A CK2 site is reversibly phosphorylated in the photosystem II subunit CP29

The origin of pigment-binding differences in CP29 and LHCII: the role of protein structure and dynamics

The first step of photosynthesis in plants is performed by the light-harvesting complexes (LHC), a large family of pigment-binding proteins embedded in the photosynthetic membranes. These complexes are conserved across species, suggesting that each has a distinct role. However, they display a high degree of sequence homology and their static structures are almost identical. What are then the structural features that determine their different properties? In this work, we compared the two best-characterized LHCs of plants: LHCII and CP29. Using molecular dynamics simulations, we could rationalize the difference between them in terms of pigment-binding properties. The data also show that while the loops between the helices are very flexible, the structure of the transmembrane regions remains very similar in the crystal and the membranes. However, the small structural differences significantly affect the excitonic coupling between some pigment pairs. Finally, we analyzed in detail the structure of the long N-terminus of CP29, showing that it is structurally stable and it remains on top of the membrane even in the absence of other proteins. Although the structural changes upon phosphorylation are minor, they can explain the differences in the absorption properties of the pigments observed experimentally.

Changes in the Arabidopsis thaliana Proteome Implicate cAMP in Biotic and Abiotic Stress Responses and Changes in Energy Metabolism

The second messenger 3',5'-cyclic adenosine monophosphate (cAMP) is increasingly recognized as having many different roles in plant responses to environmental stimuli. To gain further insights into these roles, Arabidopsis thaliana cell suspension culture was treated with 100 nM of cell permeant 8-bromo-cAMP for 5 or 10 min. Here, applying mass spectrometry and comparative proteomics, 20 proteins were identified as differentially expressed and we noted a specific bias in proteins with a role in abiotic stress, particularly cold and salinity, biotic stress as well as proteins with a role in glycolysis. These findings suggest that cAMP is sufficient to elicit specific stress responses that may in turn induce complex changes to cellular energy homeostasis.

High light-dependent phosphorylation of photosystem II inner antenna CP29 in monocots is STN7 independent and enhances nonphotochemical quenching

Phosphorylation of the photosystem II antenna protein CP29 has been reported to be induced by excess light and further enhanced by low temperature, increasing resistance to these stressing factors. Moreover, high light-induced CP29 phosphorylation was specifically found in monocots, both C3 and C4, which include the large majority of food crops. Recently, knockout collections have become available in rice (Oryza sativa), a model organism for monocots. In this work, we have used reverse genetics coupled to biochemical and physiological analysis to elucidate the molecular basis of high light-induced phosphorylation of CP29 and the mechanisms by which it exerts a photoprotective effect. We found that kinases and phosphatases involved in CP29 phosphorylation are distinct from those reported to act in State 1-State 2 transitions. In addition, we elucidated the photoprotective role of CP29 phosphorylation in reducing singlet oxygen production and enhancing excess energy dissipation. We thus established, in monocots, a mechanistic connection between phosphorylation of CP29 and nonphotochemical quenching, two processes so far considered independent from one another.

Emerging roles of protein kinase CK2 in abscisic acid signaling

The phytohormone abscisic acid (ABA) regulates many aspects of plant growth and development as well as responses to multiple stresses. Post-translational modifications such as phosphorylation or ubiquitination have pivotal roles in the regulation of ABA signaling. In addition to the positive regulator sucrose non-fermenting-1 related protein kinase 2 (SnRK2), the relevance of the role of other protein kinases, such as CK2, has been recently highlighted. We have recently established that CK2 phosphorylates the maize ortholog of open stomata 1 OST1, ZmOST1, suggesting a role of CK2 phosphorylation in the control of ZmOST1 protein degradation (Vilela et al., 2015). CK2 is a pleiotropic enzyme involved in multiple developmental and stress-responsive pathways. This review summarizes recent advances that taken together suggest a prominent role of protein kinase CK2 in ABA signaling and related processes.

