CHLOROPLAST MEMBRANE PROTEIN PHOSPHORYLATION

Dynamics of higher plant photosystem cross-section associated with state transitions

Photosynthetic state transitions are a well-known phenomenon of short-term adaptation of the photosynthetic membrane to changes in spectral quality of light in low light environments. The principles of the monitoring and quantification of the process in higher plants are revised here. The use of the low-temperature excitation fluorescence spectroscopy for analysis of the photosystem I antenna cross-section dynamics is described. This cross section was found to increase by 20-25% exclusively due to the migration and attachment of LHCIIb complex in State 2. Analysis of the fine structure of the additional PSI cross-section spectrum revealed the 510 nm band, characteristic of Lutein 2 of LHCIIb and present only when the complex is in a trimeric state. The excitation fluorescence spectrum of the phospho-LHCII resembles the spectrum of aggregated and hence quenched LHCII. This novel observation could explain the fact that at no point in the course of the state transition high fluorescence and long lifetime components of detached trimeric LHCII have ever been observed. In the plants lacking Lhcb1 and 2 proteins and unable to perform state transitions, compensatory sustained adjustments of the photosystem I and II antennae have been revealed. Whilst the major part of the photosystem II antenna is built largely of CP26 trimers, possessing less chlorophyll b and more of the red-shifted chlorophyll a, photosystem I in these plants contains more than 20% of extra LHCI antenna enriched in chlorophyll b. Hence, both photosystems in the plants lacking state transitions have less spectrally distinct antennae, which enable to avoid energy imbalance due to the changes in the light quality. These alterations reveal remarkable plasticity of the higher plant photosynthetic antenna design providing the basis for a flexible adaptation to the light environment.

DeltapH-dependent photosystem II fluorescence quenching induced by saturating, multiturnover pulses in red algae

We have previously shown that in the red alga Rhodella violacea, exposure to continuous low intensities of light 2 (green light) or near-saturating intensities of white light induces a DeltapH-dependent PSII fluorescence quenching. In this article we further characterize this fluorescence quenching by using white, saturating, multiturnover pulses. Even though the pulses are necessary to induce the DeltapH and the quenching, the development of the latter occurred in darkness and required several tens of seconds. In darkness or in the light in the presence of 2, 5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, the dissipation of the quenching was very slow (more than 15 min) due to a low consumption of the DeltapH, which corresponds to an inactive ATP synthase. In contrast, under far-red illumination or in the presence of 3-(3,4-dichlorophenyl)-1,1'-dimethylurea (only in light), the fluorescence quenching relaxed in a few seconds. The presence of N, N'-dicyclohexyl carbodiimide hindered this relaxation. We propose that the quenching relaxation is related to the consumption of DeltapH by ATP synthase, which remains active under conditions favoring pseudolinear and cyclic electron transfer.

Evidence for light-dependent and light-independent protein dephosphorylation in chloroplasts

A number of photosystem II (PSII) associated proteins, including core proteins D1, D2 and CP43, and several proteins of the LHCII complex, are phosphorylated by a thylakoid-bound, redox-regulated kinase(s). We demonstrate here that the compound propyl gallate is an effective inhibitor of LHCII phosphorylation in vivo while having little effect on PSII core protein phosphorylation. Using this inhibitor, we demonstrate that LHCII dephosphorylation is insensitive to light in vivo. Taken together with our previous conclusion (Elich et al., EMBO J. 12 (1993) 4857-4862) that PSII core protein dephosphorylation is light-stimulated, our data suggest the presence of multiple phosphatases responsible for thylakoid protein dephosphorylation in vivo.

Modification of excitation energy distribution to photosystem I by protein phosphorylation and cation depletion during thylakoid biogenesis in wheat

The effects of protein phosphorylation and cation depletion on the electron transport rate and fluorescence emission characteristics of photosystem I at two stages of chloroplast development in light-grown wheat leaves are examined. The light-harvesting chlorophyll a/b protein complex associated with photosystem I (LHC I) was absent from the thylakoids at the early stage of development, but that associated with photosystem II (LHC II) was present. Protein phosphorylation produced an increase in the light-limited rate of photosystem I electron transport at the early stage of development when chlorophyll b was preferentially excited, indicating that LHC I is not required for transfer of excitation energy from phosphorylated LHC II to the core complex of photosystem I. However, no enhancement of photosystem I fluorescence at 77 K was observed at this stage of development, demonstrating that a strict relationship between excitation energy density in photosystem I pigment matrices and the long-wavelength fluorescence emission from photosystem I at 77 K does not exist. Depletion of Mg(2+) from the thylakoids produced a stimulation of photosystem I electron transport at both stages of development, but a large enhancement of the photosystem I fluorescence emission was observed only in the thylakoids containing LHC I. It is suggested that the enhancement of PS I electron transport by Mg(2+)-depletion and phosphorylation of LHC II is associated with an enhancement of fluorescence at 77 K from LHC I and not from the core complex of PS I.

