Determination of the aggregate size in detergent solution of the light-harvesting chlorophyll a/b-protein complex from chloroplast membranes

Towards a more accurate future for chlorophyll a and b determinations: the inaccuracies of Daniel Arnon's assay

A recent publication (Esteban in New Phytol 217:341-342, 2018) describes how the use and citation of the assay of chlorophylls a and b extracted in aqueous 80% acetone by Arnon (Plant Physiol 2:1-15, 1949) is increasing, even in journals with high impact factors. This is a very disconcerting situation: the assay is outdated because it relies on the seriously under-estimated extinction coefficients of Mackinney (J Biol Chem 140:315-322, 1941), and the assay of chlorophylls is one of the most important, and much reported, procedures in studies of photosynthesis and related plant biological fields. Using the assay has led to the accumulation of masses of inaccurate data and confusion during the resolution of some plant biological problems. A summary not only of an accurate assay of chlorophylls in aqueous 80% acetone but also of a long-known method to correct the data obtained by Arnon's procedure (cf. Porra et al. in Biochim Biophys Acta 975:384-394, 1989) is briefly described below together with references to reliable assays in this and other solvents by other authors.

Silica entrapment for significantly stabilized, energy-conducting light-harvesting complex (LHCII)

The major light-harvesting chlorophyll a/b complex (LHCII) of the photosynthetic apparatus in green plants consists of a membrane protein and numerous noncovalently bound pigments that make up about one-third of the molecular mass of the pigment-protein complex. Due to this high pigment density, LHCII is potentially interesting as a light-harvesting component in synthetic constructs. However, for such applications its stability needs to be significantly improved. In this work, LHCII was dramatically stabilized by enclosing it within polymerizing colloidal silica. The entrapped LHCII stayed functional at 50 °C for up to 24 h instead of a few minutes in detergent solution and clearly showed energy transfer between complexes. Entrapment yield was enhanced by a polycationic peptide attached to the N terminus. Both the extent of stabilization and the yield of entrapment strongly increased with decreasing diameters of the silica particles.

Comparison of quantum dot-binding protein tags: affinity determination by ultracentrifugation and FRET

BACKGROUND

Hybrid complexes of proteins and colloidal semiconductor nanocrystals (quantum dots, QDs) are of increasing interest in various fields of biochemistry and biomedicine, for instance for biolabeling or drug transport. The usefulness of protein-QD complexes for such applications is dependent on the binding specificity and strength of the components. Often the binding properties of these components are difficult and time consuming to assess.

METHODS

In this work we characterized the interaction between recombinant light harvesting chlorophyll a/b complex (LHCII) and CdTe/CdSe/ZnS QDs by using ultracentrifugation and fluorescence resonance energy transfer (FRET) assay experiments. Ultracentrifugation was employed as a fast method to compare the binding strength between different protein tags and the QDs. Furthermore the LHCII:QD stoichiometry was determined by separating the protein-QD hybrid complexes from unbound LHCII via ultracentrifugation through a sucrose cushion.

RESULTS

One trimeric LHCII was found to be bound per QD. Binding constants were evaluated by FRET assays of protein derivatives carrying different affinity tags. A new tetra-cysteine motif interacted more strongly (Ka=4.9±1.9nM(-1)) with the nanoparticles as compared to a hexahistidine tag (His6 tag) (Ka~1nM(-1)).

CONCLUSION

Relative binding affinities and binding stoichiometries of hybrid complexes from LHCII and quantum dots were identified via fast ultracentrifugation, and binding constants were determined via FRET assays.

GENERAL SIGNIFICANCE

The combination of rapid centrifugation and fluorescence-based titration will be useful to assess the binding strength between different types of nanoparticles and a broad range of proteins.

Carotenoids, versatile components of oxygenic photosynthesis

Carotenoids (CARs) are a group of pigments that perform several important physiological functions in all kingdoms of living organisms. CARs serve as protective agents, which are essential structural components of photosynthetic complexes and membranes, and they play an important role in the light harvesting mechanism of photosynthesizing plants and cyanobacteria. The protection against reactive oxygen species, realized by quenching of singlet oxygen and the excited states of photosensitizing molecules, as well as by the scavenging of free radicals, is one of the main biological functions of CARs. X-ray crystallographic localization of CARs revealed that they are present at functionally and structurally important sites of both the PSI and PSII reaction centers. Characterization of a CAR-less cyanobacterial mutant revealed that while the absence of CARs prevents the formation of PSII complexes, it does not abolish the assembly and function of PSI. CAR molecules assist in the formation of protein subunits of the photosynthetic complexes by gluing together their protein components. In addition to their aforementioned indispensable functions, CARs have a substantial role in the formation and maintenance of proper cellular architecture, and potentially also in the protection of the translational machinery under stress conditions.

Bio serves nano: biological light-harvesting complex as energy donor for semiconductor quantum dots

Light-harvesting complex (LHCII) of the photosynthetic apparatus in plants is attached to type-II core-shell CdTe/CdSe/ZnS nanocrystals (quantum dots, QD) exhibiting an absorption band at 710 nm and carrying a dihydrolipoic acid coating for water solubility. LHCII stays functional upon binding to the QD surface and enhances the light utilization of the QDs significantly, similar to its light-harvesting function in photosynthesis. Electronic excitation energy transfer of about 50% efficiency is shown by donor (LHCII) fluorescence quenching as well as sensitized acceptor (QD) emission and corroborated by time-resolved fluorescence measurements. The energy transfer efficiency is commensurable with the expected efficiency calculated according to Förster theory on the basis of the estimated donor-acceptor separation. Light harvesting is particularly efficient in the red spectral domain where QD absorption is relatively low. Excitation over the entire visible spectrum is further improved by complementing the biological pigments in LHCII with a dye attached to the apoprotein; the dye has been chosen to absorb in the "green gap" of the LHCII absorption spectrum and transfers its excitation energy ultimately to QD. This is the first report of a biological light-harvesting complex serving an inorganic semiconductor nanocrystal. Due to the charge separation between the core and the shell in type-II QDs the presented LHCII-QD hybrid complexes are potentially interesting for sensitized charge-transfer and photovoltaic applications.

