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At least 19 recordsLinked to original sources
PubMed1981

Estimation of lipid regions in a cytochrome oxidase-lipid complex using spin labeling electron spin resonance: distribution effects on the spin label.

The distribution of lipid in the cytochrome oxidase-lipid complex from beef heart mitochondria has been studied by the spin labeling electron spin resonance technique. The spectra of a phospholipid spin label incorporated in the complex reveals an immobilized (on the ESR time scale) component in addition to the fluid component which is found in aqueous dispersions of the extracted lipids. The first component corresponds to the domain of lipid influenced by the protein, and the second component to the remaining lipid. A theory taking into account not only the sizes of the lipid regions in which the spin label molecule distributes itself, but also the different affinities of the label for the two domains, has been developed. Taking advantage of the variation in spectra obtained with increasing amounts of spin label, computer calculations have been performed to estimate the distribution of lipid in the different regions of the cytochrome oxidase-lipid complex. An extrapolation of the amount of immobilized spin-labeled phospholipid to zero concentration of label allows a calculation of the number of fatty acid residues interacting with the protein to be made. It has been found that the number of aliphatic chains influenced by the protein is higher than that calculated for a single boundary layer around the protein. The approach used in this paper can be used for studies of protein-lipid interactions in other systems.

Animals↗
PubMed1978

Librational motion of an "immobilized" spin label: hemoglobin spin labeled by a maleimide derivative.

The spin label Tempo-maleimide, when "immobilized" in hemoglobin, is shown to exhibit motional fluctuation whose amplitude and/or frequency depend on temperature and solution conditions. These motional fluctuations are observable by several electron spin resonance techniques. For desalted hemoglobin the fluctuations are detectable at approximately -15 degrees C using saturation transfer techniques and at approximately +25 degrees C using line-width measurements of normal absorption spectra. In ammonium sulfate precipitated hemoglobin, however, motional fluctuations are not detectable by either technique up to at least 40 degrees C. The most probable mechanism for spin-label motion appears to be either fluctuations in protein conformation which affect the label binding site or conformational transitions of the nitroxide ring itself. These motional fluctuations are shown to introduce a librational character to the overall label motion during hemoglobin rotational diffusion, with the librational motion significantly affecting the use of spin-label spectral shapes to calculate hemoglobin rotational correlation times.

Chemical Precipitation↗
PubMed2001

Membrane insertion and lipid-protein interactions of bovine seminal plasma protein PDC-109 investigated by spin-label electron spin resonance spectroscopy.

The interaction of the major acidic bovine seminal plasma protein, PDC-109, with dimyristoylphosphatidylcholine (DMPC) membranes has been investigated by spin-label electron spin resonance spectroscopy. Studies employing phosphatidylcholine spin labels, bearing the spin labels at different positions along the sn-2 acyl chain indicate that the protein penetrates into the hydrophobic interior of the membrane and interacts with the lipid acyl chains up to the 14th C atom. Binding of PDC-109 at high protein/lipid ratios (PDC-109:DMPC = 1:2, w/w) results in a considerable decrease in the chain segmental mobility of the lipid as seen by spin-label electron spin resonance spectroscopy. A further interesting new observation is that, at high concentrations, PDC-109 is capable of (partially) solubilizing DMPC bilayers. The selectivity of PDC-109 in its interaction with membrane lipids was investigated by using different spin-labeled phospholipid and steroid probes in the DMPC host membrane. These studies indicate that the protein exhibits highest selectivity for the choline phospholipids phosphatidylcholine and sphingomyelin under physiological conditions of pH and ionic strength. The selectivity for different lipids is in the following order: phosphatidylcholine approximately sphingomyelin > or = phosphatidic acid (pH 6.0) > phosphatidylglycerol approximately phosphatidylserine approximately and rostanol > phosphatidylethanolamine > or = N-acyl phosphatidylethanolamine >> cholestane. Thus, the lipids bearing the phosphocholine moiety in the headgroup are clearly the lipids most strongly recognized by PDC-109. However, these studies demonstrate that this protein also recognizes other lipids such as phosphatidylglycerol and the sterol androstanol, albeit with somewhat reduced affinity.

Animals↗
PubMed1979

Neisseria gonorrhoeae membrane microenvironment studied by spin-label electron spin resonance: comparison of colony types.

