Spin-label ESR studies of lipid-protein interactions in thylakoid membranes

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.

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 10× 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.

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.

Isolation and characterization of lipids strictly associated to PSII complexes: Focus on cardiolipin structural and functional role

In this work, lipid extracts from spinach membrane fragments enriched in Photosystem II (PSII) and from spinach PSII dimers were analyzed, by means of Thin Layer Chromatography (TLC) and Electro-Spray Ionization Mass Spectrometry. Cardiolipin found in association with PSII was isolated and purified by preparative TLC, then characterized by mass and mass-mass analyses. Cardiolipin structures with four unsaturated C18 acyl chains and variable saturation degrees were evidenced. Structural and functional effects of different phospholipids on PSII complexes were investigated by Fluorescence, Resonance Light Scattering and Oxygen Evolution Rate measurements. An increment of PSII thermal stability was observed in the presence of cardiolipin and phosphatidylglycerol.

Structural-functional state of thylakoid membranes of wheat genotypes under water stress

Plants were grown in field conditions in the wide area under normal water supply and severe water deficit. Two wheat (Triticum aestivum L.) genotypes contrasting by architectonics and differing in drought-resistance were used: Giymatli-2/17, short stature, with broad and drooping leaves, drought-sensitive, and Azamatli-95, short stature, with vertically oriented small leaves, drought-tolerant). It was found out that Giymatli-2/17 was characterized by relatively low content of Chl a-protein of PS I (CP I) and beta-subunit of ATP-synthase complex, the high content of proteins in the 33-30.5 kDa region and LHC polypeptides (28-24.5 kDa), the intensive fluorescence at 740 nm and more high photochemical activity of PS II under normal irrigation compared with Azamatli-95. However, the content of CP I (M(r) 115 kDa) and apoprotein of P700 with M(r) 63 kDa insignificantly increases in the drought-resistant genotype Azamatli-95 under extreme water supply condition while their content decreases in drought-sensitive cv Giymatli-2/17. Intensity of synthesis alpha- and beta-subunits of CF(1) (55 and 53.5 kDa) also decreases in Giymatli-2/17. The levels of the core antenna polypeptides of FS II with M(r) 46 and 44.5 kDa (CP47 and CP43) remains stable both in normal, and stressful conditions. At the same time the significant reduction is observed in the content of polypeptides in the 33-30.5 kDa region in the more sensitive genotype Giymatli-2/17. There is an increase in the LHC II polypeptides level in tolerant genotype Azamatli-95 in contrast to Giymatli-2/17 (where the content of these subunits is observed decreasing). The intensity of short wavelength peaks at 687 and 695 nm sharply increases in the fluorescence spectra (77 K) of chloroplasts from sensitive genotype Giymatli-2/17 under water deficiency and there is a stimulation of the ratio of fluorescence band intensity F687/F740. After exposure to drought, cv Giymatli-2/17 shows a larger reduction in the actual PS II photochemical efficiency of chloroplasts than cv Azamatli-95.

Protein composition and native state of pigments of thylakoid membrane of wheat genotypes differently tolerant to water stress

Protein composition and native state of chlorophylls were analyzed in two wheat (Triticum durum L.) genotypes with different tolerance to drought, Barakatli-95 (drought-tolerant) and Garagylchyg-2 (drought-sensitive), during water deficit. It is shown that the plants subjected to water deficit appear to have a slight increase in alpha- and beta-subunits of CF1 ATP-synthase complex (57.5 and 55 kD, respectively) in Barakatli-95 and their lower content in Garagylchyg-2. Steady-state levels of the core antenna of PS II (CP47 and CP43) and light-harvesting Chl a/b-apoproteins (LHC) II in the 29.5-24 kD region remained more or less unchanged in both wheat genotypes. The synthesis of 36 kD protein and content of low-molecular-weight polypeptides (21.5, 16.5, and 14 kD) were noticeably increased in the tolerant genotype Barakatli-95. Drought caused significant changes in the carotenoid region of the spectrum (400-500 nm) in drought-sensitive genotype Garagylchyg-2 (especially in the content of pigments of the violaxanthin cycle). A shift of the main band from 740-742 to 738 nm is observed in the fluorescence spectra (77 K) of chloroplasts from both genotypes under water deficiency, and there is a stimulation of the ratio of fluorescence band intensity F687/F740.