Evolution and functional properties of photosystem II light harvesting complexes in eukaryotes

Photoautotrophic organisms, the major agent of inorganic carbon fixation into biomass, convert light energy into chemical energy. The first step of photosynthesis consists of the absorption of solar energy by pigments binding protein complexes named photosystems. Within photosystems, a family of proteins called Light Harvesting Complexes (LHC), responsible for light harvesting and energy transfer to reaction centers, has evolved along with eukaryotic organisms. Besides light absorption, these proteins catalyze photoprotective reactions which allowed functioning of oxygenic photosynthetic machinery in the increasingly oxidant environment. In this work we review current knowledge of LHC proteins serving Photosystem II. Balance between light harvesting and photoprotection is critical in Photosystem II, due to the lower quantum efficiency as compared to Photosystem I. In particular, we focus on the role of each antenna complex in light harvesting, energy transfer, scavenging of reactive oxygen species, chlorophyll triplet quenching and thermal dissipation of excess energy. This article is part of a Special Issue entitled: Photosystem II.

The photoprotective molecular switch in the photosystem II antenna

We have reviewed the current state of multidisciplinary knowledge of the photoprotective mechanism in the photosystem II antenna underlying non-photochemical chlorophyll fluorescence quenching (NPQ). The physiological need for photoprotection of photosystem II and the concept of feed-back control of excess light energy are described. The outline of the major component of nonphotochemical quenching, qE, is suggested to comprise four key elements: trigger (ΔpH), site (antenna), mechanics (antenna dynamics) and quencher(s). The current understanding of the identity and role of these qE components is presented. Existing opinions on the involvement of protons, different LHCII antenna complexes, the PsbS protein and different xanthophylls are reviewed. The evidence for LHCII aggregation and macrostructural reorganization of photosystem II and their role in qE are also discussed. The models describing the qE locus in LHCII complexes, the pigments involved and the evidence for structural dynamics within single monomeric antenna complexes are reviewed. We suggest how PsbS and xanthophylls may exert control over qE by controlling the affinity of LHCII complexes for protons with reference to the concepts of hydrophobicity, allostery and hysteresis. Finally, the physics of the proposed chlorophyll-chlorophyll and chlorophyll-xanthophyll mechanisms of energy quenching is explained and discussed. This article is part of a Special Issue entitled: Photosystem II.

The GDC1 gene encodes a novel ankyrin domain-containing protein that is essential for grana formation in Arabidopsis

In land-plant chloroplasts, the grana play multiple roles in photosynthesis, including the potential increase of photosynthetic capacity in light and enhancement of photochemical efficiency in shade. However, the molecular mechanisms of grana formation remain elusive. Here, we report a novel gene, Grana-Deficient Chloroplast1 (GDC1), required for chloroplast grana formation in Arabidopsis (Arabidopsis thaliana). In the chloroplast of knockout mutant gdc1-3, only stromal thylakoids were observed, and they could not stack together to form appressed grana. The mutant exhibited seedling lethality with pale green cotyledons and true leaves. Further blue native-polyacrylamide gel electrophoresis analysis indicated that the trimeric forms of Light-Harvesting Complex II (LHCII) were scarcely detected in gdc1-3, confirming previous reports that the LHCII trimer is essential for grana formation. The Lhcb1 protein, the major component of the LHCIIb trimer, was substantially reduced, and another LHCIIb trimer component, Lhcb2, was slightly reduced in the gdc1-3 mutant, although their transcription levels were not altered in the mutant. This suggests that defective LHCII trimer formation in gdc1-3 is due to low amounts of Lhcb1 and Lhcb2. GDC1 encodes a chloroplast protein with an ankyrin domain within the carboxyl terminus. It was highly expressed in Arabidopsis green tissues, and its expression was induced by photosignaling pathways. Immunoblot analysis of the GDC1-green fluorescent protein (GFP) fusion protein in 35S::GDC1-GFP transgenic plants with GFP antibody indicates that GDC1 is associated with an approximately 440-kD thylakoid protein complex instead of the LHCII trimer. This shows that GDC1 may play an indirect role in LHCII trimerization during grana formation.

Structural and functional diversification of the light-harvesting complexes in photosynthetic eukaryotes

Eukaryotes acquired photosynthetic metabolism over a billion years ago, and during that time the light-harvesting antennae have undergone significant structural and functional divergence. The antenna systems are generally used to harvest and transfer excitation energy into the reaction centers to drive photosynthesis, but also have the dual role of energy dissipation. Phycobilisomes formed the first antenna system in oxygenic photoautotrophs, and this soluble protein complex continues to be the dominant antenna in extant cyanobacteria, glaucophytes, and red algae. However, phycobilisomes were lost multiple times during eukaryotic evolution in favor of a thylakoid membrane-integral light-harvesting complex (LHC) antenna system found in the majority of eukaryotic taxa. While photosynthesis spread across different eukaryotic kingdoms via endosymbiosis, the antenna systems underwent extensive modification as photosynthetic groups optimized their light-harvesting capacity and ability to acclimate to changing environmental conditions. This review discusses the different classes of LHCs within photosynthetic eukaryotes and examines LHC diversification in different groups in a structural and functional context.

