Phosphorylation of chlamydomonas reinhardi chloroplast membrane proteins in vivo and in vitro

Thylakoid Protein Phosphorylation in Chloroplasts

Because of their abundance and extensive phosphorylation, numerous thylakoid proteins stand out amongst the phosphoproteins of plants and algae. In particular, subunits of light-harvesting complex II (LHCII) and of photosystem II (PSII) are dynamically phosphorylated and dephosphorylated in response to light conditions and metabolic demands. These phosphorylations are controlled by evolutionarily conserved thylakoid protein kinases and counteracting protein phosphatases, which have distinct but partially overlapping substrate specificities. The best characterized are the kinases STATE TRANSITION 7 (STN7/STT7) and STATE TRANSITION 8 (STN8), and the antagonistic phosphatases PROTEIN PHOSPHATASE 1/THYLAKOID-ASSOCIATED PHOSPHATASE 38 (PPH1/TAP38) and PHOTOSYSTEM II CORE PHOSPHATASE (PBCP). The phosphorylation of LHCII is mainly governed by STN7 and PPH1/TAP38 in plants. LHCII phosphorylation is essential for state transitions, a regulatory feedback mechanism that controls the allocation of this antenna to either PSII or PSI, and thus maintains the redox balance of the electron transfer chain. Phosphorylation of several core subunits of PSII, regulated mainly by STN8 and PBCP, correlates with changes in thylakoid architecture, the repair cycle of PSII after photodamage as well as regulation of light harvesting and of alternative routes of photosynthetic electron transfer. Other kinases, such as the PLASTID CASEIN KINASE II (pCKII), also intervene in thylakoid protein phosphorylation and take part in the chloroplast kinase network. While some features of thylakoid phosphorylation were conserved through the evolution of photosynthetic eukaryotes, others have diverged in different lineages possibly as a result of their adaptation to varied environments.

The Use of Advanced Mass Spectrometry to Dissect the Life-Cycle of Photosystem II

Photosystem II (PSII) is a photosynthetic membrane-protein complex that undergoes an intricate, tightly regulated cycle of assembly, damage, and repair. The available crystal structures of cyanobacterial PSII are an essential foundation for understanding PSII function, but nonetheless provide a snapshot only of the active complex. To study aspects of the entire PSII life-cycle, mass spectrometry (MS) has emerged as a powerful tool that can be used in conjunction with biochemical techniques. In this article, we present the MS-based approaches that are used to study PSII composition, dynamics, and structure, and review the information about the PSII life-cycle that has been gained by these methods. This information includes the composition of PSII subcomplexes, discovery of accessory PSII proteins, identification of post-translational modifications and quantification of their changes under various conditions, determination of the binding site of proteins not observed in PSII crystal structures, conformational changes that underlie PSII functions, and identification of water and oxygen channels within PSII. We conclude with an outlook for the opportunity of future MS contributions to PSII research.

The Antarctic Psychrophile Chlamydomonas sp. UWO 241 Preferentially Phosphorylates a Photosystem I-Cytochrome b6/f Supercomplex

Chlamydomonas sp. UWO 241 (UWO 241) is a psychrophilic green alga isolated from Antarctica. A unique characteristic of this algal strain is its inability to undergo state transitions coupled with the absence of photosystem II (PSII) light-harvesting complex protein phosphorylation. We show that UWO 241 preferentially phosphorylates specific polypeptides associated with an approximately 1,000-kD pigment-protein supercomplex that contains components of both photosystem I (PSI) and the cytochrome b₆/f (Cyt b₆/f) complex. Liquid chromatography nano-tandem mass spectrometry was used to identify three major phosphorylated proteins associated with this PSI-Cyt b₆/f supercomplex, two 17-kD PSII subunit P-like proteins and a 70-kD ATP-dependent zinc metalloprotease, FtsH. The PSII subunit P-like protein sequence exhibited 70.6% similarity to the authentic PSII subunit P protein associated with the oxygen-evolving complex of PSII in Chlamydomonas reinhardtii. Tyrosine-146 was identified as a unique phosphorylation site on the UWO 241 PSII subunit P-like polypeptide. Assessment of PSI cyclic electron transport by in vivo P700 photooxidation and the dark relaxation kinetics of P700(+) indicated that UWO 241 exhibited PSI cyclic electron transport rates that were 3 times faster and more sensitive to antimycin A than the mesophile control, Chlamydomonas raudensis SAG 49.72. The stability of the PSI-Cyt b₆/f supercomplex was dependent upon the phosphorylation status of the PsbP-like protein and the zinc metalloprotease FtsH as well as the presence of high salt. We suggest that adaptation of UWO 241 to its unique low-temperature and high-salt environment favors the phosphorylation of a PSI-Cyt b₆/f supercomplex to regulate PSI cyclic electron transport rather than the regulation of state transitions through the phosphorylation of PSII light-harvesting complex proteins.

