A possible role of rhodopsin in maintaining bilayer structure in the photoreceptor membrane

Transmembrane peptides stabilize inverted cubic phases in a biphasic length-dependent manner: implications for protein-induced membrane fusion

WALP peptides consist of repeating alanine-leucine sequences of different lengths, flanked with tryptophan "anchors" at each end. They form membrane-spanning alpha-helices in lipid membranes, and mimic protein transmembrane domains. WALP peptides of increasing length, from 19 to 31 amino acids, were incorporated into N-monomethylated dioleoylphosphatidylethanolamine (DOPE-Me) at concentrations up to 0.5 mol % peptide. When pure DOPE-Me is heated slowly, the lamellar liquid crystalline (L(alpha)) phase first forms an inverted cubic (Q(II)) phase, and the inverted hexagonal (H(II)) phase at higher temperatures. Using time-resolved x-ray diffraction and slow temperature scans (1.5 degrees C/h), WALP peptides were shown to decrease the temperatures of Q(II) and H(II) phase formation (T(Q) and T(H), respectively) as a function of peptide concentration. The shortest and longest peptides reduced T(Q) the most, whereas intermediate lengths had weaker effects. These findings are relevant to membrane fusion because the first step in the L(alpha)/Q(II) phase transition is believed to be the formation of fusion pores between pure lipid membranes. These results imply that physiologically relevant concentrations of these peptides could increase the susceptibility of biomembrane lipids to fusion through an effect on lipid phase behavior, and may explain one role of the membrane-spanning domains in the proteins that mediate membrane fusion.

Ocular manifestations of equine motor neuron disease

The characteristics of the ocular manifestations of equine motor neuron disease (EMND) are described. Forty-two horses with histories, clinical signs and necropsies compatible with EMND were the subjects of this study. Ophthalmoscopic lesions that varied in severity were found in 40 of 42 horses and appeared as a distinct pigmented reticulated pattern at the tapetal-nontapetal junction or throughout the fundus, depending upon severity. The pattern colours ranged from yellow brown to black. Areas of hyperreflectivity formed mosaic patterns in the tapetal fundus. ERG B-wave amplitudes were usually at least 50% reduced and many animals showed extinguished amplitudes. None of the horses had apparent visual impairment. Histopathologically, all 42 horses had retinal pigment epithelial (RPE) congestion with ceroid-lipofuscin. Retinal degeneration was variable even within the eyes. Thin layer chromatography (TLC) analysis of the RPE and neural retina identified both green and orange emitting fluorescent compounds not found in normal horses. All unsupplemented horses had plasma vitamin E levels <1.0 microg/ml. The potential significance of this report is the pathognomonic role the ocular manifestations exhibit in helping to diagnose equine motor neuron disease.

Macroscopic orientation of natural and model membranes for structural studies

One approach for obtaining high-resolution structural and functional information for biomembranes and their proteins is by static solid-state NMR of oriented systems. Here, a general procedure to align fully functional biological membranes containing large membrane proteins (Mr >30,000) is described. The method, based on the isopotential spin-dry ultracentrifugation technique, relies on the centrifugation of membrane fragments onto a support with simultaneous, or subsequent, partial evaporation of the solvent which aids alignment. The quality of orientation, as shown by the mosaic spread of the samples, was monitored by static solid-state 31P NMR for the phospholipids and by 2H NMR for a deuterated retinal in bovine rhodopsin. The generality of this method is demonstrated with three different membranes containing bovine rhodopsin in reconstituted bilayers, natural membranes with the red cell anion exchange transport protein in erythrocytes, band 3, and the nicotinic acetylcholine receptor.