Three-dimensional electron diffraction of plant light-harvesting complex

Electron diffraction patterns of two-dimensional crystals of light-harvesting chlorophyll a/b-protein complex (LHC-II) from photosynthetic membranes of pea chloroplasts, tilted at different angles up to 60 degrees , were collected to 3.2 A resolution at -125 degrees C. The reflection intensities were merged into a three-dimensional data set. The Friedel R-factor and the merging R-factor were 21.8 and 27.6%, respectively. Specimen flatness and crystal size were critical for recording electron diffraction patterns from crystals at high tilts. The principal sources of experimental error were attributed to limitations of the number of unit cells contributing to an electron diffraction pattern, and to the critical electron dose. The distribution of strong diffraction spots indicated that the three-dimensional structure of LHC-II is less regular than that of other known membrane proteins and is not dominated by a particular feature of secondary structure.

Global target analysis of picosecond chlorophyll fluorescence kinetics from pea chloroplasts: A new approach to the characterization of the primary processes in photosystem II alpha- and beta-units

In this study, we have used the method of target analysis to analyze the ps fluorescence kinetics of pea chloroplasts with open (F(0)) and closed (F(max)) photosystem II (PS II) centers. Extending the exciton/radical pair equilibrium model (Schatz, G. H., H. Brock, and A. R. Holzwarth. 1988. Biophys. J. 54:397-405) to allow for PS II heterogeneity, we show that two types of PS II (labeled alpha and beta) must be accounted for, each pool being characterized by its own set of molecular rate constants within the model. Simultaneous global target analysis of the data at F(0) and F(max) results in a detailed description of the molecular kinetics and energetics of the primary processes in both types of PS II units. This characterization revealed that the PS IIalpha pool accounts for twice as many Chl molecules as PS IIbeta, which suggests a PSIIalpha/PSIIbeta reaction center stoichiometry of close to unity. By extrapolation it is shown that the primary charge separation in hypothetical "isolated" beta reaction centers is slower than in isolated alpha reaction centers: in open centers by a factor of 4 (1/k(1) (int) = 11 vs 2.9 ps), in closed centers by a factor of 2 (1/k(1) (int) = 34 vs 19 ps). Despite this slower charge separation process in PS IIbeta, the quantum efficiency of the charge separation process is hardly affected: a charge stabilization yield at F(0), (i.e., P(+)IQ(A) (-)) of 86% (as compared to 90% in PS IIalpha). Reduction of Q(A) (closing PS II) has distinctly different effects on the primary kinetics of PS IIbeta, as compared to PS IIalpha. In PS IIalpha the charge separation rate drops by a factor of 6, whereas the charge recombination process is hardly affected. In PS IIbeta the charge separation is slowed down by a factor of 3, whereas the charge recombination rate increases by a factor of 5. In terms of changes in standard free energy, the reduction to Q(A) (-) lifts the free energy of the radical pair P(+)I(-), relative to the excited state (Chl(n)/P)(*), by 47 meV in PS IIalpha and by 67 meV in PS IIbeta. The concomitant increase in fluorescence quantum yield is the same for both types of PS II. These results show that PS IIalpha and PS IIbeta exhibit a different molecular functioning with respect to the primary processes, which might have its origin in a different molecular structure of the reaction centers and/or a different local environment of these centers. Location in different parts of the thylakoid membrane might be involved. We also applied different error analysis procedures to determine the error ranges of the values found for the molecular rate constants. It is shown that the commonly used standard error has very little meaning, as it assumes independence of the fit parameters. Instead, an exhaustive search procedure, accounting for all possible correlations between the fit parameters, gives a more realistic view on the accuracy of the fit parameters.

Filling the "green gap" of the major light-harvesting chlorophyll a/b complex by covalent attachment of Rhodamine Red

The major light-harvesting chlorophyll a/b complex (LHCII) greatly enhances the efficiency of photosynthesis in green plants. Recombinant LHCII can be assembled in vitro from its denatured, bacterially expressed apoprotein and plant pigments. This makes it an interesting candidate for biomimetic light-harvesting in photovoltaic applications. Due to its almost 20 pigments bound per apoprotein, LHCII absorbs efficiently in the blue and red spectral domains of visible light but less efficiently in the green domain, the so-called "green gap" in its absorption spectrum. Here we present a hybrid complex of recombinant LHCII with organic dyes that add to LHCII absorption in the green spectral region. One or three Rhodamine Red dye molecules were site-specifically attached to cysteine side chains in the apoprotein and did not interfere with LHCII assembly, function and stability. The dyes transferred their excitation energy virtually completely to the chlorophylls in LHCII, partially filling in the green gap. Thus, organic dyes can be used to increase the absorption cross section and, thus, the light-harvesting efficiency of recombinant LHCII.

Determination of membrane protein molecular weight using sedimentation equilibrium analytical ultracentrifugation

Both the stoichiometry and stability of native membrane protein complexes pose challenges to understanding the biology of these proteins. Sedimentation equilibrium analytical ultracentrifugation is well recognized as a thermodynamically rigorous technique for determining these quantities for macromolecules. This unit describes the experimental strategies that can be used to extract this information for membrane proteins reconstituted in vitro in detergent micelle or detergent/lipid mixed micelle solutions.

Immobilization of light-harvesting chlorophyll a/b complex (LHCIIb) studied by surface plasmon field-enhanced fluorescence spectroscopy

The major light-harvesting chlorophyll a/ b complex (LHCIIb) of the photosynthetic apparatus in green plants can be viewed as a protein scaffold binding and positioning a large number of pigment molecules that engage in rapid excitation energy transfer. This property makes LHCIIb potentially interesting as a light harvester (or a model thereof) in photoelectronic applications. Such applications would require the immobilization of LHCIIb (or similar dye-protein complexes) on a solid surface. In this work, the immobilization of recombinant LHCIIb is tested and optimized on functionalized gold surfaces via a histidine 6 tag (His tag) in the protein moiety. Immobilization efficiency and kinetics are analyzed by using surface plasmon resonance (SPR) and surface plasmon field-enhanced fluorescence spectroscopy (SPFS). The latter was also used to assess the integrity of immobilized LHCIIb by recording Chl b-sensitized Chl a emission spectra. Since His tags have been included in a substantial number of recombinant proteins, the immobilization technique developed here for LHCIIb presumably can be extended to a large range of other membrane and water-soluble proteins.