Isolation, characterization, phosphorylation and site of synthesis of Spinacia chloroplast ribosomal proteins

We have characterized the ribosomal proteins from Spinacia chloroplasts using two-dimensional gel electrophoresis. The 30S and 50S subunits contain 23-25 and 36 ribosomal proteins, respectively. In contrast to prokaryotic ribosomes, chloroplast ribosomes contain at least one (and possibly two) phosphorylated ribosomal proteins. Isolated chloroplasts synthesize in the presence of ((35)S) labeled methionine and cysteine at least seven 30S and thirteen 50S ribosomal proteins which are assembled into (pre)ribosomes. This suggests that about one third of the chloroplast ribosomal proteins is encoded by the chloroplast DNA itself. The identity of several labeled proteins in the two-dimensional gel electrophoretic patterns which did not comigrate with stained chloroplast ribosomal proteins is discussed.

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.

Environmentally modulated phosphoproteome of photosynthetic membranes in the green alga Chlamydomonas reinhardtii

Mapping of in vivo protein phosphorylation sites in photosynthetic membranes of the green alga Chlamydomonas reinhardtii revealed that the major environmentally dependent changes in phosphorylation are clustered at the interface between the photosystem II (PSII) core and its light-harvesting antennae (LHCII). The photosynthetic membranes that were isolated form the algal cells exposed to four distinct environmental conditions affecting photosynthesis: (i) dark aerobic, corresponding to photosynthetic State 1; (ii) dark under nitrogen atmosphere, corresponding to photosynthetic State 2; (iii) moderate light; and (iv) high light. The surface-exposed phosphorylated peptides were cleaved from the membrane by trypsin, methyl-esterified, enriched by immobilized metal affinity chromatography, and sequenced by nanospray-quadrupole time-of-flight mass spectrometry. A total of 19 in vivo phosphorylation sites were mapped in the proteins corresponding to 15 genes in C. reinhardtii. Amino-terminal acetylation of seven proteins was concomitantly determined. Sequenced amino termini of six mature LHCII proteins differed from the predicted ones. The State 1-to-State 2 transition induced phosphorylation of the PSII core components D2 and PsbR and quadruple phosphorylation of a minor LHCII antennae subunit, CP29, as well as phosphorylation of constituents of a major LHCII complex, Lhcbm1 and Lhcbm10. Exposure of the algal cells to either moderate or high light caused additional phosphorylation of the D1 and CP43 proteins of the PSII core. The high light treatment led to specific hyperphosphorylation of CP29 at seven distinct residues, phosphorylation of another minor LHCII constituent, CP26, at a single threonine, and double phosphorylation of additional subunits of a major LHCII complex including Lhcbm4, Lhcbm6, Lhcbm9, and Lhcbm11. Environmentally induced protein phosphorylation at the interface of PSII core and the associated antenna proteins, particularly multiple differential phosphorylations of CP29 linker protein, suggests the mechanisms for control of photosynthetic state transitions and for LHCII uncoupling from PSII under high light stress to allow thermal energy dissipation.

CO2 limitation induces specific redox-dependent protein phosphorylation inChlamydomonas reinhardtii

Acclimation of the green alga Chlamydomonas reinhardtii to limiting environmental CO2 induced specific protein phosphorylation at the surface of photosynthetic thylakoid membranes. Four phosphopeptides were identified and sequenced by nanospray quadrupole TOF MS from the cells acclimating to limiting CO2. One phosphopeptide originated from a protein that has not been annotated. We found that this unknown expressed protein (UEP) was encoded in the genome of C. reinhardtii. Three other phosphorylated peptides belonged to Lci5 protein encoded by the low-CO2-inducible gene 5 (lci5). The phosphorylation sites were mapped in the tandem repeats of Lci5 ensuring phosphorylation of four serine and three threonine residues in the protein. Immunoblotting with Lci5-specific antibodies revealed that Lci5 was localized in chloroplast and confined to the stromal side of the thylakoid membranes. Phosphorylation of Lci5 and UEP occurred strictly at limiting CO2; it required reduction of electron carriers in the thylakoid membrane, but was not induced by light. Both proteins were phosphorylated in the low-CO2-exposed algal mutant deficient in the light-activated protein kinase Stt7. Phosphorylation of previously unknown basic proteins UEP and Lci5 by specific redox-dependent protein kinase(s) in the photosynthetic membranes reveals the early response of green algae to limitation in the environmental inorganic carbon.