The significance of CP29 reversible phosphorylation in thylakoids of higher plants under environmental stresses

Reversible phosphorylation of proteins is a key event in many fundamental cellular processes. Under stressful conditions, many thylakoid membrane proteins in photosynthetic apparatus of higher plants undergo rapid phosphorylation and dephosphorylation in response to environmental changes. CP29 is the most frequently phosphorylated protein among three minor antennae complexes in higher plants. CP29 phosphorylation in dicotyledons has been known for several decades and is well characterized. However, CP29 phosphorylation in monocotyledons is less studied and appears to have a different phosphorylation pattern. In this review, we discuss recent advancements in CP29 phosphorylation and dephosphorylation studies and its physiological significance under environmental stresses in higher plants, especially in the monocotyledonous crops. Physiologically, the phosphorylation of CP29 is likely to be a prerequisite for state transitions and the disassembly of photosystem II supercomplexes, but not involved in non-photochemical quenching (NPQ). CP29 is phosphorylated in monocots exposed to environmental cues, with its subsequent lateral migration from grana stacks to stroma lamellae. However, neither CP29 phosphorylation nor its lateral migration occurs in dicotyledonous plants after drought, cold, or salt stress. Since the molecular mechanisms of differential CP29 phosphorylation under stresses are not fully understood, this review provides insights for future studies regarding the physiological function of CP29 reversible phosphorylation.

Dynamics of reversible protein phosphorylation in thylakoids of flowering plants: the roles of STN7, STN8 and TAP38

Phosphorylation is the most common post-translational modification found in thylakoid membrane proteins of flowering plants, targeting more than two dozen subunits of all multiprotein complexes, including some light-harvesting proteins. Recent progress in mass spectrometry-based technologies has led to the detection of novel low-abundance thylakoid phosphoproteins and localised their phosphorylation sites. Three of the enzymes involved in phosphorylation/dephosphorylation cycles in thylakoids, the protein kinases STN7 and STN8 and the phosphatase TAP38/PPH1, have been characterised in the model species Arabidopsis thaliana. Differential protein phosphorylation is associated with changes in illumination and various other environmental parameters, and has been implicated in several acclimation responses, the molecular mechanisms of which are only partly understood. The phenomenon of State Transitions, which enables rapid adaptation to short-term changes in illumination, has recently been shown to depend on reversible phosphorylation of LHCII by STN7-TAP38/PPH1. STN7 is also necessary for long-term acclimation responses that counteract imbalances in energy distribution between PSII and PSI by changing the rates of accumulation of their reaction-centre and light-harvesting proteins. Another aspect of photosynthetic acclimation, the modulation of thylakoid ultrastructure, depends on phosphorylation of PSII core proteins, mainly executed by STN8. Here we review recent advances in the characterisation of STN7, STN8 and TAP38/PPH1, and discuss their physiological significance within the overall network of thylakoid protein phosphorylation. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.

Phosphorylation of photosynthetic antenna protein CP29 and photosystem II structure changes in monocotyledonous plants under environmental stresses

Kinetic studies of protein dephosphorylation in thylakoid membranes showed that the minor light-harvesting antenna protein CP29 could be phosphorylated in barley (C3) and maize (C4) seedlings, but not in spinach under water [Liu, W. J., et al. (2009) Biochim. Biophys. Acta 1787, 1238-1245], salt, or cold stress [Pursiheimo, S., et al. (2003) Plant Cell Environ. 26, 1995-2003], suggesting that phosphorylation of CP29 is a general phenomenon in monocots, but not in dicots under environmental stresses. Abscisic acid (ABA), reactive oxygen species (ROS), salicylic acid (SA), jasmonic acid (JA), ethylene (ET), NO, and the scavenger of H(2)O(2) had weak effects on CP29 phosphorylation. However, three protein kinase inhibitors, U0126, W7, and K252a (for mitogen-activated protein kinase, Ca(2+)-dependent protein kinase, and Ser/Thr protein kinases, respectively), decrease the level of CP29 phosphorylation in barley apparently under environmental stresses. Therefore, these three protein kinases are involved in CP29 phosphorylation. We also found that most CP29 phosphorylation was accompanied by its lateral migration from granum membranes to stroma-exposed thylakoid regions, and the instability of PSII supercomplexes and LHCII trimers under environmental stresses.