Determination of membrane protein molecular weights and association equilibrium constants using sedimentation equilibrium and sedimentation velocity

Regulated molecular interactions are essential for cellular function and viability, and both homo- and hetero-interactions between all types of biomolecules play important cellular roles. This chapter focuses on interactions between membrane proteins. Knowing both the stoichiometries and stabilities of these interactions in hydrophobic environments is a prerequisite for understanding how this class of proteins regulates cellular activities in membranes. Using examples from the authors' work, this chapter highlights the application of analytical ultracentrifugation methods in the determination of these parameters for integral membrane proteins. Both theoretical and practical aspects of carrying out these experiments are discussed.

Importance of trimer-trimer interactions for the native state of the plant light-harvesting complex II

Aggregates and solubilized trimers of LHCII were characterized by circular dichroism (CD), linear dichroism and time-resolved fluorescence spectroscopy and compared with thylakoid membranes in order to evaluate the native state of LHCII in vivo. It was found that the CD spectra of lamellar aggregates closely resemble those of unstacked thylakoid membranes whereas the spectra of trimers solubilized in n-dodecyl-beta,D-maltoside, n-octyl-beta,D-glucopyranoside, or Triton X-100 were drastically different in the Soret region. Thylakoid membranes or LHCII aggregates solubilized with detergent exhibited CD spectra similar to the isolated trimers. Solubilization of LHCII was accompanied by profound changes in the linear dichroism and increase in fluorescence lifetime. These data support the notion that lamellar aggregates of LHCII retain the native organization of LHCII in the thylakoid membranes. The results indicate that the supramolecular organization of LHCII, most likely due to specific trimer-trimer contacts, has significant impact on the pigment interactions in the complexes.

Assembly of the major light-harvesting chlorophyll-a/b complex: Thermodynamics and kinetics of neoxanthin binding

The major light-harvesting chlorophyll-a/b complex in most higher plants contains three carotenoids, lutein, neoxanthin, and violaxanthin. How these pigments are assembled into the complex during its biogenesis is largely unknown. Here we show that neoxanthin but not lutein can dissociate from the fully assembled complex. Its equilibrium binding constant in a detergent system (0.1% n-dodecyl-beta-D-maltoside) was determined to be > or = 10(6) m(-1). Neoxanthin insertion into light-harvesting chlorophyll-a/b complex prefolded from overexpressed apoprotein (Lhcb1*2 from Pisum sativum) in the presence of chlorophylls a, b, and lutein as the sole carotenoid is kinetically controlled by an activation energy barrier of approximately 120 kJ mol(-1). This is the first thermodynamic and kinetic description of a binding equilibrium between a non-covalently bound pigment of the photosynthetic apparatus and its protein complex. Dissociation of neoxanthin from the major light-harvesting chlorophyll-a/b complex upon temperature increase is discussed in terms of providing a readily available substrate pool for synthesizing abscisic acid as part of a heat and drought stress response.

Supramolecular organization of thylakoid membrane proteins in green plants

The light reactions of photosynthesis in green plants are mediated by four large protein complexes, embedded in the thylakoid membrane of the chloroplast. Photosystem I (PSI) and Photosystem II (PSII) are both organized into large supercomplexes with variable amounts of membrane-bound peripheral antenna complexes. PSI consists of a monomeric core complex with single copies of four different LHCI proteins and has binding sites for additional LHCI and/or LHCII complexes. PSII supercomplexes are dimeric and contain usually two to four copies of trimeric LHCII complexes. These supercomplexes have a further tendency to associate into megacomplexes or into crystalline domains, of which several types have been characterized. Together with the specific lipid composition, the structural features of the main protein complexes of the thylakoid membranes form the main trigger for the segregation of PSII and LHCII from PSI and ATPase into stacked grana membranes. We suggest that the margins, the strongly folded regions of the membranes that connect the grana, are essentially protein-free, and that protein-protein interactions in the lumen also determine the shape of the grana. We also discuss which mechanisms determine the stacking of the thylakoid membranes and how the supramolecular organization of the pigment-protein complexes in the thylakoid membrane and their flexibility may play roles in various regulatory mechanisms of green plant photosynthesis.

Pigment Binding, Fluorescence Properties, and Oligomerization Behavior of Lhca5, a Novel Light-harvesting Protein

A new potential light-harvesting protein, named Lhca5, was recently detected in higher plants. Because of the low amount of Lhca5 in thylakoid membranes, the isolation of a native Lhca5 pigment-protein complex has not been achieved to date. Therefore, we used in vitro reconstitution to analyze whether Lhca5 binds pigments and is actually an additional light-harvesting protein. By this approach we could demonstrate that Lhca5 binds pigments in a unique stoichiometry. Analyses of pigment requirements for light-harvesting complex formation by Lhca5 revealed that chlorophyll b is the only indispensable pigment. Fluorescence measurements showed that ligated chlorophylls and carotenoids are arranged in a way that allows directed energy transfer within the light-harvesting complex. Reconstitutions of Lhca5 together with other Lhca proteins resulted in the formation of heterodimers with Lhca1. This result demonstrates that Lhca5 is indeed a protein belonging to the light-harvesting antenna of photosystem I. The properties of Lhca5 are compared with those of previously characterized Lhca proteins, and the consequences of an additional Lhca protein for the composition of the light-harvesting antenna of photosystem I are discussed in view of the recently published photosystem I structure of the pea.