NaCl-induced phosphorylation of light harvesting chlorophyll a/b proteins in thylakoid membranes from the halotolerant green alga, Dunaliella salina

Light could induce phosphorylation of light harvesting chlorophyll a/b binding proteins (LHCII) in Dunaliella salina and spinach thylakoid membranes. We found that neither phosphorylation was affected by glycerol, whereas treatment with NaCl significantly enhanced light-induced LHCII phosphorylation in D. salina thylakoid membranes and inhibited that in spinach. Furthermore, even in the absence of light, NaCl and several other salts induced LHCII phosphorylation in D. salina thylakoid membranes, but not in spinach thylakoid membranes. In addition, hypertonic shock induced LHCII phosphorylation in intact D. salina under dark conditions and cells adapted to different NaCl concentrations exhibited similar LHCII phosphorylation levels. Taken together, these results show for the first time that while LHCII phosphorylation of D. salina thylakoid membranes resembles that of spinach thylakoid membranes in terms of light-mediated control, the two differ with respect to NaCl sensitivity under light and dark conditions.

Redox regulation of thylakoid protein phosphorylation

The photosystem II of chloroplast thylakoid membranes contains several proteins phosphorylated by redox-activated protein kinases. The mechanism of the reversible activation of the light-harvesting antenna complex II (LHCII) kinase(s) is one of the best understood and related to the regulation of energy transfer to photosystem II or I, thereby optimizing their relative excitation (state transition). The deactivated LHCII protein kinase(s) is associated with cytochrome b(6)f and dissociates from the complex upon activation. Activation of the LHCII protein kinase occurs via dynamic conformational changes in the cytochrome b(6)f complex taking place during plastoquinol oxidation. Deactivation of the kinase involves its reassociation with an oxidized cytochrome complex. A fine-tuning redox-dependent regulatory loop inhibits the activation of the kinase via reduction of protein disulfide groups, possibly involving the thioredoxin complex. Phosphorylation of LHCII is further modulated by light-induced conformational changes of the LHCII substrate. The reversible phosphorylation of LHCII and other thylakoid phosphoproteins, catalyzed by respective kinases and phosphatases, is under strict regulation in response to environmental changes.

Correlation between persistent forms of zeaxanthin-dependent energy dissipation and thylakoid protein phosphorylation

High light stress induced not only a sustained form of xanthophyll cycle-dependent energy dissipation but also sustained thylakoid protein phosphorylation. The effect of protein phosphatase inhibitors (fluoride and molybdate ions) on recovery from a 1-h exposure to a high PFD was examined in leaf discs of Parthenocissus quinquefolia (Virginia creeper). Inhibition of protein dephosphorylation induced zeaxanthin retention and sustained energy dissipation (NPQ) upon return to low PFD for recovery, but had no significant effects on pigment and Chl fluorescence characteristics under high light exposure. In addition, whole plants of Monstera deliciosa and spinach grown at low to moderate PFDs were transferred to high PFDs, and thylakoid protein phosphorylation pattern (assessed with anti-phosphothreonine antibody) as well as pigment and Chl fluorescence characteristics were examined over several days. A correlation was obtained between dark-sustained D1/D2 phosphorylation and dark-sustained zeaxanthin retention and maintenance of PS II in a state primed for energy dissipation in both species. The degree of these dark-sustained phenomena was more pronounced in M. deliciosa compared with spinach. Moreover, M. deliciosa but not spinach plants showed unusual phosphorylation patterns of Lhcb proteins with pronounced dark-sustained Lhcb phosphorylation even under low PFD growth conditions. Subsequent to the transfer to a high PFD, dark-sustained Lhcb protein phosphorylation was further enhanced. Thus, phosphorylation patterns of D1/D2 and Lhcb proteins differed from each other as well as among plant species. The results presented here suggest an association between dark-sustained D1/D2 phosphorylation and sustained retention of zeaxanthin and energy dissipation (NPQ) in light-stressed, and particularly 'photoinhibited', leaves. Functional implications of these observations are discussed.

Isolation and characterization of photoautotrophic mutants of Chlamydomonas reinhardtii deficient in state transition

In photosynthetic cells of higher plants and algae, the distribution of light energy between photosystem I and photosystem II is controlled by light quality through a process called state transition. It involves a reorganization of the light-harvesting complex of photosystem II (LHCII) within the thylakoid membrane whereby light energy captured preferentially by photosystem II is redirected toward photosystem I or vice versa. State transition is correlated with the reversible phosphorylation of several LHCII proteins and requires the presence of functional cytochrome b(6)f complex. Most factors controlling state transition are still not identified. Here we describe the isolation of photoautotrophic mutants of the unicellular alga Chlamydomonas reinhardtii, which are deficient in state transition. Mutant stt7 is unable to undergo state transition and remains blocked in state I as assayed by fluorescence and photoacoustic measurements. Immunocytochemical studies indicate that the distribution of LHCII and of the cytochrome b(6)f complex between appressed and nonappressed thylakoid membranes does not change significantly during state transition in stt7, in contrast to the wild type. This mutant displays the same deficiency in LHCII phosphorylation as observed for mutants deficient in cytochrome b(6)f complex that are known to be unable to undergo state transition. The stt7 mutant grows photoautotrophically, although at a slower rate than wild type, and does not appear to be more sensitive to photoinactivation than the wild-type strain. Mutant stt3-4b is partially deficient in state transition but is still able to phosphorylate LHCII. Potential factors affected in these mutant strains and the function of state transition in C. reinhardtii are discussed.