Fourier transform infrared study of the rod outer segment disk and plasma membranes of vertebrate retina

Phospholipid composition and structure of disk and plasma membranes purified from bovine rod outer segments (ROS) are examined using Fourier transform infrared spectroscopy. Vibrational data indicate that both disk and plasma membranes lack sphingophospholipids, in contrast to the lens membranes. The hydrocarbon chains of the disk lipids are unsaturated by a factor of 5 over the acyl chains of the plasma lipids. The plasma lipids with 3-fold higher cholesterol and 5-fold higher saturation melt at a higher temperature (26 degrees C) than the disk lipids which melt at 16 degrees C. The transition temperature decreases by more than 20 degrees C in going from disk lipids to disk membrane, indicating a large drop in the enthalpy of the ROS membrane-matrix, presumably due to enhanced rhodopsin-lipid interaction. The lipid composition predisposes the disk and plasma membranes to be fluid and structurally disordered (about 84%) around physiological temperature. The fluid phospholipid environment of the disk membrane (i.e., just a few degrees above subzero temperatures) is considered to be vital for the ROS photoreceptor function. The amide I band profile of rhodopsin indicates an extensive alpha-helical (53%) peptide chain, with little beta-sheet (21%) and beta-turns (18%) in ROS membranes. This structure and/or conformation is conserved between 0-60 degrees C even though disk and plasma lipids undergo a phase change. The H-D exchange data indicate that as much as 84% of the peptide residues of ROS membranes in partially bleached retinas is accessible to D2O solvent after 1 h.(ABSTRACT TRUNCATED AT 250 WORDS)

S-adenosyl-L-methionine-mediated enzymatic methylations in the rat retinal membranes

Enzymatic step-wise methylation of membrane phosphatidylethanolamine (PE) to phosphatidyl-N-methylethanolamine (PME) and then phosphatidyl-choline (PC) has been known to alter membrane properties and responsiveness of cells for activation of receptors by chemical transmitters. Conversion of PE to PME and PME to PC in the presence of S-adenosyl-L-methionine (SAM) are catalyzed by two phospholipid N-methyltransferases, PMT I and PMT II, of which PMT I is the rate limiting enzyme. Retina is a good neuronal model for chemical transmission. However, retina was not studied for PMT activity. Therefore, we studied the rat retina for PMT I activity. Methylation of PE in the rat retinal sonicates was assayed using 3H-SAM (2 microM) at 37 degrees C in Tris-glycylglycine buffer (50 mM, pH 8.0) and methylated phospholipids were extracted with chloroform/methanol/HCl (2/1/0.02, v/v) and separated by thin layer chromatography on Silica Gel G plates. Chromatograms were developed in a solvent system of propionic acid/n-propyl alcohol/chloroform/water (2/2/1/1, v/v). This study gave the following results: (a) the total methylated phospholipids were (M +/- SE, N = 5) 19.90 +/- 4.03 fmol/mg protein/min; (b) the major methylated phospholipid was PME (4.21 +/- 0.68 fmol/mg protein/min; (c) the fatty acid methylesters formed by fatty acid carboxymethylase (FACM) which accumulated in the solvent front amounted to 18.82 +/- 2.84 fmol/mg protein/min. Both PMT I and FACM were inhibited by S-adenosyl-L-homocysteine (I50, 1.2-5 microM). These observations indicate that rat retina contains both PMTs and FACM.

Synergistic effects of Ca2+ and wheat germ agglutinin on the lamellar-hexagonal (HII) phase transition of glycophorin-containing egg-phosphatidylethanolamine membranes

Glycophorin has been reconstituted into egg phosphatidylethanolamine (egg-PtdEtn) membranes. Stable vesicles were obtained at a molar ratio of 8 x 10(-4) of inserted protein/lipid. This macroscopic change from lipid aggregates to lipid vesicles was followed by density gradient centrifugation. Vesicles formed in the presence of protein enclose the dye calcein and are stable with time and temperature. Membrane aggregation does not occur, as was demonstrated by energy-transfer experiments. The phase transition from the fluid lamellar L alpha phase to the inverted hexagonal HII phase observed at 29 degrees C in pure egg-PtdEtn membranes is suppressed and finally disappears in the presence of glycophorin. The transition enthalpy decreases linearly from delta H = 4 kJ/mol in pure lipids to zero at a protein/lipid molar ratio of 1:1000. Ca2+ ions and wheat germ lectin act synergistically on the phase behavior of vesicles containing glycophorin and phosphatidylethanolamine. Differential scanning calorimetry scans show that the lamellar-to-hexagonal phase transition is reinduced. The membranes aggregate and exchange lipid as could be demonstrated by energy transfer experiments. The dye calcein is released but only if the temperature exceeds the lamellar-to-hexagonal phase transition temperature of the pure egg-PtdEtn.