Spin-label electron spin resonance was used to characterize the microenvironment around spin probes which localize (i) in membranes, (ii) at the membrane surface, or (iii) in the cytoplasm of living Neisseria gonorrhoeae. Four colony types (T1, T2, T3, and T4) of gonococci were compared on the basis of the electron spin resonance parameters 2T parallel to, S (order parameter), and tau c (microviscosity). The concentration of spin label used had little or no effect on viability. T1 and T2 gonococci were found to have a more restricted environment for molecular motion of a membrane surface spin label than did T3 and T4. The membrane fluidity, as measured by a membrane lipid spin label, of T4 (S = 0.571) was significantly greater than that of T1 or T3 (S = 0.580). This difference was detected at 37 degrees C, at 25 degrees C, in agar-grown bacteria, and in exponential-phase cells. Studies using spin labels which probe different levels of the membrane indicated the presence of a membrane flexibility gradient. Cytoplasmic spin-label studies indicated that the cytoplasm of all gonococcal colony types was three to five times more viscous than water.

Cell Membrane↗
PubMed1994

Spin-label electron spin resonance studies on the dynamics of the different phases of N-biotinylphosphatidylethanolamines.

The chain dynamics and phase behavior of a homologous series of diacyl-N-biotinylphosphatidylethanolamines of chain lengths from C(12:0) to C(20:0) were investigated by spin-label electron spin resonance (ESR) spectroscopy using both phosphatidylcholine and N-biotinylphosphatidylethanolamine spin-label probes. Chain-melting phase transition temperatures determined from the ESR spectral measurements, for all the five lipids in the presence as well as in the absence of 1 M NaCl, correlate with the endothermic phase transition temperatures determined by differential scanning calorimetry [Swamy, M. J., Angerstein, B., & Marsh, D. (1994) Biophys. J. 66, 31-39], confirming that the latter correspond to chain-melting transitions. ESR spectra obtained in the gel phase from phosphatidylcholine probes with the spin-label near the terminal methyl of the hydrocarbon chain showed a similar degree of immobilization (as reflected by the outer hyperfine splittings) to that for spin-labels positioned close to the glycerol backbone, indicating that the biotin-lipids of chain lengths from 14 to 20 carbon atoms form interdigitated gel phases in the presence of salt, as also do those of chain lengths between 16 and 20 carbon atoms in the absence of salt. For dispersions of the C(16:0) chain length biotin-lipid in 1 M NaCl, continuous monitoring of the central ESR peak intensity as a function of temperature detects a cooperative decrease in mobility in the fluid phase at ca. 65 degrees C with a spin-label in the lipid head group and an accompanying increase in mobility at the same temperature with spin-labels positioned toward the terminal methyl of the hydrocarbon chain.(ABSTRACT TRUNCATED AT 250 WORDS)

Biotin↗
PubMed1976

Effect of membrane protein on lipid bilayer structure: a spin-label electron spin resonance study of vesicular stomatitis virus.

Spin-label electron spin resonance (ESR) methods have been used to study the structure of the envelope of vesicular stomatitis virus (VSV). The data indicate that the lipid is organized in a bilayer structure. Proteolytic digestion of the glycoproteins which are the spike-like projections on the outer surface of the virus particle increases the fluidity of the lipid bilayer. Since the lipid composition of the virion reflects the composition of the host plasma membrane and the protein composition is determined by the viral genome, VSV was grown in both MDBK and BHK21-F cells to determine the effect of a change in lipid composition on the structure of the lipid bilayer of VSV. The lipid bilayer of the virion was found to be more rigid when derived from MDBK cells than from BHK21-F cells. Studies comparing spin-labeled intact cells and cell membrane fractions suggest that upon labeling the whole cell the spin label probes the plasma membrane. Comparison of spin-labeled VSV particles and their host cells indicates that the lipid bilayer of the plasma membrane is considerably more fluid than that of the virion. These results are discussed in terms of the effect of membrane-associated protein on the structure of the lipid bilayer.

Binding Sites↗
PubMed2002

Membrane water-penetration profiles from spin labels.

Spin label hyperfine splittings in mixtures of protic and aprotic solvents are used to obtain association constants K(A,h) for hydrogen bonding to oxazolidine nitroxides. With the Onsager approach to account for the variation in local dielectric constant, these results are used to determine the effective penetration profile of water into fluid phospholipid membranes, from recent electron paramagnetic resonance (EPR) studies on phospholipids spin-labelled systematically down the sn-2 chain. Water penetration is appreciable, depends on chain unsaturation, and is strongly affected by cholesterol.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed2000

Spin-label electron spin resonance studies on the mode of anchoring and vertical location of the N-acyl chain in N-acylphosphatidylethanolamines.