Regulation of thylakoid protein phosphorylation at the substrate level: reversible light-induced conformational changes expose the phosphorylation site of the light-harvesting complex II

Light-dependent activation of thylakoid protein phosphorylation regulates the energy distribution between photosystems I and II of oxygen-evolving photosynthetic eukaryotes as well as the turnover of photosystem II proteins. So far the only known effect of light on the phosphorylation process is the redox-dependent regulation of the membrane-bound protein kinase(s) activity via plastoquinol bound to the cytochrome bf complex and the redox state of thylakoid dithiols. By using a partially purified thylakoid protein kinase and isolated native chlorophyll (chl) a/b light-harvesting complex II (LHCII), as well as recombinant LHCII, we find that illumination of the chl-protein substrate exposes the phosphorylation site to the kinase. Light does not activate the phosphorylation of the LHCII apoprotein nor the recombinant pigment-reconstituted complex lacking the N-terminal domain that contains the phosphothreonine site. The suggested light-induced conformational change exposing the N-terminal domain of LHCII to the kinase is evidenced also by an increase in its accessibility to tryptic cleavage after light exposure. Light activates preferentially the trimeric form of LHCII, and the process is paralleled by chl fluorescence quenching. Both phenomena are slowly reversible in darkness. Light-induced exposure of the LHCII N-terminal domain to the endogenous protein kinase(s) and tryptic cleavage occurs also in thylakoid membranes. These results demonstrate that light may regulate thylakoid protein phosphorylation not only via the signal transduction chain connecting redox reactions to the protein kinase activation, but also by affecting the conformation of the chl-protein substrate.

Characterization of mutants with alterations of the phosphorylation site in the D2 photosystem II polypeptide of chlamydomonas reinhardtii

We have changed the potential phosphorylation site, a threonine residue at position 2 of the D2 polypeptide of the photosystem II complex of Chlamydomonas reinhardtii, to alanine, valine, aspartate, proline, glycine, or glutamate. Mutants with neutral amino acid changes did not display any phenotype with regard to photoautotrophic growth, light sensitivity, fluorescence transients, or photoinhibition. Pulse labeling of these mutants with 32P indicated that a phosphorylated protein of the same size as D2 is absent in these mutants, suggesting that threonine-2 is indeed the unique phosphorylation site of D2. In contrast, mutants in which threonine-2 has been replaced with acidic residues are deficient in photosystem II. Use of chimeric genes containing the psbD 5'-untranslated region revealed that the initiation of translation was not affected in these mutants, but the mutations interfered with a later step of D2 synthesis and accumulation.

Mutation of residue threonine-2 of the D2 polypeptide and its effect on photosystem II function in Chlamydomonas reinhardtii

The D2 polypeptide of the photosystem II (PSII) complex in the green alga Chlamydomonas reinhardtii is thought to be reversibly phosphorylated. By analogy to higher plants, the phosphorylation site is likely to be at residue threonine-2 (Thr-2). We have investigated the role of D2 phosphorylation by constructing two mutants in which residue Thr-2 has been replaced by either alanine or serine. Both mutants grew photoautotrophically at wild-type rates, and noninvasive biophysical measurements, including the decay of chlorophyll fluorescence, the peak temperature of thermoluminescence bands, and rates of oxygen evolution, indicate little perturbation to electron transfer through the PSII complex. The susceptibility of mutant PSII to photoinactivation as measured by the light-induced loss of PSII activity in whole cells in the presence of the protein-synthesis inhibitors chloramphenicol or lincomycin was similar to that of wild type. These results indicate that phosphorylation at Thr-2 is not required for PSII function or for protection from photoinactivation. In control experiments the phosphorylation of D2 in wild-type C. reinhardtii was examined by 32P labeling in vivo and in vitro. No evidence for the phosphorylation of D2 in the wild type could be obtained. [14C]Acetate-labeling experiments in the presence of an inhibitor of cytoplasmic protein synthesis also failed to identify phosphorylated (D2.1) and nonphosphorylated (D2.2) forms of D2 upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Our results suggest that the existence of D2 phosphorylation in C. reinhardtii is still in question.

