Kinase-induced changes in electron transport rates of spinach chloroplasts

Sharing light between two photosystems: mechanism of state transitions

In the thylakoid membrane, the two photosystems act in series to promote linear electron flow, with the concomitant production of ATP and reducing equivalents such as NADPH. Photosystem I, which is preferentially activated in far-red light, also energizes cyclic electron flow which generates only ATP. Thus, changes in light quality and cellular metabolic demand require a rapid regulation of the activity of the two photosystems. At low light intensities, this is mediated by state transitions. They allow the dynamic allocation of light harvesting antennae to the two photosystems, regulated through protein phosphorylation by a kinase and phosphatase pair that respond to the redox state of the electron transfer chain. Phosphorylation of the antennae leads to remodeling of the photosynthetic complexes.

Evidence that phosphorylation and dephosphorylation regulate the distribution of excitation energy between the two photosystems of photosynthesis in vivo: Photoacoustic and fluorimetric study of an intact leaf

State 1-state 2 transitions in an intact tobacco leaf were monitored by the photoacoustic method. Modulated oxygen evolution yield and its enhancement by continuous far-red light ("Emerson enhancement") were used to characterize the balance of light distribution between the two photosystems. These measurements were additionally supported by fluorimetry. Adaptation of the leaf to far-red light (lambda [unk] 700 nm), mainly absorbed in photosystem I (light 1), results in state 1, where short-wavelength light (light 2) is distributed in favor of photosystem II. This is shown by a low yield of oxygen evolution, a high extent of Emerson enhancement, a concomitantly high extent of fluorescence quenching by far-red light, and a low ratio of the 77 K emission peaks at 730 and 695 nm. The magnitudes of these parameters were reversed when the leaf was adapted to light 2 (state 2), indicating a change towards a more equal distribution of the excitation between the two photosystems. Preincubation of an intact leaf with NaF, a specific phosphatase inhibitor, stimulated the extent of adaptation to light 2, shown by all the above criteria, and completely abolished adaptation to light 1. Light 1 preillumination prior to NaF treatment resulted initially in state 1, but then a transition to state 2 was irreversibly induced by any light. The NaF effect was specific because NaCl did not affect the state 1-state 2 transitions. Leaching out the NaF restored the original physiological transitions of the leaf. NaF presumably acts here in the same way as it acts in isolated thylakoids-by blocking the dephosphorylation of membranal proteins (particularly the chlorophyll a/b-protein complex) phosphorylated by a light 2-activated kinase. Our results give direct support to the suggestion [Allen, J. F., Bennett, J., Steinback, K. E. & Arntzen, C. J. (1981) Nature (London) 291, 25-29] that it is the phosphorylation level of thylakoid proteins that controls the light distribution between the two photosystems in vivo, shown previously in isolated thylakoids.

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.

Protein phosphorylation and Mg2+ influence light harvesting and electron transport in chloroplast thylakoid membrane material containing only the chlorophyll-a/b-binding light-harvesting complex of photosystem II and photosystem I

A material containing only photosystem I (PSI) and the chlorophyll-a/b-binding light-harvesting complex of PSII (LHC-II) has been isolated from the chloroplast thylakoid membrane by solubilization with Triton X-100. Fluorescence spectroscopy shows that, within the material, LHC-II is coupled to PSI for excitation-energy transfer and that this coupling is decreased by the presence of Mg2+, which also decreased PSI electron transport specifically at limiting light intensity. Inclusion of phosphorylated LHC-II within the material did not alter its structure, but gave decreased energy transfer to PSI and inhibition of electron transport which was independent of light intensity, implying effects of phosphorylation on both light harvesting and directly on electron transport. Inclusion of Mg2+ within the phosphorylated material gave decreased energy transfer, but slightly increased PSI electron transport. A cation-induced direct promotion of PSI electron transport was also observed in isolated PSI particles. The PSI/LHC-II material represents a model system for examining protein interactions during light-state adaptations and the possibility that LHC-II can contribute to the antenna of PSI in light state 2 in vivo is discussed.