Photoinhibition, 77K chlorophyll fluorescence quenching and phosphorylation of the light-harvesting chlorophyll-protein complex of photosystem II in soybean leaves

When the capacity of leaves for orderly dissipation of excitation energy in photosynthesis is exceeded, one mechanism by which the excess energy appears to be dissipated is through a nonradiative decay process. This process is observed as a reversible quenching of chlorophyll fluorescence emission (77K) from both photosystem II and photosystem I which persists in darkness (Demmig and Björkman 1987, Planta 171, 171-184). Fluorescence quenching was induced in soybean (Glycine max (L.) Merr.) leaves by two methods: 1) changing the composition of the gas surrounding the leaf from normal air to 2% O2, 0% CO2 at a low, constant photon flux density (PFD=photon fluence rate), and 2) increasing the PFD in the presence of normal air. In either case the quenching was fully reversible after return to the original condition (low PFD, normal air). The half-time of the relaxation of the quenching was in the order of 30 min. Both treatments resulted in reversible dephosphorylation of the light-harvesting chlorophyll-protein complex of photosystem II (LHC-II). Treatment under photoinhibitory conditions (high PFD plus chloramphenicol) also caused dephosphorylation of LHC-II. Therefore, phosphorylation of LHC-II cannot account for the observed fluorescence quenching. In addition, our results indicate that in vivo a factor other than the redox state of the plastoquinone pool controls LHC-II phosphorylation. This factor may be ΔpH, the pH gradient across the thylakoid membranes.

High-temperature damage and acclimation of the photosynthetic apparatus : II. Effect of mono- and divalent cations and pH on the temperature sensitivity of some functional characteristics of chloroplasts isolated from heat-acclimated and non-acclimated bean plants

The influence of mono- (K(+)) and divalent (Mg(2+)) cations and protons (pH) on the temperature sensitivity of thylakoid membranes was investigated in three groups of young bean plants (control, heat-acclimated and non-acclimated). Thylakoid-membrane function was monitored by second and millisecond delayed fluorescence and 9-aminoacridine fluorescence quenching. It was established that metal ions at investigated concentrations decreased the thermostability of the photosynthetic parameters - an increase of MgSO4 concentration from 0.1 to 20 mM decreased the temperature of their half-inactivation (T50) by 13°C. At the same time the pH dependence of the thermal stability of these parameters showed a maximum at pH 5.5-6.5. The half-inactivation temperatures of those photosynthetic parameters connected with the ability of the thylakoid membrane to form light-induced proton gradients increased by 6-7°C in the heat-acclimated plants compared with the control. It was assumed that the temperature inactivation of photosynthetic electron transfer and the energization of the thylakoid membrane was determined both by the thermoinduced dissociation of the light-harvesting chlorophyll a/b protein complex from PSII, leading to destruction of the excitation energy transfer to the reaction centres, and by the thermal denaturation of the membrane-protein components. The rate of these processes was probably controlled by the size of the negative surface charge and the viscosity of the thylakoid membrane.

Correlation between thylakoid protein phosphorylation and molecular organization of the photosynthetic apparatus in a dynamic system