Methylation of the Imidazole Side Chains of the Alzheimer Disease Amyloid-β Peptide Results in Abolition of Superoxide Dismutase-like Structures and Inhibition of Neurotoxicity

The toxicity of the amyloid-beta peptide (Abeta) is thought to be responsible for the neurodegeneration associated with Alzheimer disease. Generation of hydrogen peroxide has been implicated as a key step in the toxic pathway. Abeta coordinates the redox active metal ion Cu2+ to catalytically generate H2O2. Structural studies on the interaction of Abeta with Cu have suggested that the coordination sphere about the Cu2+ resembles the active site of superoxide dismutase 1. To investigate the potential role for such structures in the toxicity of Abeta, two novel Abeta40 peptides, Abeta40(HistauMe) and Abeta40(HispiMe), have been prepared, in which the histidine residues 6, 13, and 14 have been substituted with modified histidines where either the pi- or tau-nitrogen of the imidazole side chain is methylated to prevent the formation of bridging histidine moieties. These modifications did not inhibit the ability of these peptides to form fibrils. However, the modified peptides were four times more effective at generating H2O2 than the native sequence. Despite the ability to generate more H2O2, these peptides were not neurotoxic. Whereas the modifications to the peptide altered the metal binding properties, they also inhibited the interaction between the peptides and cell surface membranes. This is consistent with the notion that Abeta-membrane interactions are important for neurotoxicity and that inhibiting these interactions has therapeutic potential.

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.

Phosphatidylglycerol is involved in the dimerization of photosystem II

Photosystem II core dimers (450 kDa) and monomers (230 kDa) consisting of CP47, CP43, the D1 and D2 proteins, the extrinsic 33-kDa subunit, and the low molecular weight polypeptides PsbE, PsbF, PsbH, PsbI, PsbK, PsbL, PsbTc, and PsbW were isolated by sucrose density gradient centrifugation. The photosystem II core dimers were treated with phospholipase A2 (PL-A2), which cuts phosphatidylglycerol (PG) and phosphatidylcholine molecules at the sn-2 position. The PL-A2-treated dimers dissociated into two core monomers and further, yielding a CP47-D1-D2 subcomplex and CP43. Thin layer chromatography showed that photosystem II dimers contained four times more PG than their monomeric counterparts but with similar levels of phosphatidylcholine. Consistent with this was the finding that, compared with monomers, the dimers contained a higher level of trans-hexadecanoic fatty acid (C16:1Delta3tr), which is specific to PG of the thylakoid membrane. Moreover, treatment of dimers with PL-A2 increased the free level of this fatty acid specific to PG compared with untreated dimers. Further evidence that PG is involved in stabilizing the dimeric state of photosystem II comes from reconstitution experiments. Using size exclusion chromatography, it was shown that PG containing C16:1Delta3tr, but not other lipid classes, induced significant dimerization of isolated photosystem II monomers. Moreover, this dimerization was observed by electron crystallography when monomers were reconstituted into thylakoid lipids containing PG. The unit cell parameters, p2 symmetry axis, and projection map of the reconstituted dimer was similar to that observed for two-dimensional crystals of the native dimer.

The phospholipid-deficient pho1 mutant of Arabidopsis thaliana is affected in the organization, but not in the light acclimation, of the thylakoid membrane

The pho1 mutant of Arabidopsis has been shown to respond to the phosphate deficiency in the leaves by decreasing the amount of phosphatidylglycerol (PG). PG is thought to be of crucial importance for the organization and function of the thylakoid membrane. This prompted us to ask what the consequences of the PG deficiency may be in the pho1 mutant when grown under low or high light. While in the wild-type, the lipid pattern was almost insensitive to changes in the growth light, PG was reduced to 45% under low light in the mutant, and it decreased further to 35% under high light. Concomitantly, sulfoquinovosyl diacylglycerol (SQDG) and to a lesser extent digalactosyl diacylglycerol (DGDG) increased. The SQDG increase correlated with increased amounts of the SQD1 protein, an indicator for an actively mediated process. Despite of alterations in the ultrastructure, mutant thylakoids showed virtually no effects on photosynthetic electron transfer, O2 evolution and excitation energy allocation to the reaction centers. Our results support the idea that PG deficiency can at least partially be compensated for by the anionic lipid SQDG and the not charged lipid DGDG. This seems to be an important strategy to maintain an optimal thylakoid lipid milieu for vital processes, such as photosynthesis, under a restricted phosphate availability.

