Lipid-protein interactions in thylakoid membranes of chilling-resistant and -sensitive plants studied by spin label electron spin resonance spectroscopy

Lipid and protein dynamics of stacked and cation-depletion induced unstacked thylakoid membranes

Chloroplast thylakoid membranes in plants and green algae form 3D architectures of stacked granal membranes interconnected by unstacked stroma lamellae. They undergo dynamic structural changes as a response to changing light conditions that involve grana unstacking and lateral supramolecular reorganization of the integral membrane protein complexes. We assessed the dynamics of thylakoid membrane components and addressed how they are affected by thylakoid unstacking, which has consequences for protein mobility and the diffusion of small electron carriers. By a combined nuclear and electron paramagnetic-resonance approach the dynamics of thylakoid lipids was assessed in stacked and cation-depletion induced unstacked thylakoids of Chlamydomonas (C.) reinhardtii. We could distinguish between structural, bulk and annular lipids and determine membrane fluidity at two membrane depths: close to the lipid headgroups and in the lipid bilayer center. Thylakoid unstacking significantly increased the dynamics of bulk and annular lipids in both areas and increased the dynamics of protein helices. The unstacking process was associated with membrane reorganization and loss of long-range ordered Photosystem II- Light-Harvesting Complex II (PSII-LHCII) complexes. The fluorescence lifetime characteristics associated with membrane unstacking are similar to those associated with state transitions in intact C. reinhardtii cells. Our findings could be relevant for understanding the structural and functional implications of thylakoid unstacking that is suggested to take place during several light-induced processes, such as state transitions, photoacclimation, photoinhibition and PSII repair.

Specific Composition of Lipid Phases Allows Retaining an Optimal Thylakoid Membrane Fluidity in Plant Response to Low-Temperature Treatment

Thylakoid membranes isolated from leaves of two plant species, the chilling tolerant (CT) pea and chilling sensitive (CS) runner bean, were assessed for the composition of lipids, carotenoids as well as for the arrangement of photosynthetic complexes. The response to stress conditions was investigated in dark-chilled and subsequently photo-activated detached leaves of pea and bean. Thylakoids of both species have a similar level of monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), but different sulfoquinovosyldiacylglycerol to phosphatidylglycerol (PG) ratio. In pea thylakoid fraction, the MGDG, DGDG and PG, have a higher double bond index (DBI), whereas bean thylakoids contain higher levels of high melting point PG. Furthermore, the lutein to the β-carotene ratio is higher in bean thylakoids. Smaller protein/lipid ratio in pea than in bean thylakoids suggests different lipid-protein interactions in both species. The differences between species are also reflected by the course of temperature-dependent plots of chlorophyll fluorescence pointing various temperatures of the lipid phase transitions of pea and bean thylakoids. Our results showed higher fluidity of the thylakoid membrane network in pea than in bean in optimal temperature conditions. Dark-chilling decreases the photochemical activity and induces significant degradation of MGDG in bean but not in pea leaves. Similarly, substantial changes in the arrangement of photosynthetic complexes with increase in LHCII phosphorylation and disturbances of the thylakoid structure take place in bean thylakoids only. Changes in the physical properties of bean thylakoids are manifested by the conversion of a three-phase temperature-dependent plot to a one-phase plot. Subsequent photo-activation of chilled bean leaves caused a partial restoration of the photochemistry and of membrane physical properties, but not of the photosynthetic complexes arrangement nor the thylakoid network structure. Summarizing, the composition of the thylakoid lipid matrix of CT pea allows retaining the optimal fluidity of its chloroplast membranes under low temperatures. In contrast, the fluidity of CS bean thylakoids is drastically changed, leading to the reorganization of the supramolecular structure of the photosynthetic complexes and finally results in structural remodeling of the CS bean thylakoid network.