Thylakoid protein phosphorylation in evolutionally divergent species with oxygenic photosynthesis

Phosphothreonine antibody was used to explore reversible thylakoid protein phosphorylation in vivo in evolutionally divergent organisms with oxygenic photosynthesis. Three distinct groups of organisms were found. Cyanobacteria and red algae, both with phycobilisome antenna system, did not show phosphorylation of any of the photosystem II (PSII) proteins and belong to group 1. Group 2 species, consisting of a moss, a liverwort and a fern, phosphorylated both the light-harvesting chlorophyll alb proteins (LHCII) and the PSII core proteins D2 and CP43, but not the D1 protein. Reversible phosphorylation of the D1 protein seems to be the latest event in the evolution of PSII protein phosphorylation and was found only in seed plants, in group 3 species. Light-intensity-dependent regulation of LHCII protein phosphorylation was similar in group 2 and 3 species, with maximal phosphorylation of LHCII at low light and nearly complete dephosphorylation at high light.

Primary structure characterization of the photosystem II D1 and D2 subunits

Mass spectrometry techniques have been applied in a protein mapping strategy to elucidate the majority of the primary structures of the D1 and D2 proteins present in the photosystem II reaction center. Evidence verifying the post-translational processing of the initiating methionine residue and acetylation of the free amino group, similar to those reported for other higher plant species, are presented for the two subunits from pea plants (Pisum sativum L.). Further covalent modifications observed on the D1 protein include the COOH-terminal processing with a loss of nine amino acids and phosphorylation of Thr2. In addition, the studies reported in this paper provide the first definitive characterization of oxidations on specific amino acids of the D1 and D2 proteins. We believe that these oxidations, and to a much lesser extent the phosphorylations, are major contributors to the heterogeneity observed during the electrospray analysis of the intact subunits reported in the accompanying paper (Sharma, J., Panico, M., Barber, J., and Morris, H. R. (1997) J. Biol. Chem. 272, 33153-33157). Significantly, all of the regions that have been identified as those particularly susceptible to oxidation are anticipated (from current models) to be in close proximity to the redox active components of the photosystem II complex.

Decreased Photosystem II Core Phosphorylation in a Yellow-Green Mutant of Wheat Showing Monophasic Fluorescence Induction Curve

In the present work we study the regulation of the distribution of the phosphorylated photosystem II (PSII) core populations present in grana regions of the thylakoids from several plant species. The heterogeneous nature of PSII core phosphorylation has previously been reported (M.T. Giardi, F. Rigoni, R. Barbato [1992] Plant Physiol 100: 1948-1954; M.T. Giardi [1993] Planta 190: 107-113). The pattern of four phosphorylated PSII core populations in the grana regions appears to be ubiquitous in higher plants. In the dark, at least two phosphorylated PSII core populations are always detected. A mutant of wheat (Triticum durum) that shows monophasic room-temperature photoreduction of the primary quinone electron acceptor of PSII as measured by chlorophyll fluorescence increase in the presence and absence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea and by fluorescence upon flash illumination in intact leaves also lacks the usual distribution of phosphorylated PSII core populations. In this mutant, the whole PSII core population pattern is changed, probably due to altered threonine kinase activity, which leads to the absence of light-induced phosphorylation of CP43 and D2 proteins. The results, correlated to previous experiments in vivo, support the idea that the functional heterogeneity observed by fluorescence is correlated in part to the PSII protein phosphorylation in the grana.

Assessing modulation of stromal and thylakoid light-harvesting complex-II phosphatase activities with phosphopeptide substrates

The study of the light-harvesting complex II (LHC-II) phosphatase activity has been difficult due to the membrane association of its substrate. Thylakoid membranes labeled with [γ-(32)P]ATP were incubated with chymotrypsin, releasing phosphopeptides which served as labeled substrates for LHC-II phosphatase. Utilizing these phosphopeptides as substrates, protein phosphatase activities have been identified in both the thylakoid membrane and the stromal fraction. The thylakoid-bound phosphatase was liberated from the membrane with a sub-solubilizing concentration of Brij 35. The membrane and the stromal protein phosphatases were inhibited by NaF and EDTA, but not inhibited by microcystin-LR. The stromal phosphatase differed from the membrane phosphatase in pH optimum, in its lack of inhibition by molybdate ions, and by its response to magnesium and manganese ions. Using the soluble chymotryptic peptide substrate, the effect of light on pea thylakoid-bound LHC-II phosphatase activity was also assessed. Incubation of the thylakoid membranes in the light caused a 35% inhibition of LHC-II phosphatase activity. The inhibition was diminished by the addition of DCMU. Addition of 10 mM dithiothreitol stimulated the activity in darkness and obviated the inhibition when exposed to light. These studies suggest that positive or negative regulation of the LHC-II phosphatase activity is possible in vivo.