Electron spin resonance (ESR) studies have been performed on N-myristoyl dimyristoylphosphatidylethanolamine (N-14-DMPE) membranes using both phosphatidylcholines spin-labeled at different positions in the sn-2 acyl chain and N-acyl phosphatidylethanolamines spin-labeled in the N-acyl chain to characterize the location and mobility of the N-acyl chain in the lipid membranes. Comparison of the positional dependences of the spectral data for the two series of spin-labeled lipids suggests that the N-acyl chain is positioned at approximately the same level as the sn-2 chain of the phosphatidylcholine spin-label. Further, similar conclusions are reached when the ESR spectra of the N-acyl PE spin-labels in dimyristoylphosphatidylcholine (DMPC) or dimyristoylphosphatidylethanolamine (DMPE) host matrixes are compared with those of phosphatidylcholine spin-labels in these two lipids. Finally, the chain ordering effect of cholesterol has also been found to be similar for the N-acyl PE spin-label and PC spin-labels, when the host matrix is either DMPC and cholesterol or N-14-DMPE and cholesterol at a 6:4 mole ratio. In both cases, the gel-to-liquid crystalline phase transition is completely abolished but cholesterol perturbs the gel-phase mobility of N-14-DMPE more readily than that of DMPC. These results demonstrate that the long N-acyl chains are anchored firmly in the hydrophobic interior of the membrane, in an orientation that is parallel to that of the O-acyl chains, and are located at nearly the same vertical position as that of the sn-2 acyl chains in the lipid bilayer. There is a high degree of dynamic compatibility between the N-acyl chains and the O-acyl chains of the lipid bilayer core, although bilayers of N-acyl phosphatidylethanolamines possess a more hydrophobic interior than phosphatidylcholine bilayers. These results provide a structural basis for rationalizing the biological properties of NAPEs.

Cholesterol↗
PubMed2001

Lipid membrane expansion and micelle formation by polymer-grafted lipids: scaling with polymer length studied by spin-label electron spin resonance.

Spin-label electron spin resonance (ESR) spectroscopy and auxiliary optical density measurements are used to study lipid dispersions of N-poly(ethylene glycol)-dipalmitoyl phosphatidylethanolamine (PEG:5000-DPPE) mixed with dipalmitoyl phosphatidylcholine (DPPC). PEG:5000-DPPE bears a large hydrophilic polymer headgroup (with approximately 114 oxyethylene monomers) and is commonly used for steric stabilization of liposomes used in drug delivery. Comparison is made with results from mixtures of DPPC with polymer lipids bearing shorter headgroups (approximately 45 and 8 oxyethylene monomers). ESR spectra of phosphatidylcholine spin-labeled on the 5-C atom position of the sn-2 chain are shown to reflect the area expansion of the lipid membranes by the lateral pressure exerted in the polymer brush, in a way that is consistent with theory. The lipid chain packing density at the onset of micelle formation is the same for all three PEG-lipids, although the mole fraction at which this occurs differs greatly. The mole fraction at onset scales inversely with the size of the polymer headgroup, where the experimental exponent of 0.7 is close to theoretical predictions (viz. 0.55-0.6). The mole fraction of PEG-lipid at completion of micelle formation is more weakly dependent on polymer size, which conforms with theoretical predictions. At high mole fractions of PEG:5000-DPPE the dependence of lipid packing density on mole fraction is multiphasic, which differs qualitatively from the monotonic decrease in packing density found with the shorter polymer lipids. Lipid spin-label ESR is an experimental tool that complements theoretical analysis using polymer models combined with the lipid equation of state.

1,2-Dipalmitoylphosphatidylcholine↗
PubMedDate not supplied

[Analysis of existing models of spin label movement during spin labeling of biological macromolecules].

The spin-labeled bovine serum albumin and IgG were studied in search of an experimental approach for comparison of different models of rotational mobility of spin label. These models are: the model of isotropic motion of spin label together with the macromolecule (IM); the model of highly anisotropic motion of spin label (HAM); and the model of slow isotropic motion of label around the binding site (SIML). The experimental spectra were measured on a common X-band ESR spectrometer and on the unique 140 GHZ (lambda = 2 mm) ESR spectrometer under the same conditions. Theoretical spectra were computer-calculated according to Freed's theory. We have found, that the results of temperature-viscosity experiments in X-band are contradictory to the model of IM both for the BSA and IgG species. The models of HAM and SIML for the BSA give identical X-band spectra. The bovine serum albumin spectra in the 2 mm region strongly contradict to the assumptions of the HAM model. Also, the SIML model fails to describe the experimental spectra in terms of isotropic motion of the spin label around the binding site. X-band spectra of IgG can not be explained by the SIML model, while the same spectra in the 2 mm region can not be explained by the HAM model.