Exploring the structure of the N-terminal domain of CP29 with ultrafast fluorescence spectroscopy

A high-throughput Förster resonance energy transfer (FRET) study was performed on the approximately 100 amino acids long N-terminal domain of the photosynthetic complex CP29 of higher plants. For this purpose, CP29 was singly mutated along its N-terminal domain, replacing one-by-one native amino acids by a cysteine, which was labeled with a BODIPY fluorescent probe, and reconstituted with the natural pigments of CP9, chlorophylls and xanthophylls. Picosecond fluorescence experiments revealed rapid energy transfer (~20–70 ps) from BODIPY at amino-acid positions 4, 22, 33, 40, 56, 65, 74, 90, and 97 to Chl a molecules in the hydrophobic part of the protein. From the energy transfer times, distances were estimated between label and chlorophyll molecules, using the Förster equation. When the label was attached to amino acids 4, 56, and 97, it was found to be located very close to the protein core (~15 Å), whereas labels at positions 15, 22, 33, 40, 65, 74, and 90 were found at somewhat larger distances. It is concluded that the entire N-terminal domain is in close contact with the hydrophobic core and that there is no loop sticking out into the stroma. Most of the results support a recently proposed topological model for the N-terminus of CP29, which was based on electron-spin-resonance measurements on spin-labeled CP29 with and without its natural pigment content. The present results lead to a slight refinement of that model.

Site-directed spin-labeling study of the light-harvesting complex CP29

The topology of the long N-terminal domain (approximately 100 amino-acid residues) of the photosynthetic Lhc CP29 was studied using electron spin resonance. Wild-type protein containing a single cysteine at position 108 and nine single-cysteine mutants were produced, allowing to label different parts of the domain with a nitroxide spin label. In all cases, the apoproteins were either solubilized in detergent or they were reconstituted with their native pigments (holoproteins) in vitro. The spin-label electron spin resonance spectra were analyzed in terms of a multicomponent spectral simulation approach, based on hybrid evolutionary optimization and solution condensation. These results permit to trace the structural organization of the long N-terminal domain of CP29. Amino-acid residues 97 and 108 are located in the transmembrane pigment-containing protein body of the protein. Positions 65, 81, and 90 are located in a flexible loop that is proposed to extend out of the protein from the stromal surface. This loop also contains a phosphorylation site at Thr81, suggesting that the flexibility of this loop might play a role in the regulatory mechanisms of the light-harvesting process. Positions 4, 33, 40, and 56 are found to be located in a relatively rigid environment, close to the transmembrane protein body. On the other hand, position 15 is located in a flexible region, relatively far away from the transmembrane domain.

Environmentally modulated phosphorylation and dynamics of proteins in photosynthetic membranes

Recent advances in vectorial proteomics of protein domains exposed to the surface of photosynthetic thylakoid membranes of plants and the green alga Chlamydomonas reinhardtii allowed mapping of in vivo phosphorylation sites in integral and peripheral membrane proteins. In plants, significant changes of thylakoid protein phosphorylation are observed in response to stress, particularly in photosystem II under high light or high temperature stress. Thylakoid protein phosphorylation in the algae is much more responsive to the ambient redox and light conditions, as well as to CO(2) availability. The light-dependent multiple and differential phosphorylation of CP29 linker protein in the green algae is suggested to control photosynthetic state transitions and uncoupling of light harvesting proteins from photosystem II under high light. The similar role for regulation of the dynamic distribution of light harvesting proteins in plants is proposed for the TSP9 protein, which together with other recently discovered peripheral proteins undergoes specific environment- and redox-dependent phosphorylation at the thylakoid surface. This review focuses on the environmentally modulated reversible phosphorylation of thylakoid proteins related to their membrane dynamics and affinity towards particular photosynthetic protein complexes.