Quantitative study of state 1-state 2 transitions in broken chloroplasts-comparison to in-vivo properties

A detailed quantitative study was conducted on state 1-state 2 transition and its reversal in broken chloroplasts by modulated fluorimetry. The characteristics of the transition obtained supported other previous in-vitro findings. More importantly, a very close quantitative similarity was obtained under suitable conditions to previous in-vivo studies, particularly in approaching a constancy of Fm/F0 during the transition and the equality of the fractional change of these fluorescence parameters with the calculated light distribution fraction to PS II. This confirms that in broken chloroplasts too, the state transitions involve reciprocal changes in the absorption cross-sections of PS II and PS I.

State transitions, photosystem stoichiometry adjustment and non-photochemical quenching in cyanobacterial cells acclimated to light absorbed by photosystem I or photosystem II

Cells of the cyanobacterium Synechococcus 6301 were grown in yellow light absorbed primarily by the phycobilisome (PBS) light-harvesting antenna of photosystem II (PS II), and in red light absorbed primarily by chlorophyll and, therefore, by photosystem I (PS I). Chromatic acclimation of the cells produced a higher phycocyanin/chlorophyll ratio and higher PBS-PS II/PS I ratio in cells grown under PS I-light. State 1-state 2 transitions were demonstrated as changes in the yield of chlorophyll fluorescence in both cell types. The amplitude of state transitions was substantially lower in the PS II-light grown cells, suggesting a specific attenuation of fluorescence yield by a superimposed non-photochemical quenching of excitation. 77 K fluorescence emission spectra of each cell type in state 1 and in state 2 suggested that state transitions regulate excitation energy transfer from the phycobilisome antenna to the reaction centre of PS II and are distinct from photosystem stoichiometry adjustments. The kinetics of photosystem stoichiometry adjustment and the kinetics of the appearance of the non-photochemical quenching process were measured upon switching PS I-light grown cells to PS II-light, and vice versa. Photosystem stoichiometry adjustment was complete within about 48 h, while the non-photochemical quenching occurred within about 25 h. It is proposed that there are at least three distinct phenomena exerting specific effects on the rate of light absorption and light utilization by the two photoreactions: state transitions; photosystem stoichiometry adjustment; and non-photochemical excitation quenching. The relationship between these three distinct processes is discussed.

Regulation of excitation energy transfer in organisms containing phycobilins

The mechanism of excitation energy redistribution (state transition) in organisms containing phycobilins is reviewed. Recent measurements using time-resolved fluorescence spectroscopy in the picosecond range confirm that the state transition in cyanobacteria and red algae is controlled by changes in the kinetics of energy transfer from PS 2 to PS 1 (spillover) rather than by physical dislocation of the phycobilisome and reassociation between the two photosystems (mobile antenna model). Contrary to the analogous situation in higher plants, there is no compelling evidence for the involvement of a protein phosphorylation event in the rapid time range of the state transition, but a variety of data indicate that a membrane conformational change occurs that might change the relative distance between, and/or orientation of the two photosystems within the thylakoid. The state transition is most probably initiated by the redox state of the intersystem electron transport chain, and the conversion to state 1 is driven by coupled PS1 cyclic electron transport. The cryptomonads also undergo wavelength dependent changes in excitation energy distribution by a mechanism very similar to that observed in the red algae and cyanobacteria. However, the changes in energy distribution in this group are most likely related to a photoprotection mechanism for PS2 rather than to a state transition.

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.

Changes in topography and function of thylakoid membranes following membrane protein phosphorylation

Changes in topography and function of pea (Pisum sativum L.) thylakoid membrane fractions following membrane protein phosphorylation have been studied. After protein phosphorylation the stromal membrane fraction had a higher chlorophyll a/b ratio, an increased content of light-harvesting chlorophyll protein and a higher ratio of chlorophyll to cytochrome f. This indicates that a pool of light-harvesting chlorophyll protein migrates from the photosystem II-enriched grana regions to the photosystem I-enriched stroma lamellae, in agreement with Kyle et al. (1984, Biochim. Biophys. Acta 765, 89-96) and Larsson et al. (1983, Eur. J. Biochem. 136, 25-29). Phosphorylation caused a stimulation in the rate of light-limited photosystem-I electron transfer in the unappressed membrane fraction, indicating that the translocated LHC-II becomes functionally associated with photosystem I.

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.