In-vitro thylakoid protein phosphorylation has been studied in synchronized cells of Scenedesmus obliquus at the 8- and 16-h of the life cycle, stages which are characterized by the maximum and minimum photosynthetic activities, respectively. The stage of maximum photosynthetic activity (8-h) is characterized by the highest protein phosphorylation in vitro and in vivo, by the largest proportion of the heavy subfraction of thylakoids, and by maximum oligomerization of the light-harvesting chlorophyll a/b-protein complex, altogether creating the highest energy charge of the thylakoid membranes. Protein phosphorylation in vitro decreases the amount of the heavy subfraction and increases the amount of oligomerization of the antenna of photosystem I (PSI) (increase of chlorophyll b in the light fraction). Concomittantly, PSII units become smaller (longer time for the rise in fluorescence induction) and photosynthetic efficiency increases (decrease of fluorescence yield). In-vivo protein phosphorylation is controlled mainly endogenously during the 8-h of the life cycle but is exogenously modulated by light to optimize the photosynthetic activity by redistribution of pigment-protein complexes. In-vitro protein phosphorylation seems to restore partially the conditions prevalent in vivo and lost during the preparation of membranes. The effect is greater in 16-h cells which have less-stable membranes. The regulatory mechanism between membrane stabilization and oligomerization on the one hand and redistribution of the light-harvesting chlorophyll a/b-protein complex from PSII to PSI on the other hand remains unexplained. We have confirmed that the mechanism of protein phosphorylation is regulated via plastohydroquinone, but experiments with the plastohydroquinone analogue 2,3,5,6-tetramethyl-p-benzoquinone demonstrated that plastohydroquinone is not solely responsible for the differences in protein phosphorylation of 8- and 16-h thylakoids. The inhibitory effect of ADP and the distinct rates of kinase reaction indicate that the adenylate energy charge and changes in the organization of the photosynthetic apparatus also contribute to the observed differences in protein phosphorylation. Phosphorylation in the presence of 3-(3',4'-dichlorophenyl)-1,1-dimethylurea indicated that the 32-kDa phosphoprotein and the herbicide-binding QB protein may be the same. These experiments also indicated that 3-(3',4'-dichlorophenyl)-1,1-dimethylurea-binding reduces kinase activity directly and not only by inhibiting electron transport.

State 1/State 2 changes in higher plants and algae

Current ideas regarding the molecular basis of State 1/State 2 transitions in higher plants and green algae are mainly centered around the view that excitation energy distribution is controlled by phosphorylation of the light-harvesting complex of photosystem II (LHC-II). The evidence supporting this view is examined and the relationship of the transitions occurring in these systems to the corresponding transitions seen in red and blue-green algae is explored.

Phosphorylation of thylakoid proteins during chloroplast biogenesis in greening etiolated and light-grown wheat leaves

Phosphorylation of polypeptides in isolated thylakoids was examined during chloroplast biogenesis in greening etiolated wheat leaves and 4 day-old wheat leaves grown under a diurnal light regime. At early stages of plastid development standard thylakoid preparations were heavily contaminated with nuclear proteins, which distorted the polypeptide phosphorylation profiles. Removal of contamination from membranes by sucrose density centrifugation demonstrated that the major membrane phosphoprotein in etioplasts was at 35 kDa. During etioplast greening a number of phosphoproteins appeared, of which the 25-27 kDa apoproteins of the light-harvesting chlorophylla/b protein complex associated with photosystem II (LHCII) became the most dominant. At the early stages of thylakoid development found at the base of the 4-day-old light grown leaf the LHCII apoproteins were evident as phosphoproteins; however the major phosphoprotein was polypeptide atca. 9kDA. Phosphorylation of both the LHCII apoproteins and the 9 kDa polypeptide in these thylakoids was not light-dependent. In the older thylakoids isolated from the leaf tip the LHCII apoproteins were the major phosphoproteins and their phosphorylation had become light-regulated; however phosphorylation of the 9 kDa polypeptide remained insensitive to light.

Chlorophyll fluorescence transients from the diatom Phaeodactylum tricornutum: relative rates of cyclic phosphorylation and chlororespiration

In Phaeodactylum tricornutum cells kept 30 min in the dark, induction of fluorescence showed the well-known levels OIDPSMT. The decrease of MT was the most important when the intensity of excitation light was high. It was mainly due to the photochemical quenching. After addition of DCMU (2 to 20 μM), a quenching qE was still observed: this quenching, cancelled by NH4Cl (2 to 20 mM) is attributed to ΔpH. This qE was also inhibited by antimycin, an inhibitor of cyclic phosphorylation and may be of chlororespiration above plastoquinones. Anaerobiosis also decreased it. We can infer that chlororespiration also plays a part in the formation of the ΔpH in the presence of DCMU. After 30 mn of preillumination in red light, the levels P and M were lower and the quenching in presence of DCMU was no more observed: thus, neither the chlororespiration nor the cyclic phosphorylation were active, unless the activity of ATPase was much more important. So, in diatoms, one at least of the above cited phenomena can be modulated by light.