Magnetic resonance of membranes

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The effect of salinity on the phase behaviour of total lipid extracts and binary mixtures of the major phospholipids isolated from a moderately halophilic eubacterium

The effects of molar NaCl concentrations on the phase behaviour of the total lipid extracts and binary mixtures of the major phospholipids, namely phosphatidylethanolamine (PE) and phosphatidylglycerol (PG), isolated from the moderately halophilic eubacterium, Vibrio costicola, grown in 1 M and 3 M NaCl containing media have been studied using X-ray diffraction and freeze-fracture electron microscopy. The effect of both the PE/PG ratio and alterations in fatty acid composition were examined by using binary mixtures which mimicked the PE/PG ratio found in the native bacterial membranes. We show that the samples exhibited complex phase behaviour, including the formation of non-bilayer phases, which depend upon the salinity of both the bacterial culture medium and the suspending solution. The total lipid from bacteria cultured in 1 M NaCl-containing medium and dispersed in 1 M NaCl exhibited a mixture of L alpha and hexagonal-II phases at the optimum growth temperature of the organism (i.e., 30 degrees C), whereas the same lipid dispersed in 3 M NaCl showed only a hexagonal-II phase down to a temperature of +3 degrees C. The total lipid extracted from 3 M NaCl cultures showed only lamellar phases over the temperature range studied (+50 degrees C to -50 degrees C), but the phase transition temperatures of the various lamellar phases were generally higher when the lipid was dispersed in 3 M compared with 1 M NaCl. The phase behaviour of the binary mixtures was similar but not identical to that of the corresponding total lipid extracts and it is suggested that the minor lipid components (diphosphatidylglycerol, lysophosphatidylethanolamine and lysophosphatidylglycerol) play a part in determining the phase behaviour of the native membranes. These results show that the PE/PG ratio and fatty acid composition of the individual phospholipids, which are normally regulated by Vibrio costicola in vivo in response to culture medium salinity, are both important in maintaining a stable bilayer structure within the membrane.

Incorporation of cytochrome oxidase into cardiolipin bilayers and induction of nonlamellar phases

Cytochrome oxidase from beef heart has been lipid-substituted with beef heart cardiolipin. The lipid phase behavior and protein aggregation state of the reconstituted complexes have been studied with 31P NMR, freeze-fracture electron microscopy, and saturation-transfer ESR of the spin-labeled protein. In the absence of salt, the lipid has a lamellar arrangement, and the protein is integrated and uniformly distributed in the membrane vesicles and undergoes rapid rotational diffusion. The presence of the protein stabilizes the cardiolipin lamellar phase against salt-induced transitions to the inverted hexagonal phase. The threshold salt concentration becomes higher and the extent of conversion becomes lower with decreasing lipid:protein ratio. In high salt, lamellar-phase lipid with integrated protein coexists with hexagonal-phase lipid free of protein, and the rotational diffusion of the protein is drastically reduced as a result of the high packing density.

Bilayer-stabilizing properties of myelin basic protein in dioleoylphosphatidylethanolamine systems

