The interaction of intrinsic proteins and lipids in biomembranes

The Myth of The Annular Lipids

In the early 1970s, the existence of a "lipid annulus" stably surrounding the individual intrinsic protein molecules was proposed by several authors. They referred to a number of lipid molecules in slow exchange with the bulk lipid in the bilayer, i.e., more or less protein-bound, and more ordered than the bulk lipid. The annular lipids would control enzyme activity. This idea was uncritically accepted by most scientists working with intrinsic membrane proteins at the time, so that the idea operated like a myth in the field. However, in the following decade, hard spectroscopic and biochemical evidence showed that the proposed annular lipids were not immobilized for a sufficiently long time to influence enzyme or transporter activity, nor were they ordered by the protein. Surprisingly, forty years later, the myth survives, and the term 'annular lipid' is still in use, in a different, but even more illogical sense.

Interaction of the C-terminal domain of Bcl-2 family proteins with model membranes

Bcl-2 family proteins are involved in the cell homeostasis by regulating programmed cell death. Some of these proteins promote apoptosis, while others inhibit the same process. The C-terminal hydrophobic domain of some of these proteins is predicted to be involved in anchoring them to a variety of cell membranes, such as mitochondrial, endoplasmic reticulum and nuclear membranes. We have used five synthetic peptides imitating the C-terminal domain from both anti-apoptotic (Bcl-2) and pro-apoptotic members (Bak, Bax, and two mutants of this last protein) of this family to study their interaction with model membranes. Some differences were detected in the interaction with these peptides. The addition of all the peptides to large unilamellar vesicles destabilized them and released encapsulated carboxyfluorescein to different degrees, so that fluidity and the increase in negative curvature favoured the extent in the release of carboxyfluorescein. Bcl-2-C and Bax-C peptides produced the highest release levels in most cases, while BaxS184K-C was the least efficient in this respect. These results indicate that these C-terminal domains are able to insert themselves in the membranes, each in a different way that is probably related with their different way which can be related to their differing locations within the cell and their different roles in regulating apoptosis.

Interaction of the C2 Domain from Protein Kinase Cε with Model Membranes

The C2 domain from protein kinase Cepsilon (PKCepsilon) binds to membranes but does not require Ca2+ to do so. This work examines the mode in which the conformation and organization of the phospholipids present in model membranes are altered by the presence of the C2 domain from PKCepsilon (C2-PKCepsilon). It is concluded from the results of differential scanning calorimetry that the protein shifted the temperature of the gel to the fluid phase transition of pure 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphate (POPA), widening the transition and increasing it to a higher temperature. When POPA was mixed with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), the changes in the transition were smaller and no phase separation was observed. Experiments performed using magic angle spinning NMR showed that this C2 domain specifically affected POPA when the phospholipid was mixed with POPC, as indicated by the downfield shift in the isotropic resonance of POPA, the widening of the resonance peak, the decrease in T2, and the decrease in T1 observed at all temperatures. All these effects were quite marked compared with the very small effect observed with POPC, indicating the specificity of the effect. The presence of the C2-PKCepsilon protein changed the conformation of the polar head group of POPA, as shown by infrared spectroscopy. All these results clearly illustrate the electrostatic interaction that takes place between this C2 domain and membranes which contain POPA in the absence of Ca2+.

Exogenously incorporated ketocarotenoids in large unilamellar vesicles. Protective activity against peroxidation

The ability of astaxanthin and canthaxanthin as chain-breaking antioxidants was studied in Cu(2+)-initiated peroxidation of phosphatidylcholine large unilamellar vesicles (LUVs). Both carotenoids increased the lag period that precedes the maximum rate of lipid peroxidation, though astaxanthin showed stronger activity. For these experiments, different amounts of xanthophylls were exogenously added to previously made LUVs, non-incorporated pigment being afterwards removed. Differential scanning calorimetry assays with L-beta,gamma-dimyristoyl-alpha-phosphatidylcholine LUVs demonstrated that xanthophylls incorporated as described interact with the lipid matrix becoming interspersed among the phospholipid molecules.