Studies on thylakoid phosphorylation and noncyclic electron transport

The effect of thylakoid phosphorylation on noncyclic electron transport in spinach chloroplasts was investigated by measuring both the reduction of nicotinamide adenine dinucleotide phosphate (NADP) and the steady-state redox level of the primary electron acceptor quinone of photosystem II (Q) during electron flow to NADP. These data are compared with the theoretical predictions for an electron transport model which relates both the redox levels of Q and the photosystem II optical cross section to the overall velocity of noncyclic electron flow. It is demonstrated that transfer of 15-20% of the photosystem II antenna to photosystem I may stimulate electron flow to NADP only if Q is less than 60-70% oxidized (this condition exists with our thylakoids, even at extremely low absorption fluxes, when the illumination is not specifically enriched in photosystem I absorbed wavelengths); in phosphorylated thylakoids the steady-state redox level Q is substantially shifted to a more oxidized one (measurements of this parameter using light of different wavelengths quantitatively support the idea that thylakoid phosphorylation leads to increased photosystem I and decreased photosystem II cross sections); thylakoid phosphorylation leads to stimulated noncyclic electron flow to NADP only when the increased photosystem I antenna is able to bring about large increases in the steady-state level of oxidized Q.

Thylakoid-protein phosphorylation during the life cycle of Scenedesmus obliquus in synchronous culture

The phosphorylation of thylakoid proteins, which comprise apoproteins of the light-harvesting chlorophyll a/b-protein complex (LHCP), was investigated in vivo and in vitro during the development of Scenedesmus obliquus in synchronous cultures. The in-vitro and in-vivo protein phosphorylation exhibited a maximum activity in cells with maximum photosynthetic capacity (8th hour) and miximum activity in cells with minimum photosynthetic capacity (16th hour). The major phosphorylated polypeptides in vivo were the 24/25-kDa and 28-30-kDa apoprotein of the LHCP, a protein of about 32 kDa, and some smaller polypeptides within the range 10 to 20 kDa. In vitro, the main phosphoproteins were the 28-30-kDa apoprotein and the protein characterized by an apparent molecular weight of 32 kDa. Pulse-chase experiments in vivo established that the latter had the fastest radioactivity turnover of the thylakoidal phosphoproteins.

Regulation of energy transfer by cations and protein phosphorylation in relation to thylakoid membrane organisation

A brief review is given of the state of knowledge which indicates that the State I-State II transition in higher plants and green algae is due to the reversible phosphorylation of the chlorophyll a/b light harvesting complex. The importance of membrane reorganisational changes in this process is discussed in terms of changes in electrostatic parameters as emphasised by the interplay of the effect of phosphorylation and the background levels of cations surrounding the membrane. It is argued that recognition of this interplay is vital when using the bipartite or tripartite models of Butler to obtain quantitative information of energy transfer between the various pigment complexes.

Purification and partial sequence of the Mr 10,000 phosphoprotein from spinach thylakoids

The Mr 10,000 phosphoprotein was purified from photosystem II particles by solubilization of the particles in 5% (w/v) dodecyl dimethylamine oxide, centrifugation in 10% (w/v) sucrose, and three chromatography steps. The purified phosphoprotein showed a unique NH2 terminus indicating a highly purified polypeptide. The amino acid sequence for the first nine residues is NH2-Ala-Thr-Gln-Thr-Val-Glu-Ser-Ser-Ser . . . COOH. The amino acid composition was determined and could also be used to help distinguish the polypeptide from other known thylakoid proteins. The sequence and composition data indicated that the Mr 10,000 phosphoprotein is neither the hydrophobic 8-kDa subunit of the energy coupling complex nor cytochrome b-559, but rather a unique, as yet unidentified, polypeptide associated with photosystem II.

Inhibition of photosynthetic reactions under water stress: interaction with light level

When the shrub Nerium oleander L., growing under full natural daylight outdoors, was subjected to water stress, stomatal conductance declined, and so did non-stomatal components of photosynthesis, including the CO2-saturated rate of CO2 uptake by intact leaves and the activity of electron transport by chloroplasts isolated from stressed plants. This inactivation of photosynthetic activity was accompanied by changes in the fluorescence characteristics determined at 77 K (-196°C) for the upper leaf surface and from isolated chloroplasts. The maximum (F M) and the variable (F V) fluorescence yield at 692 nm were strongly quenched but there was little effect on the instantaneous (F O) fluorescence. There was a concomitant quenching of the maximum and variable fluorescence at 734 nm. These results indicate an inactivation of the primary photochemistry associated with photosystem II. The lower, naturally shaded surfaces of the same leaves were much less affected than the upper surfaces and water-stress treatment of plants kept in deep shade had little or no effect on the fluorescence characteristics of either surface, or of chloroplasts isolated from the water-stressed leaves. The effects of subjecting N. oleander plants, growing in full daylight, to water stress are indistinguishable from those resulting when plants, grown under a lower light regime, are exposed to full daylight (photoinhibition). Both kinds of stress evidently cause an inactivation of the primary photochemistry associated with photosystem II. The results indicate that water stress predisposes the leaves to photoinhibition. Recovery from this inhibition, following restoration of favorable water relations, is very slow, indicating that photoinhibition is an important component of the damage to the photosynthetic system that takes place when plants are exposed to water stress in the field. The underlying causes of this water-stress-induced susceptibility to photoinhibition are unknown; stomatal closure or elevated leaf temperature cannot explain the increased susceptibility.