31P-NMR and X-ray diffraction techniques are used to study the comparative ability of myelin basic protein (MBP) vs. other basic proteins to convert hexagonal (HII) phases to stable lamellar (L alpha) structures. Pure dioleoylphosphatidylethanolamine (DOPE) at pH 9 and 7, and mixtures of DOPE/phosphatidylserine (PS) (95:5 and 80:20% w/w) at pH 7 were employed for this investigation. The polymorphic behavior of the lipid suspensions was evaluated in the presence and absence of several basic proteins (MBP, calf thymus histone, lysozyme, melittin) and the cationic polypeptide, polylysine (PL). Each of the proteins and PL was capable of binding the pure DOPE HII phase at pH 9 but with varying morphological consequences, i.e., lamellar stabilization (MBP, histone, PL), formation of new protein-DOPE HII phases (lysozyme) or lipid disordering/vesiculation (melittin). Reduction to pH 7 resulted in the dissociation of protein from DOPE - with the exception of melittin - and the reformation of a pure lipid HII phase. Additions of PS to DOPE at pH 7 facilitated protein binding, but among the proteins examined, only MBP was capable of converting the lipid suspension into a stable multilamellar form. Differences in the lipid morphology produced by each protein are discussed in terms of protein physicochemical characteristics. In addition, a possible relationship between MBP-lipid interactions and the stability of myelin sheath lipid multilayers is inferred from the significant bilayer-stabilizing capacity of MBP.

Principles of membrane stability and phase behavior under extreme conditions

Biological membranes consist of a complex assortment of lipids and proteins. The arrangement of the components, particularly in regard to their lateral disposition in the plane of the membrane under physiological conditions, is dependent on the phase behavior of the different membrane lipids and the way that this behavior is modified by interaction with other membrane components and electrolytes in the aqueous medium. Irreversible phase separation of components within the membrane may result from exposure to extreme environmental conditions including temperature, pressure, or electrolyte concentration. The principles underlying the phase-mixing behavior of model membrane systems can be used to provide useful information about the factors that determine the stability of biomembranes under physiological and non-physiological conditions. These data are reviewed and used to predict events that take place when membranes are exposed to environmental stress.

Lipid polymorphism of model and cellular membranes as revealed by electron microscopy

This review surveys the current state of knowledge relating to lipid polymorphism within both model lipid membrane and cellular membrane systems. Emphasis is placed upon the contribution of data obtained by transmission electron microscopy of freeze-fractured specimens. Some consideration is also given to the other important methods for the study of lipid polymorphism, namely X-ray diffraction and NMR spectroscopy. A detailed account of the different phases found in lipid mixtures within model membranes (bilayer, cubic or tetragonal, tubular or hexagonal) provides the background to the understanding of the factors involved in polymorphic phase transitions. The sequential steps involved in lipid polymorphism are defined from electron microscopical data and are related to the structural changes that can be detected within biological membranes. It is proposed that the fine structural changes detected at the initial stages of polymorphic transition in vivo may be highly relevant in relation to membrane fusion events, to the formation of tight junctions, and even to physiological transport processes. Since the later stages of polymorphic transition generally destroy the permeability barrier of model and cellular membranes, extensive rather than localized phase transition of the lipid bilayer is not at the moment considered to be compatible with cellular viability.

Spontaneous vesicularization of myelin lipids is counteracted by myelin basic protein

Hand-vortexed dispersions of several lipids (cerebrosides, sulfatides, PC, PE, PS and sphingomyelin), mixed in the ratios found for these categories of lipids in myelin, exhibit 31P-NMR spectra which have contributions from both isotropic and lamellar resonances. Investigation of this system by freeze-fracture electron microscopy and X-ray diffraction revealed that this lipid mixture has spontaneously formed small unilamellar vesicles (SUVs) (diam. approximately 400 A) and large highly convoluted unilamellar vesicles (LUVs) (diam. approximately 1000 A), the latter possibly resulting from aggregation and fusion of the SUV structures. This vesicularization of the myelin lipids was reversed by the addition of myelin basic protein: only large multilamellar aggregates were formed in the presence of protein, as shown by all three experimental methods. Although no rigorous physical-chemical explanation for these phenomena is yet available, the possibility is suggested that the high concentration of cerebrosides and/or phosphatidylethanolamine in this particular mixture of myelin lipids play pivotal roles in the formation of these unusual vesicles. Spontaneous vesicularization of myelin lipids is discussed as a potential pathway toward destabilization of the myelin sheath.