Studying Lipid-Protein Interactions with Electron Paramagnetic Resonance Spectroscopy of Spin-Labeled Lipids

Spin label electron paramagnetic resonance (EPR) of lipid-protein interactions reveals crucial features of the structure and assembly of integral membrane proteins. Spin-label EPR spectroscopy is the technique of choice to characterize the protein solvating lipid shell in its highly dynamic nature, because the EPR spectra of lipids that are spin-labeled close to the terminal methyl end of their acyl chains display two spectral components, those corresponding to lipids directly contacting the protein and those corresponding to lipids in the bulk fluid bilayer regions of the membrane. In this chapter, typical spin label EPR procedures are presented that allow determination of the stoichiometry of interaction of spin-labeled lipids with the intramembranous region of membrane proteins or polypeptides, as well as the association constant of the spin-labeled lipid with respect to the host lipid. The lipids giving rise to a so-called immobile spectral component in the EPR spectrum of such samples are identified as the motionally restricted first-shell lipids solvating membrane proteins in biomembranes. Stoichiometry and selectivity are directly related to the structure of the intramembranous sections of membrane-associated proteins or polypeptides and can be used to study the state of assembly of such proteins in the membrane. Since these characteristics of lipid-protein interactions are discussed in detail in the literature (see ref. Marsh, Eur Biophys J 39:513-525, 2010 for a recent review), here we focus more on how to spin label model membranes and biomembranes and how to measure and analyze the two-component EPR spectra of spin-labeled lipids in phospholipid bilayers that contain proteins or polypeptides. After a description of how to prepare spin-labeled model and native biological membranes, we present the reader with computational procedures for determining the molar fraction of motionally restricted lipids when both, one or none of the pure isolated-mobile or immobile-spectral components are available. With these topics, this chapter complements a previous methodological paper (Marsh, Methods 46:83-96, 2008). The interpretation of the data is discussed briefly, as well as other relevant and recent spin label EPR techniques for studying lipid-protein interactions, not only from the point of view of lipid chain dynamics.

Studying lipid-protein interactions with electron paramagnetic resonance spectroscopy of spin-labeled lipids

Spin label electron paramagnetic resonance (EPR) of lipid-protein interactions reveals crucial features of the structure and assembly of integral membrane proteins. Spin label EPR spectroscopy is the technique of choice to characterize the protein-solvating lipid shell in its highly dynamic nature, because the EPR spectra of lipids that are spin labeled close to the terminal methyl end of their acyl chains display two spectral components, those corresponding to lipids directly contacting the protein and those corresponding to lipids in the bulk fluid bilayer regions of the membrane. In this chapter, typical spin label EPR procedures are presented that allow determination of the stoichiometry of interaction of spin-labeled lipids with the intra-membranous region of membrane proteins or polypeptides, as well as the association constant of the spin-labeled lipid with respect to the host lipid. The lipids giving rise to the so-called immobile spectral component in the EPR spectrum of such samples are identified as the motionally restricted first-shell lipids solvating membrane proteins in biomembranes. Stoichiometry and selectivity are directly related to the structure of the intra-membranous sections of membrane-associated proteins or polypeptides and can be used to study the state of assembly of such proteins in the membrane. Since these characteristics of lipid-protein interactions are discussed in detail in the literature [see Marsh (Eur Biophys J 39:513-525, 2010) for a most recent review], here we focus more on how to spin label model and biomembranes and how to measure and analyze the two-component EPR spectra of spin-labeled lipids in phospholipid bilayers that contain proteins or polypeptides. After a description of how to prepare spin-labeled model and native biological membranes, we present the reader with computational procedures for determining the molar fraction of motionally restricted lipids when both, one, or none of the pure isolated-mobile or immobile-spectral components are available. With these topics, this chapter complements a recent methodological paper [Marsh (Methods 46:83-96, 2008)]. The interpretation of the data is discussed briefly, as well as other relevant and recent spin label EPR techniques for studying lipid-protein interactions, not only from the point of view of lipid chain dynamics.