Micelle-induced folding of spinach thylakoid soluble phosphoprotein of 9 kDa and its functional implications

Thylakoid soluble phosphoprotein of 9 kDa (TSP9) has been identified as a plant-specific protein in the photosynthetic thylakoid membrane (Carlberg et al. (2003) Proc. Natl. Acad. Sci. 100, 757-762). Nonphosphorylated TSP9 is associated with the membrane, whereas, after light-induced phosphorylation, a fraction of the phosphorylated TSP9 is released into the aqueous stroma. By NMR spectroscopy, we have determined the structural features of nonphosphorylated TSP9 both in aqueous solution and in membrane mimetic micelles. The results show that both wild type nonphosphorylated TSP9 and a triple-mutant (T46E + T53E + T60E) mimic of the triphosphorylated form of TSP9 are disordered under aqueous conditions, but adopt an ordered conformation in the presence of detergent micelles. The micelle-induced structural features, which are similar in micelles either of SDS or dodecylphosphocholine (DPC), consist of an N-terminal alpha-helix, which may represent the primary site of interaction between TSP9 and binding partners, and a less structured helical turn near the C-terminus. These structured elements contain mainly hydrophobic residues. NMR relaxation data for nonphosphorylated TSP9 in SDS micelles indicated that the molecule is highly flexible with the highest order in the N-terminal alpha-helix. Intermolecular NOE signals, as well as spin probe-induced broadening of NMR signals, demonstrated that the SDS micelles contact both the structured and a portion of the unstructured regions of TSP9, in particular, those containing the three phosphorylation sites (T46, T53, and T60). This interaction may explain the selective dissociation of phosphorylated TSP9 from the membrane. Our study presents a structural model for the role played by the structured and unstructured regions of TSP9 in its membrane association and biological function.

Photosynthesis research in Italy: a review

This historical review was compiled and edited by Giorgio Forti, whereas the other authors of the different sections are listed alphabetically after his name, below the title of the paper; they are also listed in the individual sections. This review deals with the research on photosynthesis performed in several Italian laboratories during the last 50 years; it includes research done, in collaboration, at several international laboratories, particularly USA, UK, Switzerland, Hungary, Germany, France, Finland, Denmark, and Austria. Wherever pertinent, references are provided, especially to other historical papers in Govindjee et al. [Govindjee, Beatty JT, Gest H, Allen JF (eds) (2005) Discoveries in Photosynthesis. Springer, Dordrecht]. This paper covers the physical and chemical events starting with the absorption of a quantum of light by a pigment molecule to the conversion of the radiation energy into the stable chemical forms of the reducing power and of ATP. It describes the work done on the structure, function and regulation of the photosynthetic apparatus in higher plants, unicellular algae and in photosynthetic bacteria. Phenomena such as photoinhibition and the protection from it are also included. Research in biophysics of photosynthesis in Padova (Italy) is discussed by G.M. Giacometti and G. Giacometti (2006).

Novel Protein Phosphorylation Site Identification in Spinach Stroma Membranes by Titanium Dioxide Microcolumns and Tandem Mass Spectrometry

In this work, spinach stroma membrane, instead of thylakoid, has been investigated for the presence of phosphorylated proteins. We identified seven previously unknown phosphorylation sites by taking advantage of TiO(2) phosphopeptides enrichment coupled to mass spectrometric analysis. Upon illumination at 100 micromol m(-2) s(-1), two novel phosphopeptides belonging to the N-terminal region of Lhcb1 light-harvesting protein were detected: NVSSGS(p)PWYGPDR and T(p)VQSSSPWYGPDR. Moreover, three new threonine residues in CP43 (Thr-6, Thr-8, and Thr-346) and, for the first time, two amino acid residues of the N-terminus of Rieske Fe-S protein of the cytochrome b(6)f complex (Thr-2 and Ser-3) were revealed to be phosphorylated. Since Lhcb1 and CP43 have been reported as mobile proteins, it may be suggested that illumination derived phosphorylation, and consequently the addition of negatively charged groups to the protein, is a necessary condition to induce a significant protein structural change.