Infrared studies of protein-induced perturbation of lipids in lipoproteins and membranes

The paper reviews the main recent publications concerning infrared (IR) spectroscopy as applied to the study of lipid-protein interactions in model and cell membranes, lipoproteins, and related systems (e.g. lung surfactant). The review focuses mainly on transmission IR. Based on the available data, a number of general conclusions are presented on the perturbations caused by proteins on either the hydrocarbon chains, the polar headgroups or the interface region. Lipid-protein interactions in native cell membranes do not reveal significant differences from what is observed in semisynthetic model systems.

Triple helix formation of procollagen type I can occur at the rough endoplasmic reticulum membrane

One key problem in understanding the biosynthesis of collagens remains the assembly of the three alpha-chains. How and when are the different gene products selected, aligned, and folded into a triple helix? As the spatial arrangement during biosynthesis might be important, we concentrated on whether the rough endoplasmic reticular membrane is involved in this process. Microsomes were prepared from biosynthetically labeled chick tendon fibroblasts. Vesicles were spread as a monomolecular film which was then transferred over several compartments of a filmbalance containing fresh subphase. Fluorograms of the surface film showed that the monolayer contains procollagen chains. When the monolayer was transferred onto a chymotrypsin/trypsin-containing subphase, the gel bands of the proalpha-chains were shifted into the position of mature alpha-chains, indicating that only the propeptides were digested and the collagenous regions were protected due to triple helix formation. Our results suggest that newly synthesized proalpha-chains can associate as trimers and fold into a triple helical conformation while they are still associated with the membranes of the rough endoplasmic reticulum. These processes also occur when interchain disulfide linkage is inhibited, indicating that chain selection and registration is not dependent on formation of covalent bonds among the carboxyl propeptides.

On the principles of functional ordering in biological membranes

Integrating the available data on lipid-protein interactions and ordering in lipid mixtures allows to emanate a refined model for the dynamic organization of biomembranes. An important difference to the fluid mosaic model is that a high degree of spatiotemporal order should prevail also in liquid crystalline, "fluid" membranes and membrane domains. The interactions responsible for ordering the membrane lipids and proteins are hydrophobicity, coulombic forces, van der Waals dispersion, hydrogen bonding, hydration forces and steric elastic strain. Specific lipid-lipid and lipid-protein interactions result in a precisely controlled yet highly dynamic architecture of the membrane components, as well as in its selective modulation by the cell and its environment. Different modes of organization of the compositionally and functionally differentiated domains would correspond to different functional states of the membrane. Major regulators of membrane architecture are proposed to be membrane potential controlled by ion channels, intracellular Ca2+, pH, changes in lipid composition due to the action of phospholipase, cell-cell coupling, as well as coupling of the membrane with the cytoskeleton and the extracellular matrix. Membrane architecture is additionally modulated due to the membrane association of ions, lipo- and amphiphilic hormones, metabolites, drugs, lipid-binding peptide hormones and amphitropic proteins. Intermolecular associations in the membrane and in the membrane-cytoskeleton interface are further selectively controlled by specific phosphorylation and dephosphorylation cascades involving both proteins and lipids, and regulated by the extracellular matrix and the binding of growth factors and hormones to their specific receptor tyrosine kinases. A class of proteins coined architectins is proposed, as a notable example the pp60src kinase. The functional role of architectins would be in causing specific changes in the cytoskeleton-membrane interface, leading to specific configurational changes both in the membrane and cytoskeleton architecture and corresponding to (a) distinct metabolic/differentiation states of the cell, and (b) the formation and maintenance of proper three dimensional membrane structures such as neurites and pseudopods.