Influence of thylakoid protein phosphorylation on photosynthetic electron transport and photophosphorylation

Data are reported which show that thylakoid protein phosphorylation decreases photosystem II fluorescence yield and enhances the photosystem I dependent photophosphorylation catalyzed by phenazinemethosulphate in the presence of DCMU. The stimulation is larger at low light intensity, but is still observed at high intensity. These observations are interpreted to demonstrate that thylakoid protein phosphorylation causes a transfer of excitation energy from PS II to PS I, but may also have an independent stimulatory effect on PS I dependent photophosphorylation.

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.

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.

Regulation of chloroplast membrane function: protein phosphorylation changes the spatial organization of membrane components

A chlorophyll-protein complex of chloroplast membranes, which simultaneously serves as light-harvesting antenna and membrane adhesion factor, undergoes reversible, lateral diffusion between appressed and nonappressed membrane regions under the control of a protein kinase. The phosphorylation-dependent migration process regulates the amount of light energy that is delivered to the reaction centers of photosystems I and II (PS I and PS II), and thereby regulates their rate of turnover. This regulatory mechanism provides a rationale for the finding that the two photosystems are physically separated in chloroplast membranes (PS II in appressed, grana membranes, and PS I in nonappressed, stroma membranes). The feedback system involves the following steps: a membrane-bound kinase senses the rate of PS II vs. PS I turnover via the oxidation-reduction state of the plastoquinone pool, which shuttles electrons from PS II via cytochrome f to PS I. If activated, the kinase adds negative charge (phosphate) to a grana-localized pigment-protein complex. The change in its surface charge at a site critical for promoting membrane adhesion results in increased electrostatic repulsion between the membranes, unstacking, the lateral movement of the complex to adjacent stroma membranes, which differ in their functional composition. The general significance of this type of membrane regulatory mechanism is discussed.

Changes in the lateral distribution of the light-harvesting chlorophyll-a/b--protein complex induced by its phosphorylation

The effect of protein phosphorylation on the distribution of chlorophyll-protein complexes between appressed and non-appressed thylakoid regions of spinach chloroplasts has been investigated. Stroma lamellae vesicles and inside-out vesicles, representative of non-appressed and appressed thylakoids, respectively, were isolated from thylakoid membranes before and after phosphorylation. The fractions were analyzed with respect to the yield of vesicles, incorporation of [32P]phosphate into the light-harvesting chlorophyll-a/b--protein complex, chlorophyll composition, and the relative content of the main chlorophyll-protein complexes. The yield of inside-out vesicles from phosphorylated thylakoids was 20% lower than that from control thylakoids, indicating a partial destacking. The specific incorporation of [32P]phosphate into the light-harvesting chlorophyll-a/b--protein complex was at least four times higher in stroma lamellae vesicles than in inside-out vesicles. The proportion of the light-harvesting chlorophyll-a/b--protein complex in stroma lamellae vesicles increased from 13% to 21% of their total chlorophyll after phosphorylation of the thylakoids. There was also a corresponding increase in the level of chlorophyll b. These observations strongly suggest that phosphorylated light-harvesting complexes migrate from appressed thylakoids rich in photosystem 2 to non-appressed thylakoids rich in photosystem 1. In contrast, there was no evidence for a lateral migration of the chlorophyll-a--protein complex of photosystem 2 after phosphorylation. Our results indicate that a lateral migration of phosphorylated light-harvesting complexes in combination with a lateral separation of photosystem 1 and 2 to the different thylakoid regions, is a basis for the regulation of excitation energy between the two photosystems.

Protein phosphorylation-induced State I-State II transitions are dependent on thylakoid membrane microviscosity

Incorporation of cholesterol hemisuccinate into thylakoid membranes decreased the membrane fluidity as measured by polarized fluorescence from 1,6-diphenyl-1,3,5-hexatriene. Increasing membrane viscosity in this manner did not inhibit the thylakoid membrane protein kinase. In contrast the effects of the protein phosphorylation on State I-State II transitions, which were observed in untreated membranes, were abolished. This observation is interpreted as indicating that protein phosphorylation-induced energy transfer changes are sensitive to membrane viscosity because they might require a lateral migration of the light-harvesting complex serving Photosystem II from grana to stromal lamellae. Cation effects on room- and low-temperature fluorescence emission properties and membrane adhesion were not abolished in these cholesterol hemisuccinate-treated membranes.

Regulation of photosynthesis by reversible phosphorylation of the light-harvesting chlorophyll a/b protein

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