Light-harvesting complex II protein CP29 binds to photosystem I of Chlamydomonas reinhardtii under State 2 conditions

The State 1 to State 2 transition in the photosynthetic membranes of plants and green algae involves the functional coupling of phosphorylated light-harvesting complexes of photosystem II (LHCII) to photosystem I (PSI). We present evidence suggesting that in Chlamydomonas reinhardtii this coupling may be aided by a hyper-phosphorylated form of the LHCII-like CP29 protein (Lhcbm4). MS analysis of CP29 showed that Thr6, Thr16 and Thr32, and Ser102 are phosphorylated in State 2, whereas in State 1-exposed cells only phosphorylation of Thr6 and Thr32 could be detected. The LHCI-PSI supercomplex isolated from the alga in State 2 was found to contain strongly associated CP29 in phosphorylated form. Electron microscopy suggests that the binding site for this highly phosphorylated CP29 is close to the PsaH protein. It is therefore postulated that redox-dependent multiple phosphorylation of CP29 in green algae is an integral part of the State transition process in which the structural changes of CP29, induced by reversible phosphorylation, determine the affinity of LHCII for either of the two photosystems.

Chlorophyll b to chlorophyll a energy transfer kinetics in the CP29 antenna complex: a comparative femtosecond absorption study between native and reconstituted proteins

The energy transfer processes between Chls b and Chls a have been studied in the minor antenna complex CP29 by femtosecond transient absorption spectroscopy. Two samples were analyzed: the native CP29, purified from higher plants, and the recombinant one, reconstituted in vitro with the full pigment complement. The measurements indicate that the transfer kinetics in the two samples are virtually identical, confirming that the reconstituted CP29 has the same spectroscopic properties as the native one. In particular, three lifetimes (150 fs, 1.2 ps, and 5-6 ps) were identified for Chl b-652 nm to Chl a energy transfer and at least one for Chl b-640 nm (600-800 fs). Considering that the complexes bind two Chls b per polypeptide, the observation of more than two lifetimes for the Chl b to Chl a energy transfer, in both samples, clearly indicates the presence of the so-called mixed Chl binding sites--sites which are not selective for Chl a or Chl b, but can accommodate either species. The kinetic components and spectra are assigned to specific Chl binding sites in the complex, which provides further information on the structural organization.

Redox regulation of chloroplast transcription

Chloroplasts are the important plant cell organelles where photosynthesis takes place. Throughout this process, reaction center proteins are degraded and subsequently replenished by redox-responsive gene expression. In addition to well defined posttranscriptional mechanisms at the RNA and protein level, the transcription of chloroplast DNA into RNA precursors has been a focal point of studies in this area. Evidence has become available for a central role of a redox-responsive protein kinase named plastid transcription kinase (PTK) because of its association with the chloroplast transcription complex. The recent cloning of the PTK gene has resulted in a full-length cDNA for a protein related to the catalytic alpha subunit of nucleocytoplasmic casein kinase (CK2), yet with an additional chloroplast transit peptide. The corresponding protein, termed cpCK2alpha, was shown to be associated with the major organellar RNA polymerase, PEP-A. Both authentic PTK and recombinant cpCK2alpha have comparable general properties in vitro, and both are subject to regulation by the redox-reactive reagent glutathione. Based on the physical and functional equivalence, it is anticipated that the cloned protein can help clarify the functional role of the transcription kinase in vivo, including the identification of interaction partners at the interface between photosynthetic redox signaling and gene expression.

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.