Crystallization and identification of an assembly defect of recombinant antenna complexes produced in transgenic tobacco plants

A chimeric Lhcb gene encoding light-harvesting chlorophyll a/b-binding protein (LHCII) was expressed in transgenic tobacco plants. To separate native from recombinant LHCII, the protein was extended by six histidines at its C terminus. Recombinant LHCII was isolated by detergent-mediated monomerization of pure trimers followed by affinity-chromatography on Ni(2+)-NTA-agarose (NTA is nitrilotriacetic acid). Elution with imidazole yielded recombinant monomers that formed trimers readily after dilution of the detergent without further in vitro manipulations. LHCII subunits showed the typical chlorophyll a/b ratio at all steps of purification indicating no significant loss of pigments. Transgenic tobacco overexpressed amounts of recombinant protein that corresponded to about 0.7% of total LHCII. This yield suggested that expression in planta might be an alternative to the expression of eukaryotic membrane proteins in yeast. Recombinant LHCII was able to form two-dimensional crystals after addition of digalactolipids, which diffracted electrons to 3.6-A resolution. LHCII carrying a replacement of Arg-21 with Gln accumulated to only 0.004% of total thylakoid proteins. This mutant was monomeric in the photosynthetic membrane probably due to the deletion of the phosphatidylglycerol binding site and was degraded by the plastidic proteolytic system. Exchange of Asn-183 with Leu impaired LHCII biogenesis in a similar way presumably due to the lack of a chlorophyll a binding site.

Unsaturation of the membrane lipids of chloroplasts stabilizes the photosynthetic machinery against low-temperature photoinhibition in transgenic tobacco plants

Using tobacco plants that had been transformed with the cDNA for glycerol-3-phosphate acyltransferase, we have demonstrated that chilling tolerance is affected by the levels of unsaturated membrane lipids. In the present study, we examined the effects of the transformation of tobacco plants with cDNA for glycerol-3-phosphate acyltransferase from squash on the unsaturation of fatty acids in thylakoid membrane lipids and the response of photosynthesis to various temperatures. Of the four major lipid classes isolated from the thylakoid membranes, phosphatidylglycerol showed the most conspicuous decrease in the level of unsaturation in the transformed plants. The isolated thylakoid membranes from wild-type and transgenic plants did not significantly differ from each other in terms of the sensitivity of photosystem II to high and low temperatures and also to photoinhibition. However, leaves of the transformed plants were more sensitive to photoinhibition than those of wild-type plants. Moreover, the recovery of photosynthesis from photoinhibition in leaves of wild-type plants was faster than that in leaves of the transgenic tobacco plants. These results suggest that unsaturation of fatty acids of phosphatidylglycerol in thylakoid membranes stabilizes the photosynthetic machinery against low-temperature photoinhibition by accelerating the recovery of the photosystem II protein complex.

2H-NMR investigation of DMPC/glycophorin bilayers

Deuterium nuclear magnetic resonance spectroscopy was used to investigate the phase equilibria, and the temperature and concentration dependences of the phospholipid hydrocarbon chain order, of mixtures of glycophorin in dimyristoylphosphatidyl-choline. In the fluid phase it is found that the protein has only a slight effect on the first moment of the 2H spectrum, which for perdeuterated chains is a direct measure of the average chain orientational order. However, analysis of the rate of change of the first moment with respect to protein concentration, at different temperatures within the fluid phase, shows that at a molar protein concentration of about 0.0295 +/- 0.01, the lipid chain order (or M1) is essentially independent of temperature. At this concentration the chain order is determined by the lipid's interaction with the protein and one can conclude that about 34 (+/- 12) lipids are required to solvate the protein. At higher lipid concentrations these lipids are freely exchanging, on the NMR time scale, with the other lipids in the bilayer. At glycophorin concentrations below about 1 mol% there is a two-phase coexistence region at temperatures below the pure lipid's chain melting transition. The boundary between the fluid phase and this two-phase region curves downwards (is concave downwards), whereas the boundary between the two-phase region and the gel phase, while naturally occurring at lower temperatures than the upper boundary, is concave upwards. As a consequence the protein partitions preferentially into the fluid phase. This behaviour is similar to that observed in a number of other protein/lipid and peptide/lipid mixtures where it was suggested that those systems may have been close to a critical mixing point and some characteristics of a continuous phase change were noted. Indeed, at glycophorin concentrations near and above 1 mol% there are indications that the phase behaviour becomes more complex, suggesting the presence of significant protein/protein interactions and that this system may be close to a critical point.