Electron spin resonance in membrane research: protein-lipid interactions from challenging beginnings to state of the art

Conventional electron paramagnetic resonance (EPR) spectra of lipids that are spin-labelled close to the terminal methyl end of the acyl chains are able to resolve the lipids directly contacting the protein from those in the fluid bilayer regions of the membrane. This allows determination of both the stoichiometry of lipid-protein interaction (i.e., number of lipid sites at the protein perimeter) and the selectivity of the protein for different lipid species (i.e., association constants relative to the background lipid). Spin-label EPR data are summarised for 20 or more different transmembrane peptides and proteins, and 7 distinct species of lipids. Lineshape simulations of the two-component conventional spin-label EPR spectra allow estimation of the rate at which protein-associated lipids exchange with those in the bulk fluid regions of the membrane. For lipids that do not display a selectivity for the protein, the intrinsic off-rates for exchange are in the region of 10 MHz: less than 10x slower than the rates of diffusive exchange in fluid lipid membranes. Lipids with an affinity for the protein, relative to the background lipid, have off-rates for leaving the protein that are correspondingly slower. Non-linear EPR, which depends on saturation of the spectrum at high radiation intensities, is optimally sensitive to dynamics on the timescale of spin-lattice relaxation, i.e., the microsecond regime. Both progressive saturation and saturation transfer EPR experiments provide definitive evidence that lipids at the protein interface are exchanging on this timescale. The sensitivity of non-linear EPR to low frequencies of spin exchange also allows the location of spin-labelled membrane protein residues relative to those of spin-labelled lipids, in double-labelling experiments.

Contrasting effect of dark-chilling on chloroplast structure and arrangement of chlorophyll-protein complexes in pea and tomato: plants with a different susceptibility to non-freezing temperature

The effect of dark-chilling and subsequent photoactivation on chloroplast structure and arrangements of chlorophyll-protein complexes in thylakoid membranes was studied in chilling-tolerant (CT) pea and in chilling-sensitive (CS) tomato. Dark-chilling did not influence chlorophyll content and Chl a/b ratio in thylakoids of both species. A decline of Chl a fluorescence intensity and an increase of the ratio of fluorescence intensities of PSI and PSII at 120 K was observed after dark-chilling in thylakoids isolated from tomato, but not from pea leaves. Chilling of pea leaves induced an increase of the relative contribution of LHCII and PSII fluorescence. A substantial decrease of the LHCII/PSII fluorescence accompanied by an increase of that from LHCI/PSI was observed in thylakoids from chilled tomato leaves; both were attenuated by photoactivation. Chlorophyll fluorescence of bright grana discs in chloroplasts from dark-chilled leaves, detected by confocal laser scanning microscopy, was more condensed in pea but significantly dispersed in tomato, compared with control samples. The chloroplast images from transmission-electron microscopy revealed that dark-chilling induced an increase of the degree of grana stacking only in pea chloroplasts. Analyses of O-J-D-I-P fluorescence induction curves in leaves of CS tomato before and after recovery from chilling indicate changes in electron transport rates at acceptor- and donor side of PS II and an increase in antenna size. In CT pea leaves these effects were absent, except for a small but irreversible effect on PSII activity and antenna size. Thus, the differences in chloroplast structure between CS and CT plants, induced by dark-chilling are a consequence of different thylakoid supercomplexes rearrangements.

Heterogeneity of thylakoid membranes studied by EPR spin probe

A lipophilic nitroxyl radical, 1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl 1-adamantylacetate, has been applied to EPR spin probe study of chloroplasts and subchloroplast fragments of different types. The latter originate from grana and the grana core regions. The binding of the spin probe to the membranes was revealed by specific changes in a shape of the EPR spectra. A share of membrane-bound spin probe was different for chloroplasts and subchloroplast fragments, as well as its rotational correlation time and apparent enthalpy and entropy activation of nitroxide rotational motion. The binding of the spin probe induced a significant decrease in the amount of the oxidized P700 and changes in the kinetics of its light oxidation and dark recovery. This suggests that one of the sites of nitroxyl radical binding is the nearest surrounding of the pigment-protein complexes of Photosystem I (PSI). Distinctions in mobility of spin probe immobilized by chloroplasts and their fragments can be caused by the different environment of the PSI complexes located in various regions of thylakoid membranes.