A novel plant protein undergoing light-induced phosphorylation and release from the photosynthetic thylakoid membranes

The characteristics of a phosphoprotein with a relative electrophoretic mobility of 12 kDa have been unknown during two decades of studies on redox-dependent protein phosphorylation in plant photosynthetic membranes. Digestion of this protein from spinach thylakoid membranes with trypsin and subsequent tandem nanospray-quadrupole-time-of-flight mass spectrometry of the peptides revealed a protein sequence that did not correspond to any previously known protein. Sequencing of the corresponding cDNA uncovered a gene for a precursor protein with a transit peptide followed by a strongly basic mature protein with a molecular mass of 8,640 Da. Genes encoding homologous proteins were found on chromosome 3 of Arabidopsis and rice as well as in ESTs from 20 different plant species, but not from any other organisms. The protein can be released from the membrane with high salt and is also partially released in response to light-induced phosphorylation of thylakoids, in contrast to all other known thylakoid phosphoproteins, which are integral to the membrane. On the basis of its properties, this plant-specific protein is named thylakoid soluble phosphoprotein of 9 kDa (TSP9). Mass spectrometric analyses revealed the existence of non-, mono-, di-, and triphosphorylated forms of TSP9 and phosphorylation of three distinct threonine residues in the central part of the protein. The phosphorylation and release of TSP9 from the photosynthetic membrane on illumination favor participation of this basic protein in cell signaling and regulation of plant gene expression in response to changing light conditions.

Absolute measurement of phosphorylation levels in a biological membrane using atomic force microscopy: the creation of phosphorylation maps

We show that it is possible to produce phosphorylation difference maps of biological membranes under conditions which reflect those in vivo and in which proteins remain functional. We also demonstrate that absolute levels of phosphorylation are retrieved through the application of an appropriate calibration method. Finally we show that the kinetics of phosphorylation/dephosphorylation can also be monitored. These methods are demonstrated on photosynthetic membranes from higher plants, for which protein phosphorylation is the dominant regulatory mechanism. We show directly that the most recent estimates of the phosphorylation levels in this system are reasonably accurate. Phosphorylation difference maps show that the distribution of phosphates is not even, with significantly higher levels at the membrane margins and patches of high phosphate density next to patches of low charge density.

Chromophore organization in the higher-plant photosystem II antenna protein CP26

The chlorophyll a/b-xanthophyll-protein CP26 complex belongs to the Lhc protein family. It binds nine chlorophylls and two xanthophylls per 26.6 kDa polypeptide. Determination of the characteristics of each binding site is needed for the understanding of functional organization of individual proteins belonging to the photosystem II supramolecular complex. The biochemical and spectroscopic features of native CP26 are presented here together with identification of pigment binding and energy transitions in different sites. The analysis has been performed via a new approach using recombinant CP26 complexes in which the chromophore content has been experimentally modified. Data were interpreted on the basis of homology with CP29 and LHCII complexes, for which detailed knowledge is available from mutation analysis. We propose that one additional Chl b is present in CP26 as compared to CP29 and that it is located in site B2. We also found that in CP26 three chlorophyll binding sites are selective for Chl a, one of them being essential for the folding of the pigment-protein complex. Two xanthophyll binding sites were identified, one of which (L1) is essential for protein folding and specifically binds lutein. The second site (L2) has lower selectivity and can bind any of the xanthophyll species present in thylakoids.

A new subunit of cytochrome b6f complex undergoes reversible phosphorylation upon state transition

A 15.2-kDa polypeptide, encoded by the nuclear gene PETO, was identified as a novel cytochrome b(6)f subunit in Chlamydomonas reinhardtii. The PETO gene product is a bona fide subunit, subunit V, of the cytochrome b(6)f complex, because (i) it copurifies with the other cytochrome b(6)f subunits in the early stages of the purification procedure, (ii) it is deficient in cytochrome b(6)f mutants accumulating little of the complex, and (iii) it colocalizes with cytochrome f, which migrates between stacked and unstacked membrane regions upon state transition. Sequence analysis and biochemical characterization of subunit V shows that it has a one transmembrane alpha-helix topology with two large hydrophilic domains extending on the stromal and lumenal side of the thylakoid membranes, with a lumenal location of the N terminus. Subunit V is reversibly phosphorylated upon state transition, a unique feature that, together with its topological organization, points to the possible role of subunit V in signal transduction during redox-controlled short term and long term adaptation of the photosynthetic apparatus in eukaryotes.