Animals↗
PubMed1997

Spin-label electron spin resonance studies on the interactions of lysine peptides with phospholipid membranes.

The interactions of lysine oligopeptides with dimyristoyl phosphatidylglycerol (DMPG) bilayer membranes were studied using spin-labeled lipids and electron spin resonance spectroscopy. Tetralysine and pentalysine were chosen as models for the basic amino acid clusters found in a variety of cytoplasmic membrane-associating proteins, and polylysine was chosen as representative of highly basic peripherally bound proteins. A greater motional restriction of the lipid chains was found with increasing length of the peptide, while the saturation ratio of lipids per peptide was lower for the shorter peptides. In DMPG and dimyristoylphosphatidylserine host membranes, the perturbation of the lipid chain mobility by polylysine was greater for negatively charged spin-labeled lipids than for zwitterionic lipids, but for the shorter lysine peptides these differences were smaller. In mixed bilayers composed of DMPG and dimyristoylphosphatidylcholine, little difference was found in selectivity between spin-labeled phospholipid species on binding pentalysine. Surface binding of the basic lysine peptides strongly reduced the interfacial pK of spin-labeled fatty acid incorporated into the DMPG bilayers, to a greater extent for polylysine than for tetralysine or pentalysine at saturation. The results are consistent with a predominantly electrostatic interaction with the shorter lysine peptides, but with a closer surface association with the longer polylysine peptide.

Electron Spin Resonance Spectroscopy↗
PubMed1996

Membrane assembly studied by spin-label electron spin resonance.

Conventional electron spin resonance (ESR) of spin-labelled lipids and saturation transfer ESR of spin-labelled proteins are used to study lipid-protein interactions and the mobility of integral proteins, respectively, both in biological membranes and in reconstituted lipid-protein systems. Conventional ESR spectra reveal two spin-labelled lipid populations, the mobility of one of which is hindered by direct interaction with the integral membrane proteins. The proportion of the latter component increases with increasing protein content and with increasing selectivity of the lipid species for the protein. The two-component spectra are quantitated by spectral subtraction and addition, and by simulation using the exchange-coupled Bloch equations. Lipid exchange rates at the protein interface obtained by simulation are found to be consistent with fast exchange found by 2H NMR on similar systems and to reflect the lipid selectivity observed by ESR. Protein-reactive covalent spin labels have been used to study the rotational diffusion and aggregation states of membrane proteins via saturation transfer ESR. The integral protein rotation is uniaxial to the first approximation and the anisotropic motion is analyzed to obtain the principal component of the diffusion tensor. The latter is sensitively dependent on the cross-sectional dimensions of the protein in the membrane, and hence on its state of assembly. A variety of new experiments based on the power saturation properties of the spin-labelled components are also used to determine lipid exchange rates, protein translational diffusion rates, and the location and penetration of proteins in membranes.

Diffusion↗
PubMed1989

Chain configuration and flexibility gradient in phospholipid membranes. Comparison between spin-label electron spin resonance and deuteron nuclear magnetic resonance, and identification of new conformations.