Studies on the relation of the Mr 9000 phosphoprotein to cytochrome b-559 in spinach thylakoid membranes

Cytochrome b-559 was purified from phosphorylated spinach chloroplast thylakoids after activation of kinase activity in the presence of [gamma-32P]ATP in order to determine whether the 9-kDa phosphoprotein in these membranes arises from phosphorylation of the cytochrome b-559. It was established in this work that the 9-kDa phosphoprotein, like the cytochrome b-559 polypeptide, is a PS II component, and that these two proteins migrate very similarly on denaturing gels. However, the initial 2% Triton-4 M urea membrane extract contains most of the cytochrome b-559 and little 32P. A substantially larger amount of stable 32P-labeled 9-kDa phosphoprotein fraction is found in the material that is insoluble in the 2% Triton-4 M urea. Furthermore, the ratio of 32P:heme in cytochrome b-559 purified in the presence of protease inhibitors from phosphorylated membranes was on the order of 1% of that expected if cytochrome b-559 were the sole source of the radiolabel seen in the 9-kDa band. The differential extraction properties of the 32P-labeled 9-kDa phosphoprotein and cytochrome b-559, and the stoichiometry of 32P:heme in the purified cytochrome appear to exclude the cytochrome as a candidate for the 9-kDa phosphoprotein.

Influence of carbon dioxide concentration during growth on fluorescence induction characteristics of the Green Alga Chlamydomonas reinhardii

Carbon dioxide concentration during growth is commonly not considered to be a factor influencing the photochemical properties of plants. It was observed that fluorescence induction in Chlamydomonas reinhardii cells grown at air levels of CO2 was both qualitatively and quantitatively different from that of cells grown at 5% CO2. In the two cell types, measured at equivalent chlorophyll and irradiance levels, the fluorescence intensity and the ratio of the levels of peak fluorescence (Fp) to that of the initial fluorescence (Fo) were much lower in the air-adapted than in the 5% CO2 adapted cells. The maximum fluorescence (Fmax) in the presence of diuron was also lower for air-adapted cells. Roughly twice the light input was required for the air-adapted cells to give a fluorescence induction transient and intensity equivalent to that of the 5% CO2-adapted cells. Similar properties were observed in several other unicellular green algae and in cyanobacteria. Chlamydomonas grown under variable CO2 concentrations exhibit significant differences in photosynthetic carbon metabolism and are presumed to have altered energy requirements. The observed variation in fluorescence induction may be due to changes in the properties of the thylakoid reactions (e.g. cyclic electron flow) of Chlamydomonas cells, which may, in turn, be due to a response to the altered energy requirements.

Chloroplast thylakoid membrane fluidity and its sensitivity to temperature

In order to investigate membrane fluidity, the hydrophobic probe, 1,6-diphenyl-1,3,5-hexatriene (DPH), has been incorporated into intact isolated thylakoids and separated granal and stromal lamellae obtained from the chloroplasts of Pisum sativum. The steady-state polarization of DPH fluorescence was measured as a function of temperature and indicated that at physiological values the thylakoid membrane is a relatively fluid system with the stromal lamellae being less viscous than the lamellae of the grana. According to the DPH technique, neither region of the membrane, however, showed a sharp phase transition of its bulk lipids from the liquid-crystalline to the gel state for the temperature range -20° to 50° C. Comparison of intact thylakoids isolated from plants grown at cold (4°/7°C) and warm (14°/17° C) temperatures indicate that there is an adaptation mechanism operating which seems to maintain an optimal membrane viscosity necessary for growth. Using a modified Perrin equation the optimal average viscosity for the thylakoid membrane of the chill-resistant variety used in the study (Feltham First) is estimated to be about 1.8 poise.