Substrate specificity and kinetics of thylakoid phosphoprotein phosphatase reactions

A synthetic 15-amino-acid phosphopeptide analogue of an N-terminal phosphorylated segment of LHC II was found to inhibit dephosphorylation not only of phospho-LHC II but of all other thylakoid phosphoproteins resolved by phosphorimaging. The results suggest that structural features required for recognition of the phosphoprotein phosphatase are common to different thylakoid phosphoproteins as well as to the phosphopeptide itself: at least one thylakoid phosphoprotein phosphatase exhibits a broad substrate specificity. Dephosphorylation reaction rates of all 13 thylakoid phosphoproteins were determined, and the dephosphorylation half-times were found to range from 7 min to more than 180 min. Most of the phosphoprotein dephosphorylation reactions were partially inhibited by NaF, and were insensitive to antimycin A and okadaic acid. Nevertheless, both antimycin A and NaF stimulated the phosphorylation of LHC II and 9 kDa protein. Possible reasons for differences in sensitivity to these inhibitors are discussed.

Functional size of the thylakoid phosphatases determined by radiation inactivation

Radiation inactivation technique was employed to determine the functional size of phosphatases from thylakoid membrane. The enzymatic activities of phosphatases decayed in a simple function with the increase of radiation dosage. D37 values of 18.8 +/- 2.4-14.1 +/- 1.5 Mrad were obtained, using phosphoserine, phosphothreonine, p-nitrophenol phosphate, and phospho-histone V-S, respectively, as substrates. The molecular masses of 48.2 +/- 6.3-61 +/- 5.7 kDa were yielded by target theory analysis. We thus speculate that the thylakoid alkaline phosphatase is probably a monomer while acid phosphatase is functionally a dimer in situ.

Lack of Types 1 and 2A Protein Serine(P)/Threonine(P) Phosphatase Activities in Chloroplasts

Protein phosphatase activity in crude leaf extracts and in purified intact chloroplasts of wheat (Triticum aestivum) and pea (Pisum sativum) was analyzed using exogenously supplied phosphoproteins or endogenous thylakoid proteins. Leaf extracts contain readily detectable amounts of protein phosphatase activity measured with either phosphohistone or phosphorylase a, substrates of mammalian protein phosphatases. No significant chloroplast protein phosphatase activity was detected using these exogenous phosphoproteins. The dephosphorylation of endogenous thylakoid light-harvesting chlorophyll a/b binding proteins in situ was inhibited by fluoride, but not by microcystin-LR or okadaic acid, diagnostic inhibitors of mammalian types 1 and 2A protein phosphatases. Additionally, no evidence for a pea chloroplast alkaline phosphatase activity was found using beta-glycerolphosphate or 4-methylum-belliferyl phosphate as substrates. From these results, we conclude that phosphohistone and phosphorylase a are not useful substrates for chloroplast thylakoid protein phosphatase activity and that the chloroplast enzymes may not fit into one of the canonical classifications currently used for protein phosphatases.

In Vivo and In Vitro Protein Phosphorylation Studies on Ochromonas danica, an Alga with a Chlorophyll a/c/Fucoxanthin Binding Protein

The phosphorylation of thylakoid membranes in the Chromophyte alga Ochromonas danica was studied in whole cells and in vitro. Protein kinase activity was observed in the thylakoid fraction, and several membrane-bound polypeptides were found to be phosphorylated. The thylakoid protein kinase demonstrated several unusual regulatory properties. Both the polypeptides that were phosphorylated and the rate of protein phosphorylation were independent of illumination. Protein kinase activity was also unaffected by 3-(3,4-dichlorophenyl)-1,1-dimethylurea, diuron. The kinase activity was inhibited under strong reducing conditions. Whole cells labeled with (32)PO(4) (3-) were converted to light states I and II by pre-illumination favoring photosystem I or photosystem II, respectively. Analysis of the phosphoproteins from cells in state I and state II showed that no changes in phosphorylation accompanied the change in energy redistribution.

Chloroplast thylakoid protein phosphatase is a membrane surface-associated activity

Chloroplast thylakoid protein phosphatase activity was measured using (32)P-labeled histone as an exogenous substrate and an assay of the (32)Pi released involving formation of a phosphomolybdate complex and organic extraction. The activity was liberated from wheat (Triticum aestivum) thylakoids by washing the membranes in NaCl-containing solutions followed by centrifugation. The liberated phosphatase activity had a pH optimum of approximately 6.75, was inhibited by addition of 10 millimolar EDTA or EGTA, and was stimulated by addition of millimolar amounts of dithiothreitol, magnesium, manganese, or calcium ions. The rate of thylakoid protein dephosphorylation was decreased following liberation of a portion of the protein phosphatase activity and was increased by addition of salt-liberated phosphatase fraction. These results suggest that at least a portion of wheat thylakoid protein phosphatase is a peripheral, rather than an integral, membrane protein.