The electron spin resonance spectra of 1-myristoyl-2-[n-(4,4-dimethyloxazolidine-N-oxyl)myristoyl]-sn- glycero-3-phosphocholine spin-label positional isomers (n = 6, 10, and 13) have been studied in oriented, fully hydrated bilayers of dimyristoylphosphatidylcholine, as a function of temperature and magnetic field orientation. The spectra have been simulated using a line-shape model which incorporates chain rotational isomerism, as well as restricted anisotropic motion of the lipid molecules as a whole, and which is valid in all motional regimes of conventional spin-label electron spin resonance (ESR) spectroscopy. At least one component of the lipid motion is found to lie in the slow-motion regime for all label positions, even in the fluid liquid crystalline phase, well above the phase transition. In the gel phase, the chain isomerism lies in the slow-motional regime, and the overall motions are at the rigid-limit. In the fluid phase, the chain isomerism is in the fast-motional regime, and the chain axis motions are in the slow regime. This indicates that the commonly used motional-narrowing theory is not appropriate for the interpretation of spin-label spectra in biological membranes. The simulation parameters yield a consistent description for the chain order and dynamics for all label positions. The correlation times and order parameters for the overall motion are the same at all positions down the chain, whereas the chain conformation and trans-gauche isomerism rate display a characteristic flexibility gradient, with increasing motion towards the terminal methyl end of the chain. Significantly, it is found that all six distinct tetrahedral orientations of the hyperfine tensor at the labeled segment are required for a consistent description of the chain isomerism. For the C-6 segment only the 0 degree (trans) and two 60 degrees (gauche) orientations are significantly populated, for the C-10 position two further 60 degrees orientations are populated, and for the C-13 position all orientations have non-vanishing populations. Detailed comparisons have been made with the results of 2H nuclear magnetic resonance (NMR) measurements on dimyristoylphosphatidylcholine labeled at the same position in the sn-2 chain, and using an identical motional model. The parameters of overall reorientation, both order parameter and correlation times, have very similar values as determined by ESR and NMR. The major difference between the results from the two methods lies in the conformational populations at the labeled chain segment and the trans-gauche isomerization rate in the gel phase. The conformational order is much lower for the spin-labeled chain segments than for the corresponding deuterium-labeled segments, and the isomer interconversion rates in the gel phase(although displaying a mobility gradient in both cases) are found to be much slower in the former case. In addition the spin-label measurements provide information on the macro order (chain tilt), which is only available from oriented samples. These results are consistent between the different spin label positions and are in agreement with the findings from x-ray diffraction.

Dimyristoylphosphatidylcholine↗
PubMed1988

Ganglioside-protein interactions: spin-label electron spin resonance studies with (Na+,K+)-ATPase membranes.

Lipid-protein interactions in (Na+,K+)-ATPase-rich membranes from Squalus acanthias have been studied using spin-labeled derivatives of the mono- and disialogangliosides GM1, GM2, GM3, and GD1b, in conjunction with electron spin resonance (ESR) spectroscopy. Ganglioside-protein interactions are revealed by the presence of a second component in the ESR spectra of the membranes in addition to a component that corresponds closely to the ESR spectra obtained from dispersions of the extracted membrane lipids. This second component corresponds to spin-labeled gangliosides whose chain motion is significantly restricted relative to that of the fluid lipids in the membrane or the lipid extract. A small selectively for the motionally restricted component associated with the protein is found in the order GD1b greater than GM1 approximately equal to GM2 approximately equal to GM3. Comparison with previous results from spin-labeled phospholipids in the same system [Esmann, M., Watts, A., & Marsh, D. (1985) Biochemistry 24, 1386-1393] shows that the spin-labeled monosialogangliosides GM1, GM2, and GM3 display little selectivity in the lipid-protein interaction relative to spin-labeled phosphatidylcholine. The spectral characteristics of both the fluid and motionally restricted spin-labeled components differ very significantly, however, between the gangliosides and the phospholipids. The outer hyperfine splitting of the motionally restricted component is smaller for the gangliosides than for the phospholipids, indicating a smaller degree of motional restriction on interaction of the ganglioside lipid chains with the protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1988

Interaction of phospholipids with the detergent-solubilized ADP/ATP carrier protein as studied by spin-label electron spin resonance.

The interaction of spin-labeled phospholipids with the detergent-solubilized ADP/ATP carrier protein from the inner mitochondrial membrane has been investigated by electron spin resonance spectroscopy. The equilibrium binding of cardiolipin and phosphatidic acid was studied by titration of the protein with spin-labeled phospholipid analogues using a spectral subtraction protocol for the evaluation of the mobile and immobilized lipid portions. This analysis revealed the immobilization of two molecules of spin-labeled cardiolipin per protein dimer. Phosphatidic acid has a similar affinity for the protein surface as cardiolipin. The lipid-protein interaction was less pronounced with the neutral phospholipids and with phosphatidylglycerol. The importance of the electrostatic contribution to the phospholipid-protein interaction shows up with a strong dependence of the lipid binding on salt concentration. Cleavage by phospholipase A2 and spin reduction by ascorbate of the spin-labeled acidic phospholipids in contact with the protein surface suggest that these lipids are located on the outer perimeter of the protein. At reduced detergent concentration, the protein aggregated upon addition of small amounts of cardiolipin but remained solubilized when more cardiolipin was added. This result is discussed with respect to the aggregation state of the protein in the mitochondrial membrane. It is also tentatively concluded that binding of spin-labeled cardiolipin does not displace the tightly bound cardiolipin of mitochondrial origin, which was detected previously by 31P nuclear magnetic resonance spectroscopy.