Assignment of the lowest Qy-state and spectral dynamics of the CP29 chlorophyll a/b antenna complex of green plants: a hole-burning study

Low-temperature absorption, fluorescence and persistent non-photochemical hole-burned spectra are reported for the CP29 chlorophyll (Chl) a/b antenna complex of photosystem II of green plants. The absorption-origin band of the lowest Qy-state lies at 678.2 nm and carries a width of approximately 130 cm-1 that is dominated by inhomogeneous broadening at low temperatures. Its absorption intensity is equivalent to that of one of the six Chl a molecules of CP29. The absence of a significant satellite hole structure produced by hole burning, within the absorption band of the lowest state, indicates that the associated Chl a molecule is weakly coupled to the other Chl and, therefore, that the lowest-energy state is highly localized on a single Chl a molecule. The electron-phonon coupling of the 678.2 nm state is weak with a Huang-Rhys factor S of 0.5 and a peak phonon frequency (omega m) of approximately 20 cm-1. These values give a Stokes shift (2S omega m) in good agreement with the measured positions of the absorption band at 678.2 nm and a fluorescence-origin band at 679.1 nm. Zero-phonon holes associated with the lowest state have a width of approximately 0.05 cm-1 at 4.2 K, corresponding to a total effective dephasing time of approximately 400 ps. The temperature dependence of the zero-phonon holewidth indicates that this time constant is dominated at temperatures below 8 K by pure dephasing/spectral diffusion due to coupling of the optical transition to the glass-like two-level systems of the protein. Zero-phonon hole-widths obtained for the Chl b bands at 638.5 and 650.0 nm, at 4.2 K, lead to lower limits of 900 +/- 150 fs and 4.2 +/- 0.3 ps, respectively, for the Chl b-->Chl a energy-transfer times. Downward energy transfer from the Chl a state(s) at 665.0 nm occurs in 5.3 +/- 0.6 ps at 4.2 K.

Spectroscopic characterization of the spinach Lhcb4 protein (CP29), a minor light-harvesting complex of photosystem II

A spectroscopic characterization is presented of the minor photosystem II chlorophyll a/b-binding protein CP29 (or the Lhcb4 protein) from spinach, prepared by a modified form of a published protocol [Henrysson, T., Schroder, W. P., Spangfort, M. & Akerlund, H.-E. (1989) Biochim. Biophys. Acta 977, 301-308]. The isolation procedure represents a quicker, cheaper means of isolating this minor antenna protein to an equally high level of purity to that published previously. The pigment-binding protein shows similarities to other related light-harvesting complexes (LHCs), including the bulk complex LHCIIb but more particularly another minor antenna protein CP26 (Lhcb5). It is also, in the main, similar to other preparations of CP29, although some significant differences are discussed. In common with CP26, the protein binds about six chlorophyll a and two chlorophyll b molecules. Two chlorophyll b absorption bands are present at 638 and 650 nm and they are somewhat more pronounced than in a recent report [Giuffra, E., Zucchelli, G., Sandonà, D., Croce, R., Cugini, D., Garlaschi, F.M., Bassi, R. & Jennings, R.C. (1997) Biochem. 36, 12984-12993]. The bands give rise to positive and negative linear dichroism, respectively; both show negative CD bands (cf. bands with similar properties at 637 and 650 nm in CP26). Chlorophyll a absorption is dominated by a large contribution at 674 nm which also shows similarities to the major band in LHCIIb and CP26, while (as for CP26) a reduction in absorption around 670 nm is observed relative to the bulk complex. Principal differences from LHCIIb and CP26, and from other CP29 preparations, occur in the carotenoid region.