Bacterial solute transport proteins in their lipid environment

The cytoplasmic membrane of bacteria is a selective barrier that restricts entry and exit of solutes. Transport of solutes across this membrane is catalyzed by specific membrane proteins. Integral membrane proteins usually require specific lipids for optimal activity and are inhibited by other lipid species. Their activities are also sensitive to the lipid bilayer dynamics and physico-chemical state. Bacteria can adapt to changes in the environments (respective temperature, hydrostatic pressure, and pH) by altering the lipid composition of the membrane. Homeoviscous adaptation results in the maintenance of the liquid-crystalline phase through alterations in the degree of acyl chain saturation and branching, acyl chain length and the sterol content of the membrane. Homeophasic adaptation prevents the formation of non-bilayer phases, which would disrupt membrane organization and increase permeability. A balance is maintained between the lamellar phase, preferring lipids, and those that adopt a non-bilayer organization. As a result, the membrane proteins are optimally active under physiological conditions. The molecular basis of lipid-protein interactions is still obscure. Annular lipids stabilize integral membrane proteins. Stabilization occurs through electrostatic and possibly other interactions between the lipid headgroups and the charged amino acid residues close to the phospholipid-water interface, and hydrophobic interactions between the fatty acyl chains and the membrane-spanning segments. Reconstitution techniques allow manipulation of the lipid composition of the membrane in a way that is difficult to achieve in vivo. The physical characteristics of membrane lipids that affect protein-mediated transport functions have been studied in liposomal systems that separate an inner and outer compartment. The activity of most transport proteins is modulated by the bulk physical characteristics of the lipid bilayer, while specific lipid requirements appear rare.

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.

Changes in the binding and inhibitory properties of urea/triazine-type herbicides upon phospholipid and galactolipid depletion in the outer monolayer of thylakoid membranes : Different behaviour of atrazine-susceptible and-resistant biotypes of Solanum nigrum L

The binding characteristics and the inhibitory power of atrazine and DCMU towards uncoupled electron flow activity were studied in acyl lipid-depleted thylakoid membranes from atrazine-susceptible and-resistant biotypes of Solanum nigrum L. For this purpose, phospholipase A2 from Vipera russelli and the lipase from Rhizopus arrhizus were used to obtain a selective lipid class (phospholipids or galactolipids) depletion which was restricted to the outer monolayer. Neither phospholipid nor galactolipid removal affected the dissociation constant and the number of binding sites of atrazine. In contrast, the dissociation constant of DCMU was increased in phospholipid-depleted thylakoid membranes but remained unchanged after galactolipid depletion. The number of DCMU binding sites decreased significantly after both lipase treatments, but only in the resistant biotype. The inhibitory effectiveness of the herbicide was either decreased or increased (to different extents) depending on the lipid class which was removed from the membrane and on the biotype considered. These results are discussed with reference to the possible conformational changes of the 32 kDa herbicide-binding polypeptide occurring after lipase treatments.

Magnetic resonance of membranes

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Lipid-protein interactions in membranes

The interactions of lipids with integral and peripheral proteins can be studied in reconstituted and natural membranes using spin label electron spin resonance (ESR) spectroscopy. The ESR spectra reveal a reduction in mobility of the spin-labelled lipid species, and in certain cases evidence is obtained for a partial penetration of the peripheral proteins into the membrane. The latter may be relevant to the import mechanism of apocytochrome c into mitochondria. Integral proteins induce a more direct motional restriction of the spin-labelled lipid chains, allowing the stoichiometry and specificity of the interaction, and the lipid exchange rate at the protein interface to be determined from the ESR spectra. In this way, a population of very slowly exchanging cardiolipin associated with the mitochondrial ADP-ATP carrier has been identified. The residues involved in the specificity for charged lipids of the myelin proteolipid protein have been localized to the deletion in the DM-20 mutant, and the difference in lipid-protein interactions with the beta-sheet and alpha-helical conformations of the M-13 coat protein, has been characterized.

Lipid-protein interactions in stacked and destacked thylakoid membranes and the influence of phosphorylation and illumination. Spin label ESR studies

The effects of membrane destacking, protein phosphorylation, and continuous illumination have been studied in pea thylakoid membranes using ESR spectroscopy of an incorporated spin-labelled phosphatidylglycerol. This spin-labelled analogue of an endogenous thylakoid lipid has previously been shown to exhibit a selectivity of interaction with thylakoid proteins. Neither destacking, phosphorylation nor illumination was found to change the ESR spectra appreciably, suggesting that for phosphatidylglycerol at least, neither the number of protein-associated membrane lipids nor their pattern of selectivity was altered. The redistribution of the thylakoid protein complexes in the membrane, under these various conditions, therefore takes place with conservation of the properties of the lipid/protein interface.