Degradation of the main Photosystem II light-harvesting complex

Many factors trigger the degradation of proteins, including changes in environmental conditions, genetic mutations, and limitations in the availability of cofactors. Despite the importance for viability, still very little is known about protein degradation and its regulation. The degradation of the most abundant membrane protein on Earth, the light-harvesting complex of Photosystem II (LHC II), is highly regulated under different environmental conditions, e.g. light stress, to prevent photochemical damage of the reaction center. However, despite major effort to identify the protease/proteases involved in the degradation of the apoproteins of LHC II the molecular details of this important process remain obscure. LHC II belongs to the family of chlorophyll a/b binding proteins (CAB proteins) and is located in the thylakoid membrane of the plant chloroplast. The results of biochemical experiments to isolate and characterize the protease degrading LHC II are summarized here and compared to our own recent finding indicating that a metalloprotease of the FtsH family is involved in this process.

The three isoforms of the light-harvesting complex II: spectroscopic features, trimer formation, and functional roles

The major light-harvesting complex (LHC-II) of higher plants plays a crucial role in capturing light energy for photosynthesis and in regulating the flow of energy within the photosynthetic apparatus. Native LHC-II isolated from plant tissue consists of three isoforms, Lhcb1, Lhcb2, and Lhcb3, which form homo- and heterotrimers. All three isoforms are highly conserved among different species, suggesting distinct functional roles. We produced the three LHC-II isoforms by heterologous expression of the polypeptide in Escherichia coli and in vitro refolding with purified pigments. Although Lhcb1 and Lhcb2 are very similar in polypeptide sequence and pigment content, Lhcb3 is clearly different because it lacks an N-terminal phosphorylation site and has a higher chlorophyll a/b ratio, suggesting the absence of one chlorophyll b. Low temperature absorption and fluorescence emission spectra of the pure isoforms revealed small but significant differences in pigment organization. The oligomeric state of the pure isoforms and of their permutations was investigated by native gel electrophoresis, sucrose density gradient centrifugation, and SDS-PAGE. Lhcb1 and Lhcb2 formed trimeric complexes by themselves and with one another, but Lhcb3 was able to do so only in combination with one or both of the other isoforms. We conclude that the main role of Lhcb1 and Lhcb2 is in the adaptation of photosynthesis to different light regimes. The most likely role of Lhcb3 is as an intermediary in light energy transfer from the main Lhcb1/Lhcb2 antenna to the photosystem II core.

Folding in vitro of light-harvesting chlorophyll a/b protein is coupled with pigment binding

The major light-harvesting chlorophyll a/b protein (LHCIIb) of the plant photosynthetic apparatus is able to self-organise in vitro. When the recombinant apoprotein, Lhcb1, is solubilised in the denaturing detergent sodium (or lithium) dodecylsulfate (SDS or LDS) and then mixed with chlorophylls and carotenoids under renaturing conditions, structurally authentic LHCIIb forms. Assembly of functional LHCIIb, as indicated by the establishment of energy transfer between complex-bound chlorophyll molecules, occurs in two apparent kinetic steps with time constants of 10 to 30 seconds and 50 to 300 seconds, depending on the reaction conditions. Here, we use circular dichroism (CD) in the far-UV range to monitor the folding of the LHCIIb apoprotein as it is complexed with pigments. The alpha-helix content in the protein's secondary structure increases in two apparent kinetic steps with time constants similar to those observed for the establishment of chlorophyll energy transfer. When the carotenoid concentration in the reaction mixture is reduced, the time constants of alpha-helix formation increase, as do those for the appearance of chlorophyll energy transfer. This indicates that both processes, pigment assembly and secondary structure formation, are tightly coupled. A substantial amount of alpha-helix is present in dodecylsulfate-solubilised LHCIIb apoprotein and appears to be distributed among various protein domains.

The chequered history of the development and use of simultaneous equations for the accurate determination of chlorophylls a and b

Over the last half century, the most frequently used assay for chlorophylls in higher plants and green algae, the Arnon assay [Arnon DI (1949) Plant Physiol 24: 1-15], employed simultaneous equations for determining the concentrations of chlorophylls a and b in aqueous 80% acetone extracts of chlorophyllous plant and algal materials. These equations, however, were developed using extinction coefficients for chlorophylls a and b derived from early inaccurate spectrophotometric data. Thus, Arnon's equations give inaccurate chlorophyll a and b determinations and, therefore, inaccurate chlorophyll a/b ratios, which are always low. This paper describes how the ratios are increasingly and alarmingly low as the proportion of chlorophyll a increases. Accurate extinction coefficients for chlorophylls a and b, and the more reliable simultaneous equations derived from them, have been published subsequently by many research groups; these new post-Arnon equations, however, have been ignored by many researchers. This Minireview records the history of the development of accurate simultaneous equations and some difficulties and anomalies arising from the retention of Arnon's seriously flawed equations.

The chequered history of the development and use of simultaneous equations for the accurate determination of chlorophylls a and b

Over the last half century, the most frequently used assay for chlorophylls in higher plants and green algae, the Arnon assay [Arnon DI (1949) Plant Physiol 24: 1-15], employed simultaneous equations for determining the concentrations of chlorophylls a and b in aqueous 80% acetone extracts of chlorophyllous plant and algal materials. These equations, however, were developed using extinction coefficients for chlorophylls a and b derived from early inaccurate spectrophotometric data. Thus, Arnon's equations give inaccurate chlorophyll a and b determinations and, therefore, inaccurate chlorophyll a/b ratios, which are always low. This paper describes how the ratios are increasingly and alarmingly low as the proportion of chlorophyll a increases. Accurate extinction coefficients for chlorophylls a and b, and the more reliable simultaneous equations derived from them, have been published subsequently by many research groups; these new post-Arnon equations, however, have been ignored by many researchers. This Minireview records the history of the development of accurate simultaneous equations and some difficulties and anomalies arising from the retention of Arnon's seriously flawed equations.