Multiple types of association of photosystem II and its light-harvesting antenna in partially solubilized photosystem II membranes

Photosystem II is a multisubunit pigment-protein complex embedded in the thylakoid membranes of chloroplasts. It utilizes light for photochemical energy conversion, and is heavily involved in the regulation of the energy flow. We investigated the structural organization of photosystem II and its associated light-harvesting antenna by electron microscopy, multivariate statistical analysis, and classification procedures on partially solubilized photosystem II membranes from spinach. Observation by electron microscopy shortly after a mild disruption of freshly prepared membranes with the detergent n-dodecyl-alpha,D-maltoside revealed the presence of several large supramolecular complexes. In addition to the previously reported supercomplexes [Boekema, E. J., van Roon, H., and Dekker, J. P. (1998) FEBS Lett. 424, 95-99], we observed complexes with the major trimeric chlorophyll a/b protein (LHCII) in a third, L-type of binding position (C2S2M0-2L1-2), and two different types of megacomplexes, both identified as dimeric associations of supercomplexes with LHCII in two types of binding sites (C4S4M2-4). We conclude that the association of photosystem II and its associated light-harvesting antenna is intrinsically heterogeneous, and that the minor CP26 and CP24 proteins play a crucial role in the supramolecular organization of the complete photosystem. We suggest that different types of organization form the structural basis for photosystem II to specifically react to changing light and stress conditions, by providing different routes of excitation energy transfer.

Higher plants light harvesting proteins. Structure and function as revealed by mutation analysis of either protein or chromophore moieties

Mutation analysis of higher plants light harvesting proteins has been prevented for a long time by the lack of a suitable expression system providing chromophores essential for the folding of these membrane-intrinsic pigment-protein complexes. Early work on in vitro reconstitution of the major light harvesting complex of photosystem II (LHCII) indicated an alternative way to mutation analysis of these proteins. A new procedure for in vitro refolding of the four light harvesting complexes of photosystem II, namely CP24, CP29, CP26 and LHCII yields recombinant pigment-proteins indistinguishable from the native proteins isolated from leaves. This method allows both the performing of single point mutations on protein sequence and the exchange of the chromophores bound to the protein scaffold. We review here recent results obtained by this method on the pigment-binding properties, on the chlorophyll-binding residues, on the identification of proton-binding sites and on the role of xanthophylls in the regulation of light harvesting function.

Protein phosphorylation and redox sensing in chloroplast thylakoids

Transduction of light dependent signals to redox sensitive kinases in photosynthetic membranes modulates energy transfer to the photochemical reaction centres and regulates biogenesis, stability and turnover of thylakoid protein complexes. The occupancy of the quinol-oxidation site of the cytochrome bf complex by plastoquinol and the redox state of protein thiol groups act as elements of the signal transducing chains.

Ultrafast evolution of the excited states in the chlorophyll a/b complex CP29 from green plants studied by energy-selective pump-probe spectroscopy

The energy transfer process in the minor light-harvesting antenna complex CP29 of green plants was probed in multicolor transient absorption experiments at 77 K using selective subpicosecond excitation pulses at 640 and 650 nm. Energy flow from each of the chlorophyll (Chl) b molecules of the complex could thus be studied separately. The analysis of our data showed that the "blue" Chl b (absorption around 640 nm) transfers excitation to a "red" Chl a with a time constant of 350 +/- 100 fs, while the 'red' Chl b (absorption at 650 nm) transfers on a picosecond time scale (2.2 +/- 0.5 ps) toward a "blue" Chl a. Furthermore, both fast (280 +/- 50 fs) and slow (10-13 ps) equilibration processes among the Chl a molecules were observed, with rates and associated spectra very similar to those of the major antenna complex, LHC-II. Based on the protein sequence homology between CP29 and LHC-II, a basic modelling of the observed kinetics was performed using the LHC-II structure and the Förster theory of energy transfer. Thus, an assignment for the spectral properties and orientation of the two Chl's b, as well as for their closest Chl a neighbors, is put forward, and a comparison is made with the previous assignments and models for LHC-II and CP29.