both were omitted prior to freezing, inactivation of photophosphorylation was correlated with stimulation of the phosphorylating electron flow, marked increase in H⁺ permeability and loss of the ability of the thylakoids to accumulate protons in the light. In the absence of sulfate, uncoupling was mainly a consequence of the dissociation of chloroplast coupling factor (CF₁). Partial restoration of proton impermeability and pH gradient occurred upon the addition of N,N-dicyclohexylcarbodiimide (DCCD). When sulfate was present but chloride omitted, CF₁ remained attached to the membranes and the addition of DCCD had no effect, indicating that the increase in proton efflux was caused by a different mechanism. It is concluded that sulfate stabilizes the CF₁ and prevents its release from the membranes, but KCl is also necessary for maintaining the low permeability of the membranes to protons. The importance of complex media for investigations on isolated biomembrane systems is stressed.Abbreviations CF₁ chloroplast coupling factor - DCCD N,N-dicyclohexylcarbodiimide - Hepes 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid I=Santarius 1986 b     

Heat stress induces an aggregation of the light-harvesting complex of photosystem II in spinach plants

Whole spinach (Spinacia oleracea) plants were subjected to heat stress (25 degrees C-50 degrees C) in the dark for 30 min. At temperatures higher than 35 degrees C, CO2 assimilation rate decreased significantly. The maximal efficiency of photosystem II (PSII) photochemistry remained unchanged until 45 degrees C and decreased only slightly at 50 degrees C. Nonphotochemical quenching increased significantly either in the absence or presence of dithiothreitol. There was an appearance of the characteristic band at around 698 nm in 77 K fluorescence emission spectra of leaves. Native green gel of thylakoid membranes isolated immediately from heat-stressed leaves showed that many pigment-protein complexes remained aggregated in the stacking gel. The analyses of sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting demonstrated that the aggregates were composed of the main light-harvesting complex of PSII (LHCIIb). To characterize the aggregates, isolated PSII core complexes were incubated at 25 degrees C to 50 degrees C in the dark for 10 min. At temperatures over 35 degrees C, many pigment-protein complexes remained aggregated in the stacking gel of native green gel, and immunoblotting analyses showed that the aggregates were composed of LHCIIb. In addition, isolated LHCII was also incubated at 25 degrees C to 50 degrees C in the dark for 10 min. LHCII remained aggregated in the stacking gel of native green gel at temperatures over 35 degrees C. Massive aggregation of LHCII was clearly observed by using microscope images, which was accompanied by a significant increase in fluorescence quenching. There was a linear relationship between the formation of LHCII aggregates and nonphotochemical quenching in vivo. The results in this study suggest that LHCII aggregates may represent a protective mechanism to dissipate excess excitation energy in heat-stressed plants.     

Dithiol oxidant and disulfide reductant dynamically regulate the phosphorylation of light-harvesting complex II proteins in thylakoid membranes

Light-induced phosphorylation of light-harvesting chlorophyll a/b complex II (LHCII) proteins in plant thylakoid membranes requires an activation of the LHCII kinase via binding of plastoquinol to cytochrome b(6)f complex. However, a gradual down-regulation of LHCII protein phosphorylation occurs in higher plant leaves in vivo with increasing light intensity. This inhibition is likely to be mediated by increasing concentration of thiol reductants in the chloroplast. Here, we have determined the components involved in thiol redox regulation of the LHCII kinase by studying the restoration of LHCII protein phosphorylation in thylakoid membranes isolated from high-light-illuminated leaves of pumpkin (Cucurbita pepo), spinach (Spinacia oleracea), and Arabidopsis. We demonstrate an experimental separation of two dynamic activities associated with isolated thylakoid membranes and involved in thiol regulation of the LHCII kinase. First, a thioredoxin-like compound, responsible for inhibition of the LHCII kinase, became tightly associated and/or activated within thylakoid membranes upon illumination of leaves at high light intensities. This reducing activity was completely missing from membranes isolated from leaves with active LHCII protein phosphorylation, such as dark-treated and low-light-illuminated leaves. Second, hydrogen peroxide was shown to serve as an oxidant that restored the catalytic activity of the LHCII kinase in thylakoids isolated from leaves with inhibited LHCII kinase. We propose a dynamic mechanism by which counteracting oxidizing and reducing activities exert a stimulatory and inhibitory effect, respectively, on the phosphorylation of LHCII proteins in vivo via a novel membrane-bound thiol component, which itself is controlled by the thiol redox potential in chloroplast stroma.