Tandem mass spectrometric identification of spinach Photosystem II light-harvesting components

Light-harvesting proteins harness light energy for photosynthesis. Sequences of the Photosystem II (PS II) light harvesting proteins, Lhcb1-6, have been deduced from many plants. However, limited information is available for spinach Lhcb sequences, although a spinach PS II preparation (BBY) is commonly used as a model for plant photosynthetic oxygen evolution [DA Berthold, GT Babcock and CF Yocum (1981) FEBS Lett 134: 231-234]. In this work, we describe the use of tryptic digestion, liquid chromatography, tandem mass spectrometry, and database searching to identify light-harvesting proteins in the spinach BBY preparation. Using this approach, partial amino acid sequences were assigned to the PS II-associated light-harvesting proteins, Lhcb1-6. The identified stretches of sequence are predicted to contain intra-membranous chlorophyll ligands, extra-membranous loop regions, and lutein-binding sites. In addition, we find that at least two distinct Lhcb4 (CP29) polypeptides and two distinct Lhcb1 polypeptides are present in the BBY preparation. One of these Lhcb4 polypeptides has a subsequence that has not been reported for Lhcb4 in any other organism. This work demonstrates the utility of tandem mass spectrometry in the characterization of photosynthetic membrane proteins.

The chequered history of the development and use of simultaneous equations for the accurate determination of chlorophylls a and b

Over the last half century, the most frequently used assay for chlorophylls in higher plants and green algae, the Arnon assay [Arnon DI (1949) Plant Physiol 24: 1-15], employed simultaneous equations for determining the concentrations of chlorophylls a and b in aqueous 80% acetone extracts of chlorophyllous plant and algal materials. These equations, however, were developed using extinction coefficients for chlorophylls a and b derived from early inaccurate spectrophotometric data. Thus, Arnon's equations give inaccurate chlorophyll a and b determinations and, therefore, inaccurate chlorophyll a/b ratios, which are always low. This paper describes how the ratios are increasingly and alarmingly low as the proportion of chlorophyll a increases. Accurate extinction coefficients for chlorophylls a and b, and the more reliable simultaneous equations derived from them, have been published subsequently by many research groups; these new post-Arnon equations, however, have been ignored by many researchers. This Minireview records the history of the development of accurate simultaneous equations and some difficulties and anomalies arising from the retention of Arnon's seriously flawed equations.

Heterogeneity of the main light-harvesting chlorophyll a/b-protein complex of photosystem II (LHCII) at the level of trimeric subunits

To study organization of the main light-harvesting chlorophyll a/b-protein complex of photosystem II (LHCII) from spinach thylakoid membranes at the level of trimeric subcomplexes, we have applied non-denaturing isoelectric focusing (ndIEF) in vertical, slab polyacrylamide gels. When analyzed by two consecutive ndIEF/electroelution runs, spinach BBY membrane preparations (PSII(alpha)-enriched, stacked thylakoid membranes) were resolved into nine fractions of 100% purity, labelled 1-9 in order of decreasing pI values. Seven of these fractions (3-9) were shown by absorption spectroscopy to stand for LHCII subcomplexes. The subcomplexes were established - by monitoring their circular dichroism spectra and comparing them to the spectra of native LHCII trimers and monomers - to be structurally intact trimers. The analysis of polypeptide composition of the subcomplexes in terms of apparent molecular masses and Lhcb genes' products led us to the conclusion that each of the subcomplexes might be a mixed population of closely similar individual trimers, comprising of permutations of Lhcb1 and Lhcb2 (subcomplexes 3-7) or Lhcb1, Lhcb2 and Lhcb3 (subcomplexes 8 and 9).

Identification of Lhcb1/Lhcb2/Lhcb3 heterotrimers of the main light-harvesting chlorophyll a/b-protein complex of Photosystem II (LHC II)

Using non-denaturing isoelectric focusing in polyacrylamide vertical slab gel, we have purified to homogeneity three trimeric subcomplexes of LHC II from Arabidopsis thylakoid membranes. The polypeptide composition of the subcomplexes were studied by immunoblotting. Our results indicate the existence in vivo of LHC II heterotrimers containing Lhcb1, Lhcb2 and Lhcb3 gene products.

Diatom fucoxanthin chlorophyll a/c-binding protein (FCP) and land plant light-harvesting proteins use a similar pathway for thylakoid membrane Insertion

The light-harvesting proteins in plastids of different lineages including algae and land plants represent a superfamily of chlorophyll-binding proteins that seem to be phylogenetically related, although some of the light-harvesting complex (LHC) proteins bind different carotenoids. LHCs can be divided into chlorophyll a/b-binding proteins found in green algae, euglenoids, and higher plants and into chlorophyll a/c-binding proteins of various algal taxa. LHC proteins from diatoms are named fucoxanthin-chlorophyll a/c-binding proteins (FCP). In contrast to chlorophyll a/b-binding proteins, there is no information so far about the way FCPs integrate into thylakoid membranes. The diatom FCP preproteins have a bipartite presequence that is necessary to enable transport into the four membrane-bound diatom plastids, but similar to chlorophyll a/b-binding proteins there is apparently no presequence present for targeting to the thylakoid membrane. By establishing an in vitro import assay for diatom thylakoids, we demonstrated that thylakoid integration of diatom FCP depends on the presence of stromal factors and GTP. This indicates that a pathway involving signal recognition particles (SRP) is involved in membrane integration just as shown for LHCs in higher plants. We also demonstrate integration of diatom FCP into thylakoids of higher plants and vice versa SRP-dependent targeting of LHCs from pea and Arabidopsis into diatom thylakoids. The similar SRP-dependent modes of thylakoid integration of land plant LHCs and FCPs support recent analyses indicating a common origin of chlorophyll a/b- and a/c-binding proteins.