In vitro synthesis, assembly and function of a photosynthetic membrane protein

Cell-free translation of Chlamydomonas reinhardtii RNA in the presence of photosynthetic membranes resulted in association of the herbicide binding (Qb) protein with membranes. Incubation of recovered membranes with high salt did not extract the polypeptide from membranes. Tryptic digestion of in vivo labeled membranes or membranes recovered from in vitro translation mixtures showed that Qb had similar orientation. In vitro translation in the presence of chloroplast membranes from cells exposed to high light intensity restored the membrane associated kinase activity lost by photoinhibition. Thus, in vitro synthesis resulted in functional integration of the Qb protein within the photosynthetic membrane.

Is the 9 kDa thylakoid membrane phosphoprotein functionally and structurally analogous to the 'H' subunit of bacterial reaction centres?

Although the amino acid sequence of the 9 kDa (phospho)protein of chloroplasts has been determined, the function of this thylakoid membrane protein in photosynthetic electron transport and the reason for its physiological control remains unclear. In this paper, I briefly review the evidence which indicates that the phosphorylation of the 9 kDa protein results in a partial inhibition of photosynthetic oxygen evolution by increasing the stability of the semiquinone bound to QA the primary, plastoquinone-binding site of photosystem II (PS II). I propose that in its dephosphorylated state, the 9 kDa thylakoid membrane protein may serve PS II to ensure efficient photochemical charge separation by aiding the transfer of reducing equivalents out of the reaction centre to the attendant plastoquinone pool. This function is analogous to that proposed for the H-subunit of the reaction centre of photosynthetic eubacteria. Whether these two proteins have evolved from a common ancestral reaction centre protein is discussed in the light of a comparison of their amino acid sequences and predicted secondary structures.

Changes in the flash-induced oxygen yield pattern by thylakoid membrane phosphorylation

Phosphorylation of thylakoid membrane proteins results in a partial inhibition (approximately 15-20%) of the light-saturated rate of oxygen evolution. The site of inhibition is thought to be located on the acceptor side of photosystem 2 (PS2) between the primary, QA, and secondary, QB, plastoquinone acceptors (Hodges et al. 1985, 1987). In this paper we report that thylakoid membrane phosphorylation increases the damping of the quaternary oscillation in the flash oxygen yield and increases the extent of the fast component in the deactivation of the S2 oxidation state. These results support the proposal that thylakoid membrane protein phosphorylation decreases the equilibrium constant for the exchange of an electron between QA and QB. An analysis of the oxygen release patterns using the recurrence matrix model of Lavorel (1976) indicates that thylakoid membrane phosphorylation increases the probability that PS2 miss a S-state transition by 20%. This is equivalent, however, to an insignificant inhibition (approximately 2.4%) of the light-saturated oxygen evolution rate. If a double miss in the S-state transitions is included when the PS2 centres are in S2 the fit between the experimental and theoretical oxygen yield sequences is better, and sufficient to account for the 15-20% inhibition in the steady-state oxygen yield. A double miss in the S-state transition is a consequence of an increased population of PS2 centres retaining QA (-): not only will these PS2 centres fail to catalyse photochemical charge transfer until QA (-) is reoxidized, but the re-oxidation reaction will also result in the deactivation of S2 to S1.

Characterization of a chloroplast mutation in the psaA2 gene of Chlamydomonas reinhardtii

The synthesis of polypeptides related to the CPI chlorophyll-protein complex of photosystem I has been studied by pulse-labeling experiments in twenty chloroplast mutants of Chlamydomonas reinhardtii. Three mutations of the same locus (Girard-Bascou 1987) result in the absence of these CPI-related polypeptides. Among these mutations one, (FUD26) leads to the synthesis of a new polypeptide presumed to be a truncated CPI apoprotein. The molecular characterization of this mutation in the psaA2 gene has been achieved by DNA sequencing the 3' end of this gene. The FUD26 mutation is a 4 base pair deletion resulting in frameshift and premature termination of the protein.

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.