Protein assembly and heat stability in developing thylakoid membranes during greening

The development of the thylakoid membrane was studied during illumination of dark-grown barley seedlings by using biochemical methods, and Fourier transform infrared and spin label electron paramagnetic resonance spectroscopic techniques. Correlated, gross changes in the secondary structure of membrane proteins, conformation, composition, and dynamics of lipid acyl chains, SDS/PAGE pattern, and thermally induced structural alterations show that greening is accompanied with the reorganization of membrane protein assemblies and the protein-lipid interface. Changes in overall membrane fluidity and noncovalent protein-lipid interactions are not monotonic, despite the monotonic accumulation of chlorophyll, LHCII [light-harvesting chlorophyll a/b-binding (polypeptides) associated with photosystem II] apoproteins, and 18:3 fatty acids that follow a similar time course with highest rates between 12-24 h of greening. The 18:3 fatty acid content increases 2.8-fold during greening. This appears to both compensate for lipid immobilization by membrane proteins and facilitate packing of larger protein assemblies. The increase in the amount of protein-solvating immobile lipids, which reaches a maximum at 12 h, is caused by 40% decrease in the membranous mean diameter of protein assemblies at constant protein/lipid mass ratio. Alterations in the SDS/PAGE pattern are most significant between 6-24 h. The size of membrane protein assemblies increases approximately 4.5-fold over the 12-48-h period, likely caused by the 2-fold gain in LHCII apoproteins. The thermal stability of thylakoid membrane proteins increases monotonically, as detected by an increasing temperature of partial protein unfolding during greening. Our data suggest that a structural coupling between major protein and lipid components develops during greening. This protein-lipid interaction is required for the development and protection of thylakoid membrane protein assemblies.

Spin label electron paramagnetic resonance study in thylakoid membranes from a new herbicide-resistant D1 mutant from soybean cell cultures deficient in fatty acid desaturation

The effect of fatty acid desaturation on lipid fluidity in thylakoid membranes isolated from the STR7 mutant was investigated by electron paramagnetic resonance (EPR) using spin label probes. The spectra of both 5- and 16-n-doxylstearic acid probes were measured as a function of the temperature between 10 and 305 K and compared to those of the wild type. This complete thermal evolution provides a wider picture of the dynamics. The spectra of the 5-n-doxylstearic acid probe as well as their temperature evolution were identical in both STR7 mutant and wild type thylakoids. However, differences were found with the 16-n-doxylstearic acid probe at temperatures between 230 and 305 K. The differences in the thermal evolution of the EPR spectra can be interpreted as a 5-10 K shift toward higher temperatures of the probe motional rates in the STR7 mutant as compared with that in the wild type. At temperatures below 230 K no differences were observed. The results indicated that the lipid motion in the outermost region of the thylakoids is the same in the STR7 mutant as in the wild type while the fluidity in the inner region of the STR7 mutant membrane decreases. Our data point out a picture of the STR7 thylakoid membrane in which the lipid motion is slower most probably as a consequence of fatty acid desaturation deficiency.

Peroxidative reactions attenuate oxygen effect on spectroscopic properties of isolated chloroplasts

Steady-state absorption and fluorescence excitation spectra have been measured at 25 degrees C in order to elucidate the differences between isolated chloroplasts from pea (chilling-sensitive plant) and bean (chilling-tolerant plant) and their response to oxygen treatment. A weaker light harvesting in bean in comparison with pea chloroplasts is related to higher free fatty acids level and extended peroxidation activities of bean chloroplasts. Peroxidation of free fatty acids in bean chloroplasts results in an accumulation of oxygenated forms of fatty acids demonstrated by a large negative band around 400 nm in absorption difference spectra, while the excitation spectra are not significantly altered. Similar changes have been observed in the lipase-treated pea chloroplasts. In contrast, in both pea and bean chloroplasts exhibiting no peroxidation due to antimycin A treatment, oxygen induces a pronounced absorbance increase in the regions around 435, 470 and 674 nm indicating the chloroplast swelling. A decline of chlorophyll fluorescence excitation caused by oxygen, may result from a decrease in energy transfer from antennae complexes to chlorophyll species emitting at both 680 and 740 nm. The oxygen-induced changes are partially reversed upon restoration of anaerobic conditions. The presented data show for the first time, that in contrast to pea chloroplasts the peroxidation abolishes an oxygen-induced decrease in light harvesting in bean chloroplasts, i.e., a chilling-sensitive plant.