Application of molecular orbital calculations to interpret the chlorophyll spectral forms in pea photosystem II

The energy and oscillator strength of electronic transitions of chlorophyll (Chl)-amino acid complexes were calculated by using molecular orbital methods. The energies varied widely with coordinated amino acids and the difference between the maximum and minimum energy was about 830 cm-1. This energy difference was comparable with the spreading of absorption bands for light-harvesting Chl-protein complexes of photosystem II (LHC II) of green plants. The feature of the Qy band for pea LHC II was interpreted with the aid of the calculated energies and oscillator strengths. Four spectral components of the band were assigned to individual Chl-amino acid complexes.

Electric properties of thylakoid membranes from pea mutants with modified carotenoid and chlorophyll-protein complex composition

Surface electric properties of thylakoid membranes from wild type and two mutant forms, Coeruleovireus 2/16 and Costata 2/133, of pea are investigated by electric light scattering and microelectrophoretic measurements. Characterization of the chlorophyll-protein complexes in thylakoid membranes reveals that the relative ratio of oligomeric (LHC II(1)) to monomeric (LHC II(3)) forms of the light-harvesting Chl a/b complex of Photosystem II is lower (3.34) in 2/133 mutant and higher (6.62) in 2/16 mutant than in wild type (4.57). This is accompanied by elevated amounts and a considerable reduction of all carotenoids in 2/16 and 2/133 mutant, respectively, as compared to the wild type. The concomitant variations of the permanent dipole moment (transversal charge asymmetry), electric polarizability and electrokinetic charge of the thylakoid membranes from both the mutants are discussed in terms of the differences in the supramolecular (oligomeric) organization of the light-harvesting complexes II within the photosynthetic apparatus.

Impaired photosystem II in a mutant of Chlamydomonas reinhardtii defective in sulfoquinovosyl diacylglycerol

The photosynthetic apparatus was characterized in a mutant of Chlamydomonas reinhardtii, hf-2, defective in the synthesis of a chloroplast-specific lipid, sulfoquinovosyl diacylglycerol (SQui-acyl2Gro). hf-2 showed reduced photosystem II (PSII) activity with little effect on photosystem I (PSI) activity, as compared with the parent. PAGE in the presence of dodecyl beta-D-maltoside (DodGlc2) of C. reinhardtii thylakoid membranes was used to isolate chlorophyll-protein complexes without chlorophyll (Chl) release in order to examine lipid species bound to these complexes. The four complexes obtained were shown to be the PSI complex, the PSII core complex and the two groups of the light-harvesting complex of PSII by analyses of 77-K emission spectra of Chl fluorescence and of subunit compositions. Lipid analysis of Chl-protein complexes in the parent revealed the localization of SQui-acyl2Gro in the PSII core complex and the two groups of the light-harvesting complex of PSII, but not in the PSI complex. These results suggest that SQui-acyl2Gro is responsible for PSII activity by associating with the core and light-harvesting complexes of PSII.

Circadian expression of the light-harvesting protein of Photosystem II in etiolated bean leaves following a single red light pulse: Coordination with the capacity of the plant to form chlorophyll and the thylakoid-bound protease

The appearance of the light harvesting II (LHC II) protein in etiolated bean leaves, as monitored by immunodetection in LDS-solubilized leaf protein extracts, is under phytochrome control. A single red light pulse induces accumulation of the protein, in leaves kept in the dark thereafter, which follows circadian oscillations similar to those earlier found for Lhcb mRNA (Tavladoraki et al. (1989) Plant Physiol 90: 665-672). These oscillations are closely followed by oscillations in the capacity of the leaf to form Chlorophyll (Chl) in the light, suggesting that the synthesis of the LHC II protein and its chromophore are in close coordination. Experiments with levulinic acid showed that PChl(ide) resynthesis does not affect the LHC II level nor its oscillations, but new Chl a synthesis affects LHC II stabilization in thylakoids, implicating a proteolytic mechanism. A proteolytic activity against exogenously added LHC II was detected in thylakoids of etiolated bean leaves, which was enhanced by the light pulse. The activity, also under phytochrome control, was found to follow circadian oscillations in verse to those in the stabilization of LHC II protein in thylakoids. Such a proteolytic mechanism therefore, may account for the circadian changes observed in LHC II protein level, being implicated in pigment-protein complex assembly/stabilization during thylakoid biogenesis.

The resolution and biochemical characterization of subcomplexes of the main light-harvesting chlorophyll a/b-protein complex of Photosystem II (LHC II)

LHC II isolated from carnation leaves has been solubilized and resolved by a newly developed, vertical-bed non-denaturing isoelectric focusing in polyacrylamide slab gels to yield three trimeric subcomplexes focusing at pH 4.52, 4.42 and 4.37 (designated a, b and c, respectively), comprising approximately 38%, 24% and 38% of the chlorophyll. The spectroscopic data demonstrated a close similarity among LHC II subcomplexes concerning their chlorophyll content and organization. The most alkaline and the most acidic subcomplex contained the 27 kDa polypeptide of LHC II while the intermediate pI fraction contained both LHC II polypeptides, i.e. 27 kDa and 26 kDa ones associated at 2:1 stoichiometry. The 27 kDa polypeptide could be resolved by denaturing isoelectrofocusing into 10 pI molecular isoforms covering 5.90-4.20 pH range. Three of the isoforms were found in the subcomplexes a and b and eight in the subcomplex c. The 26 kDa polypeptide comprised the unique pI molecular isoform focusing at pH 5.61.