Biogenesis of photosystem II complexes: transcriptional, translational, and posttranslational regulation

The integral membrane proteins of photosystem II (PS II) reaction center complexes are encoded by chloroplast genomes. These proteins are absent from thylakoids of PS II mutants of algae and vascular plants as a result of either chloroplast or nuclear gene mutations. To resolve the molecular basis for the concurrent absence of the PS II polypeptides, protein synthesis rates and mRNA levels were measured in mutants of Chlamydomonas reinhardtii that lack PS II. The analyses show that one nuclear gene product regulates the levels of transcripts from the chloroplast gene encoding the 51-kD chlorophyll a-binding polypeptide (polypeptide 5) but is not involved in the synthesis of other chloroplast mRNAs. Another nuclear product is specifically required for translation of mRNA encoding the 32-34-kD polypeptide, D1. The absence of either D1 or polypeptide 5 does not eliminate the synthesis and thylakoid insertion of two other integral membrane proteins of PS II, the chlorophyll a-binding polypeptide of 46 kD (polypeptide 6) and the 30-kD "D1-like" protein, D2. However, these two unassembled subunits cannot be properly processed and/or are degraded in the mutants even though they reside in the membrane. In addition, pulse labeling of the nuclear mutants and a chloroplast mutant that does not synthesize D1 mRNA indicates that synthesis of polypeptide 5 and D1 is coordinated at the translational level. A model is presented to explain how absence of one of the two proteins could lead to translational arrest of the other.

Transient inactivation of the thylakoid photosystem II light-harvesting protein kinase system and concomitant changes in intramembrane particle size during photoinhibition of Chlamydomonas reinhardtii

Light-dependent reduction of the plastoquinone pool regulates the activity of the thylakoid-bound protein kinase which phosphorylates the light harvesting chlorophyll a,b-protein complex (LHC II) and regulates energy distribution between photosystems II (PS II) and I (Staehelin, L. A., and C. J. Arntzen, 1983, J. Cell Biol., 97:1327-1337). Since reduction of plastoquinone by PS II is abolished in photoinhibited thylakoids due to loss of the secondary electron acceptor QB protein (Kyle, D. J., I. Ohad, and C. J. Arntzen, 1984, Proc. Natl. Acad. Sci. USA, 81:4070-4074), it was of interest to examine the activity of the LHC II protein kinase system during photoinhibition and recovery of PS II activity. The kinase activity was assessed both in vivo and in vitro in Chlamydomonas cells exposed to high light intensity (photoinhibition) and recovery at low light intensity. The kinase activity was progressively reduced during photoinhibition and became undetectable after 90 min. The inactive LHC II-kinase system could not be reactivated in vitro either by light or by reduction of the plastoquinone pool following addition of reduced duroquinone (TMQH2). The LHC II polypeptides were dephosphorylated in vivo when cells, prelabeled with [32P]orthophosphate before exposure to high light intensity, were transferred to photoinhibiting light in the presence of [32P]orthophosphate. In vivo recovery of the LHC II-kinase activity, elicited by the addition of TMQH2 to the assay system, did not require restoration of QB-dependent electron flow or de novo protein synthesis, either in the cytoplasm or in the chloroplast. Mild sonication of thylakoids isolated from photoinhibited cells restored the ability of the LHC II protein kinase system to be activated in vitro by addition to TMQH2. Restoration of the light-activated LHC-II kinase required recovery of QB-dependent electron flow. At the structural level, photoinhibition did not affect the ratio of grana/stroma thylakoids. A reduction of approximately 20% of the 11-17-nm intramembrane particles and an equivalent increase in the number of 6-10.5-nm particles was observed on the E-fracture faces of stacked thylakoid membranes. Similar but smaller changes were observed also on the E-fracture faces of unstacked thylakoid membranes (more 10-14-nm and less 6-9-nm particles) and P-fracture faces of stacked thylakoid membranes (more 6-8- and less 9.5-13-nm particles). All these structural changes were reversed to normal values during recovery of PS II activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Stage-Specific State I-State II Transitions during the Cell Cycle of Euglena gracilis

In synchronized Euglena gracilis (light-dark regime of 14:10 hours) the successive formation of the photosynthetic apparatus during cell ontogeny is correlated with large changes in photosynthetic efficiency (P Brandt, B von Kessel 1983 Plant Physiol 72: 616-619; B Kohnke, P Brandt 1984 Biochim Biophys Acta 766: 156-160). This observation led us to investigate the functional association of the chlorophyll a/b light-harvesting protein complex (LHCP) with photosystem I or II, because changes in energy flow to photosystem I or II and in energy transfer between the two photosystems can be a reason for these alterations. As criterion for the association of the LHCP with photosystem I or II, state transitions were determined after 15 minutes preillumination using wave-lengths of 725 or 620 nanometers. The state transitions were determined from measurements of fluorescence induction at room temperature, and fluorescence kinetics at 77 K. According to the obtained data (a) mobile LHCP is present only between the 6th and the 10th hour of the light-time of the cell cycle and (b) this functional relation of the LHCP to photosystem I only at this stage of Euglena chloroplast development is not accompanied by a decrease in stacking. A model for the organization of the newly inserted LHCP within the photosynthetic apparatus of E. gracilis is discussed.