The influence of proteins and peptides on the phase properties of lipids

This paper reviews model membrane studies on the modulation of the macroscopic structure of lipids by lipid-protein interactions, with particular emphasis on the gramicidin molecule. This hydrophobic peptide has three main effects on lipid polymorphism: (1) in lysophosphatidylcholine it triggers a micellar to bilayer transition, (2) in phosphatidylethanolamine it lowers the bilayer to hexagonal HII phase transition temperature and (3) in phosphatidylcholine and other bilayer preferring lipids it is able to induce the formation of an HII phase. From experiments in which the gramicidin molecule was chemically modified it can be concluded that the tryptophan residues play a determining role in the peptide-induced changes in polymorphism. The experimental data lead to the proposal that gramicidin molecules have a tendency to self-associate, possibly mediated by tryptophan-tryptophan interactions and organize into tubular structures such as found in the HII phase.

31P-NMR investigation of magnetically oriented rod outer segments. Spectral analysis and identification of individual phospholipids

A 31P-NMR study of magnetically oriented bovine rod outer segments is presented. We demonstrate that carefully isolated bovine rod outer segments retain the capacity to orient in a magnetic field. Maximal orientation (85-90%) is achieved at field strengths over 4.7 T in the NMR spectrometer. The lineshape of the 'oriented spectra' is totally different from the 'bilayer lineshape' of randomly oriented photoreceptor membranes. The oriented spectra consist of two phospholipid peaks, a major low-field peak (75-80% of the total intensity) near 30 ppm, and a minor high-field peak near - 14 ppm as well as two sharp metabolite peaks around 0 ppm. The phospholipid peaks are a composite of three narrower partially resolved resonances assigned to the individual phospholipid classes phosphatidylserine, phosphatidylcholine and phosphatidylethanolamine. Based on the morphology and magnetic anisotropy of the rod outer segment, the major phospholipid peak is attributed to the flat part of the disk membranes while the phospholipids of the plasma membrane are thought to contribute only to the minor peak. Disk rim phospholipids and non-oriented material contribute to the minor peak and, in addition, contribute some intensity to the middle part of the spectrum. The phospholipid class composition of the major peak is estimated by spectral simulation and is consistent with the phospholipid class composition of rod outer segment membranes. Hence, 31P analysis of oriented rod outer segments resolves the main phospholipids in at least two different membrane pools in the rod outer segment and allows the differential investigation of these pools. Most of the mobile phosphate metabolite intensity resides in the Pi peak at 3.5 ppm. A slight shift in the Pi resonance position indicates a 0.2 pH unit acidification upon illumination of rhodopsin. The absence of detectable nucleotide resonances, when compared with chemical analysis, indicates that the majority of the nucleotide population present is rather immobile and probably bound to the membranes.

A lipid-phase separation model of low-temperature damage to biological membranes

An hypothesis is proposed to explain the damage caused to biological membranes exposed to low temperatures. The thesis rests on the general observation that the lipid components of most membranes are heterogeneous and undergo phase transitions from gel-phase lamellae to liquid-crystalline lamellae and some to a non-lamellar, hexagonal-II phase over a wide range of temperatures. As a consequence of these phase transitions the lateral distribution of the lipids characteristic of the growth temperature is disturbed and redistribution takes place on the basis of the temperature at which phase transitions occur. When membranes are cooled, first the non-lamellar forming lipids pass through a transition to a fluid lamellar phase and are miscible with bilayer-forming lipids into which they diffuse. On further cooling the high-melting-point lipids begin to crystallize and separate into a lamellar gel phase, in the process excluding the low-melting point lipids and intrinsic proteins. The lipids in these remaining regions form a gel phase at the lowest temperature. It is suggested that, because the non-lamellar lipids tend to undergo a liquid-crystalline to gel-phase transition at higher temperatures than lamellar-forming lipids, these will tend to phase separate into a gel phase domain rich in these lipids. Damage results when the membrane is reheated, whereupon the hexagonal-II-forming lipids give rise to non-lamellar structures. These probably take the form of inverted micelles sandwiched within the lipid bilayer and they completely destroy the permeability barrier properties of the membrane. The model is consistent with the phase behavior of membrane lipids and the action of cryoprotective agents in modifying lipid phase properties.