Binding, Competitive↗
PubMed1982

Changes in the plasma membrane of yeast observed by spin label electron spin resonance and caused by photodynamic attack.

Spin label electron spin resonance (ESR) was used to characterize the response of lipid regions of the plasma membrane of yeast to photodynamic attack. Following photodynamic attack, the structure of these lipid regions changed resulting in the disappearance of an apparent order-disorder phase transition as well as impeding the diffusion of the steric acid based spin label 12NS into and across the plasma membrane. We propose that singlet molecular oxygen reacting with unsaturated carbon bonds in the fatty acyl chains of lipid surrounding channel proteins leads to an increase in the order of the lipid array and/or a change in the channel protein's conformation and is the cause of the lethal effect of externally sensitized photodynamic action.

Cell Membrane↗
PubMed2000

Molecular and mesoscopic properties of hydrophilic polymer-grafted phospholipids mixed with phosphatidylcholine in aqueous dispersion: interaction of dipalmitoyl N-poly(ethylene glycol)phosphatidylethanolamine with dipalmitoylphosphatidylcholine studied by spectrophotometry and spin-label electron spin resonance.

Spin-label electron spin resonance (ESR) spectroscopy, together with optical density measurements, has been used to investigate, at both the molecular and supramolecular levels, the interactions of N-poly(ethylene glycol)-phosphatidylethanolamines (PEG-PE) with phosphatidylcholine (PC) in aqueous dispersions. PEG-PEs are micelle-forming hydrophilic polymer-grafted lipids that are used extensively for steric stabilization of PC liposomes to increase their lifetimes in the blood circulation. All lipids had dipalmitoyl (C16:0) chains, and the polymer polar group of the PEG-PE lipids had a mean molecular mass of either 350 or 2000 Da. PC/PEG-PE mixtures were investigated over the entire range of relative compositions. Spin-label ESR was used quantitatively to investigate bilayer-micelle conversion with increasing PEG-PE content by measurements at temperatures for which the bilayer membrane component of the mixture was in the gel phase. Both saturation transfer ESR and optical density measurements were used to obtain information on the dependence of lipid aggregate size on PEG-PE content. It is found that the stable state of lipid aggregation is strongly dependent not only on PEG-PE content but also on the size of the hydrophilic polar group. These biophysical properties may be used for optimized design of sterically stabilized liposomes.

Dimyristoylphosphatidylcholine↗
PubMed2003

Lipid-protein interactions with cardiac phospholamban studied by spin-label electron spin resonance.

Phospholamban is a cardiac regulatory protein that, in its monomeric form, inhibits the Ca(2+)-ATPase. Lipid-protein interactions with a synthetic variant of phospholamban, in which all cysteine residues are replaced with alanine, have been studied by spin-label electron spin resonance (ESR) in different lipid host membranes. Both the stoichiometry and selectivity of lipid interactions were determined from the two-component ESR spectra of phospholipid species spin-labeled on the 14 C atom of the sn-2 chain. The lipid stoichiometry is determined by the oligomeric state of the protein and the selectivity by the membrane disposition of the positively charged residues in the N-terminal section of the protein. In dimyristoylphosphatidylcholine (DMPC) membranes, the stoichiometry (N(b)) is 7 lipids/monomer for the full-length protein and 4 for the transmembrane section (residues 26-52). These stoichiometries correspond to the dimeric and pentameric forms, respectively. In palmitoyloleoylphosphatidylcholine, N(b) = 4 for both the whole protein and the transmembrane peptide. In negatively charged membranes of dimyristoylphosphatidylglycerol (DMPG), the lipid stoichiometry is N(b) = 10-11 per monomer for both the full-length protein and the transmembrane peptide. This stoichiometry corresponds to monomeric dispersion of the protein in the negatively charged lipid. The sequence of lipid selectivity is as follows: stearic acid > phosphatidic acid > phosphatidylserine = phosphatidylglycerol = phosphatidylcholine > phosphatidylethanolamine for both the full-length protein and the transmembrane peptide in DMPC. Absolute selectivities are, however, lower for the transmembrane peptide. A similar pattern of lipid selectivity is obtained in DMPG, but the absolute selectivities are reduced considerably. The results are discussed in terms of the integration of the regulatory species in the lipid membrane.

Adenosine Triphosphatases↗