Heterogenous lipid distribution among chlorophyll-binding proteins of photosystem II in maize mesophyll chloroplasts

Photosystem II membrane fractions from dark-adapted mesophyll chloroplasts of maize were solubilized in different concentrations of dodecyl beta-D-maltoside. Chlorophyll-binding proteins from photosystem II were isolated either by ultracentrifugation on a sucrose gradient, or by flat bed isoelectric focusing and identified by gel electrophoresis analysis for their polypeptide composition. Lipid and fatty acid compositions were determined in complexes prepared by both methods and also in purified light-harvesting complex II, in minor chlorophyll a/b binding complexes 29, 26, 24, in photosystem II antennae (chlorophyll-protein complexes 43, 47) and in the photosystem II reaction centers chlorophyll-protein complexes. Comparative analysis of the results suggests that a true heterogeneity exists in the lipid class distribution among the different chlorophyll-protein complexes in this region of the photosynthetic membrane. Photosystem II core fractions prepared either by ultra-centrifugation on a sucrose gradient or by isoelectric focusing were found significantly enriched in monogalactosyldiacylglycerol; fractionation of the photosystem II core in its components showed that it was the chlorophyll-protein complexes 43 and 47 which were mainly responsible for this enrichment. One of them, the chlorophyll-protein complex 47, was found containing monogalactosyldiacylglycerol and having a very high level of saturated fatty acids. The minor chlorophyll a/b binding linkers (chlorophyll-protein complexes 24, 26 and 29) retain a largely higher amount of lipids than all other complexes and especially of highly unsaturated galactolipids. Concerning the main light-harvesting antenna (LHCII), it is demonstrated that phosphatidylglycerol is strongly linked to the complex if it cannot be detached at high detergent concentration, while many galactolipids (which nevertheless represent the major lipid classes) are lost. This main light-harvesting complex has been fractionated into several families by isoelectric focusing showing a marked difference in lipid and polypeptide composition. A spectacular increase in the phosphatidylglycerol content was observed in the fraction migrating near the anode and enriched in a 26-kDa polypeptide; but this result is difficult to interpret in physiological terms as it was shown that phosphatidylglycerol alone, because of its negative charge, also migrates toward the anode in isoelectric focusing.

Photosynthesis. Many chlorophylls make light work

The structure of a plant light-harvesting complex at atomic resolution, determined recently by electron crystallography, helps to explain the efficiency and speed of the light-gathering process.

Ultrafast chlorophyll b-chlorophyll a excitation energy transfer in the isolated light harvesting complex, LHC II, of green plants. Implications for the organisation of chlorophylls

The excitation energy transfer between chlorophyll b (Chl b) and chlorophyll a (Chl a) in the isolated trimeric chlorophyll-a/b-binding protein complex of spinach photosystem 2 (LHC II) has been studied by femtosecond spectroscopy. In the main absorption band of Chl b the ground state recovery consists of two components of 0.5 ps and 2.0 ps, respectively. Also in the Chl a absorption band, at 665 nm, the ground state recovery is essentially bi-exponential. In this case is, however, the fastest relaxation lifetime is a 2.0 ps component followed by a slower component with a lifetime in the order of 10-20 ps. In the Chl b absorption band a more or less constant anisotropy of r = 0.2 was observed during the 3 ps the system was monitored. In the Chl a absorption band there was, however, a relaxation of the anisotropy from r = 0.3 to a quasi steady state level of r = 0.18 in about 1 ps. Since the 0.5 ps component is only seen upon selective excitation of Chl b we assign this component to the energy transfer between Chl b and Chl a. The other components most likely represents redistribution processes of energy among spectrally different forms of Chl a. The energy transfer process between Chl b and Chl a can well be explained by the Förster mechanism which also gives a calculated distance of 13 A between interacting chromophores. The organisation of chlorophylls in LHC II is discussed in view of the recent crystal structure data (1991) Nature 350, 130].

Biogenesis of thylakoid membranes with emphasis on the process in Chlamydomonas

Recent results obtained by electron microscopic and biochemical analyses of greening Chlamydomonas reinhardtii y1 suggest that localized expansion of the plastid envelope is involved in thylakoid biogenesis. Kinetic analyses of the assembly of light-harvesting complexes and development of photosynthetic function when degreened cells of the alga are exposed to light suggest that proteins integrate into membrane at the level of the envelope. Current information, therefore, supports the earlier conclussion that the chloroplast envelope is a major biogenic structure, from which thylakoid membranes emerge. Chloroplast development in Chlamydomonas provides unique opportunities to examine in detail the biogenesis of thylakoids.

Pigments induce folding of light-harvesting chlorophyll a/b-binding protein

The conformational behaviour of the light-harvesting chlorophyll a/b-binding protein (LHCP), the apoprotein of the major light-harvesting complex (LHCII) of photosystem II in plants, has been studied. According to the circular dichroism in the ultraviolet range measured with isolated LHCII, the protein in the complex adopts a folded structure with a high content of alpha helix (about 60%), whereas the non-pigmented, solubilized protein has a less ordered structure (about 20% alpha helix). LHCP-pigment complexes that have been reconstituted from the overexpressed protein and isolated pigments in the presence of detergents display a protein CD signal similar to that of authentic LHCII, indicating that LHCP folds into the native structure during the reconstitution procedure. Renaturation of LHCP in these experiments is dependent on the presence of pigments and the formation of stable LHCP-pigment complexes. Pigment-induced engagement of LHCP in a compact structure has also been shown by two additional experimental approaches. (a) Upon complex formation, LHCP or its precursor (pLHCP) becomes resistant to trypsin digestion with the exception of an N-terminal segment of the protein; the same protection of LHCP is known to occur in intact thylakoids. (b) Pigment binding renders a cysteine residue within the N-proximal hydrophobic domain of the protein as well as a newly introduced cysteine four amino acid positions from the C terminus inaccessible to modification with a sulfhydryl-specific label whereas the N terminus stays susceptible to specific labelling. These observations support the notion that only the N terminus protrudes from a compact protein-pigment structure in LHCII. The fact that the major part of LHCP is trypsin-resistant in pigmented complexes reconstituted in the absence of a membrane or even lipids justifies caution in using protection against trypsin as a criterion for the integration of LHCP into the thylakoid membrane.