Thermotropic behavior of retinal rod membranes and dispersions of extracted phospholipids

High sensitivity, differential scanning calorimetry studies of bovine retinal rod outer segment (ROS) disk membranes and aqueous dispersions of the extracted ROS phospholipids have been performed. ROS disk membranes were found to exhibit a broad peak of excess heat capacity with a maximum at less than about 3 degrees C, ascribable to a gel-to-liquid crystalline phase transition of a fraction of the phospholipids. A similar thermotropic transition was observed for aqueous dispersions of the total extracted and purified ROS phospholipids. Comparison of the results obtained for the dispersion of total ROS phospholipids to those of the purified head group fractions suggests that the thermotropic behavior reflects a gel-to-liquid crystalline transition, leading to lateral phase separation, involving those phosphatidylcholine (PC) molecules containing saturated fatty acyl chains, possibly together with the highest melting ROS phosphatidylethanolamine (PE) and phosphatidylserine (PS) components. The interpretation of the thermal behavior of the ROS disk membranes depends on whether the transition is assumed to derive from the ROS PC and/or PE/PS fractions, and whether the transbilayer arrangement of the ROS phospholipids is assumed to be symmetric or asymmetric. The calorimetric data can be simply explained in terms of an asymmetric distribution of the major ROS disk membrane phospholipids (G.P. Miljanich et al., J. Membrane Biol. 60:249-255, 1981). In this case, the transition would arise from the PE/PS fractions in the outer ROS disk membrane monolayer, and the anticipated transition from the PC in the inner monolayer would be broadened due to interaction with cholesterol. For the ROS membranes at higher temperatures, two additional, irreversible transitions are observed at 57 and 72 degrees C, corresponding to the thermal denaturation of opsin and rhodopsin, respectively.

Reaggregation of etioplast lipids and the formation of prolamellar bodies and thylakoids: An ultrastructural study

Etioplasts of dark-grownAvena sativa plants were used to prepare either saponin-free or saponin-containing prolamellar bodies. Lipid extracts from both fractions were studied in reaggregation experiments: extracts containing saponins showed liposomes as well as tubules, while saponin-free samples formed only liposomes. Purified PLB lipids in reaggregation experiments were either studied in the presence or in the absence of saponins. Best tubule formation was found with samples containing MGDG+saponin. However, the reconstruction of PLB-like structures was not possible. The long tubules, protruding from isolated PLBs, are seen as a result of the reaction of saponins (originally located in vacuoles) with MGDG.

Phase behavior of isolated photoreceptor membrane lipids is modulated by bivalent cations

The phase behavior of isolated photoreceptor membrane lipids is further investigated by 31P-NMR, in view of earlier discrepant results [(1979) Biochim. Biophys. Acta 558, 330-337; (1982) FEBS Lett. 124, 93-99]. We present evidence that the discrepancy is due to bivalent cations. When resuspended in aqueous media at neutral pH in the absence of bivalent cations, the isolated photoreceptor membrane lipids largely adopt the bilayer configuration. However, upon addition of such cations (Ca2+ greater than Mg2+) or when resuspended in their presence, the formation of other phases (hexagonal H11, lipidic particles) results. The rate of this transition depends on cation concentration and temperature. The transition is not easily reversed by addition of EDTA. Implications with regard to photoreceptor membrane structure and function need further study.

The effect of calcium on the bilayer stability of lipids from bovine rod outer segment disk membranes

The phase behavior of bovine rod outer segment disk lipids has been investigated using freeze-fracture and 31P nuclear magnetic resonance (NMR) techniques. 31P-NMR spectra of isolated disk membranes were taken as a function of temperature between 25 degrees C and 45 degrees C. The 31P-NMR spectrum characteristic of phospholipid bilayers was observed at all temperatures both in the absence of Ca2+ and in the presence of 10 mM and 50 mM Ca2+. A similar study was performed on lipids isolated from the disk membranes. In the absence of Ca2+ only lamellar phase behavior was observed. In the presence of less than 10 mM Ca2+, however, there was a change in morphology to non-lamellar structures. Removal of the Ca2+ caused the system to reassume the lamellar form.