Fatty acid composition of lipids which copurify with band 3

In a previous study (L. R. Maneri and P. S. Low (1988) J. Biol. Chem. 263, 16170-16178) we determined that the anion transport protein, band 3, was significantly stabilized by lipids containing saturated and/or long chain fatty acids. To determine whether this thermodynamic preference is reflected in the composition of lipids tightly associating with the anion transporter in vivo, we have analyzed the fatty acid content of phospholipids co-isolating with the purified integral domain of band 3. Our data demonstrate that although stearic acid comprises only 14% of the bulk lipid fatty acids of the red cell membrane, it constitutes -68% of the fatty acids of lipids co-isolating with band 3. Certain other long chain fatty acids were also enriched in the adherent lipids. These results suggest that the fatty acids which most effectively stabilize band 3 also have the highest affinity for the transport protein.

Lipid-protein interactions. The mitochondrial complex III-phosphatidylcholine-water system

Bovine heart mitochondrial complex III (ubiquinol-cytochrome-c reductase) has been reconstituted into phosphatidylcholine bilayers and the effect of varying lipid/protein ratios on the structure and function of the protein has been examined. Electron microscopy, differential scanning calorimetry and Arrhenius plots of enzyme activity provide evidence that the protein is incorporated in an active conformation into pure phosphatidylcholine bilayers. At low lipid/protein ratios (e.g. 80:1 molar ratio) the protein exists in the form of aggregates. As the lipid proportion is increased, electron microscopy reveals the gradual formation of lipid bilayers; structures with the appearance of closed vesicles are seen at or above 300:1 phospholipid/protein molar ratios. Changes in enzyme activity as a function of lipid contents reveal a progressive increase in activity as more lipid is added, with a tendency to reach a saturation point. From the experimental data, a kinetic model is proposed, according to which the protein has an indefinite number of unspecific, independent and identical binding sites for phospholipids, the latter acting as essential enzyme activators. Varying lipid/protein ratios induce structural changes in complex III; visible spectra indicate changes in the polarity of the heme group environment, while Fourier-transform infrared spectroscopy suggests a change in the secondary structure of the protein as the lipid proportion is increased.

Lipid-protein interaction. The incorporation of myelin proteolipid apoprotein into phosphatidylcholine bilayers

Bovine myelin proteolipid apoprotein (PLA), obtained in high yield and purity by a novel ultrafiltration procedure, has been used to study the perturbations produced by this protein on phosphatidylcholine bilayers, using infrared spectroscopy, nuclear magnetic resonance and fluorescence polarisation. PLA interacts with phospholipids in a similar manner to other intrinsic proteins. For bilayers in the fluid state, the fatty-acyl chain static order, as measured by deuterium NMR, is slightly increased in the presence of the protein, except at very high PLA concentrations. Phosphorus NMR reveals some perturbation of the phospholipid polar group by PLA, but to a smaller degree than occurs with other intrinsic proteins. An increase in static order above tc (the onset temperature for gel-to-fluid transition) is also detected by infrared spectroscopy. Studies using steady-state polarisation of diphenylhexatriene fluorescence indicate that the microviscosity of the bilayer increases as a function of the protein mole fraction. From these data an estimation of the average number of lipids perturbed per protein monomer has been made, and a figure of 37 phospholipid molecules determined. The data are compatible with a picture of a hydrophobic polypeptide, perturbing the phospholipids close to it, but allowing rapid (greater than 10(4) s-1) exchange with all the lipid molecules in the system.

The effect of bilayer order and fluidity on detergent-induced liposome fusion

Surfactants induce fusion (or increase in size) of sonicated liposomes. This phenomenon is enhanced by cholesterol and inhibited by the intrinsic polypeptide gramicidin A. By comparison with previous physical studies we conclude that liposome 'fusion' is facilitated when both fluidity and static order of the bilayer are high.