The effects of chloroplast lipids on the stability of liposomes during freezing and drying

Chloroplast thylakoids contain four classes of lipids, monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), sulfoquinovosyldiacylglycerol (SQDG), and phosphatidylglycerol (cpPG). We have investigated the effects of these lipids on the stability of large unilamellar vesicles made from egg phosphatidylcholine (EPC), by substitution of different fractions of EPC in the membranes by the various chloroplast lipids. Damage to liposomes after freezing to - 18 degrees C was measured as carboxyfluorescein leakage or fusion between vesicles. The presence of all chloroplast lipids increased leakage. However, the maximum amount of leakage and the concentration dependence were dramatically different between the different lipids. Only SQDG induced vesicle fusion, while the non-bilayer lipid MGDG did not. The presence of MGDG in the membranes led to more leakage than the presence of another non-bilayer lipid, egg phosphatidylethanolamine (EPE). In EPE-containing liposomes, leakage was strongly associated with fusion. Combinations of different chloroplast lipids had an additive effect on leakage induced by freezing. Most of the leakage from galactolipid-containing vesicles occurred during the first 15 min of freezing at - 18 degrees C. After a 3 h incubation period, most leakage occurred between 0 degrees C and - 10 degrees C. Lowering the temperature to - 22 degrees C had only a small additional effect. Incubation of liposomes at - 10 degrees C in the presence of 2.5 M NaCl without ice crystallization, approximately the same concentration obtained by freezing to - 10 degrees C, resulted in very little leakage. Air drying of liposomes to low water contents resulted in massive leakage, both from pure EPC vesicles and from vesicles containing galactolipids. The latter vesicles showed more leakage at any given water content than EPC vesicles.

Spin label EPR study of lipid solvation of supramolecular photosynthetic protein complexes in thylakoids

Lipid-protein association in the chloroplast membrane and its various thylakoid fractions from higher plants, namely pea and maize, rich in Photosystem I (PSI) and Photosystem II (PSII), respectively, were studied using EPR spectroscopy of spin-labelled lipid molecules. All the EPR spectra consisted of two spectral components corresponding to bulk fluid lipids and solvation lipids motionally restricted at the hydrophobic surface of membrane proteins. Spin-labelled stearic acid and phosphatidylglycerol exhibited marked selectivity towards the supramolecular protein complexes of both PSI and PSII although to different extent. In addition, lipid-protein titration experiments are described for partially delipidated PSII-enriched membrane fractions of pea chloroplasts, incorporating unlabelled egg phosphatidylcholine prior to or after the incorporation of spin-labelled lipids. Two sets of solvation sites were resolved by timed labelling experiments and a significant result of these studies was that a well-defined population of solvation sites (approx. 100 mol lipids/820 kDa protein) was rapidly exchanged by laterally diffusing membrane lipids, while other solvation sites (approx. 50 mol lipids/820 kDa protein) were exchanged much slower or not exchanged at all.

Changed lateral migration of phospho-LHCII in the thylakoid membrane upon acclimation of spinach to low temperatures

Movement of proteins along the plant thylakoid membrane is of importance for several physiological events, such as state transitions and turnover and repair of the photosystem II complex. Such lateral migrations are impaired at low temperatures, which could contribute to the increased sensitivity of plants to photoinhibitory damages at low temperatures. The migration behaviour of phospho-LHCII in thylakoid membranes isolated from cold-acclimated spinach was studied and compared to that in control membranes. The rate of migration of phospho-LHCII at low temperatures is increased 2- to 3-fold and the apparent activation energy of the migration is decreased after the cold acclimation.