Response of rat heart membranes and associated ion-transporting ATPases to dietary lipid

The effects of different dietary fat intake on the lipid composition and enzyme behaviour of sarcolemmal (Na+ + K+)ATPase and sarcoplasmic reticulum Ca2+-ATPase from rat heart were investigated. Rat diets were supplemented with either sunflower seed oil (unsatd./satd. 5.6) or sheep kidney fat (unsatd./satd. 0.8). Significant changes in the phospholipid fatty acid composition were observed in both membranes after 9 weeks dietary lipid treatment. For both membranes, the total saturated/unsaturated fatty acid levels were unaffected by the dietary lipid treatment, however the proportions of the major unsaturated fatty acids were altered. Animals fed the sunflower seed oil diet exhibited an increase in n-6 fatty acids, including linoleic (18:2(n-6] and arachidonic (20:4(n-6] while the sheep kidney fat dietary rats were higher in n-3 fatty acids, principally docosahexaenoic (22:6), with the net result being a higher n-6/n-3 ratio in the sunflower seed oil group compared to sheep kidney fat dietary animals. Fluorescence polarization indicated that the fluidity of sarcoplasmic reticular membrane was greater than that of sarcolemmal membrane, with a dietary lipid-induced decrease in fluidity being observed in the sarcoplasmic reticular membrane from sheep kidney fat dietary animals. Despite these significant changes in membrane composition and physical properties, neither the specific activity nor the temperature-activity relationship (Arrhenius profile) of the associated ATPases were altered. These results suggest that with regard to the parameters measured in this study, the two ion-transporting ATPases are not modulated by changes which occur in the membrane lipid composition as a result of the diet.

Infrared spectroscopy of phosphatidylcholines in aqueous suspension a study of the phosphate group vibrations

The 1000-1300 cm-1 region of the infrared spectrum of dipalmitoylphosphatidylcholine (DPPC) and other phosphate-containing molecules has been studied by the Fourier-transform technique. Three absorption bands have been assigned to various vibrational modes of the DPPC phosphate group, with maximum wavenumbers at 1060, 1086 and 1222 cm-1. These values are the same above and below Tc of the phospholipid. Dehydration produces band-shifts toward higher wavenumbers .

Freeze-fracture analysis of membrane appression and protein segregation in model membranes containing the chlorophyll-protein complexes from chloroplasts

Cation-induced membrane appression and lateral segregation of chlorophyll-protein complexes have been investigated by freeze-fracture analysis of model membranes containing photosystem 1 and the light-harvesting complex of photosystem 2. In light-harvesting complex proteoliposomes, cations caused extensive membrane adhesion and a segregation of protein into appressed regions. A marked flattening of the appressed membranes, sometimes together with a co-alignment of the particles on the opposing membrane faces, strongly suggests a direct transmembrane attraction between the protein particles. Photosystem 1 membranes were not appressed by cations but some clustering of the particles occurred, together with their exclusion from some regions of the lipid. By incorporating dipalmitoylglycerophosphocholine into the membranes, it is shown that a similar exclusion of the particles can occur due to liquid-crystalline to gel state transitions of the lipids. Proteoliposomes containing both the light-harvesting complex and photosystem 1 displayed cation-induced membrane appression, but only between regions containing the light-harvesting complex. Photosystem 1 was largely confined to unappressed membranes. Destacking occurred at low salt concentrations irrespective of whether photosystem 1 was present, showing that in proteoliposomes, and probably in thylakoids, this process does not require the presence of highly charged complexes from unappressed regions.

Modulation of cardiac glycoside inhibition of (Na+ + K+)-ATPase by membrane lipids. Difference between species

The role of lipids in the modulation of the ouabain-sensitivity of membrane (Na+ + K+)-ATPase from different species has been studied using a reconstitution procedure which promotes lipid exchange during detergent depletion by Sephadex chromatography. Hybrid reconstitution of delipidated (Na+ + K+)-ATPase preparations from bovine brain into the lipids obtained from crab nerve enzyme preparations significantly reduces the sensitivity of the brain enzyme to inhibition by ouabain. Conversely, reconstitution of crab nerve enzyme into the lipids from bovine brain enzyme preparations increases the sensitivity of the crab enzyme to ouabain inhibition. These opposing effects demonstrate the role of membrane lipids in modulating the enzyme-inhibition relationship in preparations from these different species.