Phospholipid domains in bovine retinal rod outer segment disk membranes

Phospholipid behavior in bovine retinal rod outer segment disk membranes and in phosphatidylcholine membranes containing the photopigment rhodopsin is explored. 31P NMR spectra of these systems show two distinguishable resonances. One resembles closely the 31P NMR resonance normally obtained from phospholipid bilayers. The other resonance is much broader. Thus, there appear to be two phospholipid head-group domains in this retinal membrane. Each environment confers different properties on the head groups. Phosphatidylcholine membranes containing the disk photopigment also show two phospholipid domains. Therefore, the environment in the retinal rod outer segment disk membranes characterized by the broad resonance may arise from the influence of the integral membrane protein rhodopsin on the membrane phospholipid bilayer.

Ca2+-cardiolipin interaction in a model system. Selectivity and apparent high affinity

The interaction of cardiolipin with Ca2+ was assessed by measuring the cardiolipin-mediated extraction of 45Ca2+ from an aqueous to an organic (methylene chloride) phase. Cardiolipin binds Ca2+ with high affinity [Kd(apparent) = 0.70 +/- 0.17 microM (S.D.)]. Cation-cardiolipin interactions are selective. Interaction of cardiolipin with Ca2+ is insensitive to Na+, but is inhibited by divalent cations with Mn2+ greater than Zn2+ greater than Mg2+. In addition La3+ and Ruthenium red are particularly potent inhibitors of Ca2+ binding by cardiolipin. Cardiolipin-mediated extraction of Ca2+ into an aqueous phase is also inhibited by phosphatidylcholine. Inhibition of Ca2+-cardiolipin interaction by phosphatidylcholine (a phospholipid known to stabilize the bilayer conformation) may implicate inverted, non-bilayer lipid structures in the binding.

Morphology of the intermediate stages in the lamellar to hexagonal lipid phase transition

The addition of calcium to suspensions of egg phosphatidylcholine and cardiolipin converts multiwalled liposomes to the hexagonal (HII) phase (Rand, R.P., Sengupta, S. (1972) Biochim. Biophys. Acta 255:484-492). We have studied this lamellar to hexagonal phase transition by freeze-fracture, thin-section electron microscopy, and X-ray diffraction and have morphologically characterized the intermediate stages. The first step in the transition involves the invagination and fusion of bilayers, marked by the appearance of lipidic intramembrane particles and "crater-like" indentations, as the large liposomes are converted to smaller flattened and elongated vesicles. The next step is the formation of tightly packed hexagonal arrays of tubules, each tubule being about 11 to 15 nm in diameter. These tubules are filled with fluid and a lipid bilayer forms the wall of each cylinder. Finally this tubular bilayer phase is converted to the hexagonal (HII) phase, where the distance between tubes is 5.5 to 7.5 nm.

X-ray diffraction and electron microscope study of phase separation in rod outer segment photoreceptor membrane multilayers

Phase separation in artificially stacked multilayers of isolated bovine retinal rod outer segment (ROS) membranes has been examined via x-ray diffraction and electron microscopy. Specimens were prepared by isopotential spin drying followed with partial hydration by equilibration against moist gas streams. Upon dehydration, the multilamellar membrane phase assumes a binary phase composition consisting of concentrated protein-containing lamellae interspersed with microdomains of hexagonally packed tubes of lipid in a HII configuration. The HII lattice is geometrically coupled to the lamellar phase with one set of hexagonal crystal planes co-planar to the local membrane lamellae. The hexagonal microdomains bear a striking resemblance to the "paracrystalline inclusions" observed in fast-frozen, intact frog ROS (Corless and Costello. 1981. Exp. Eye Res. 32:217). The lamellar lattice is characterized by an unusually small degree of disorder. Sharp lamellar diffraction with a 120 A unit cell is observed (at near total dehydration) to a resolution of 6 A. A model consistent with the data is that a multilamellar array of ROS disks is stable as long as the external disk surfaces are kept sufficiently far apart. If the distance between the membranes is allowed to shrink below a certain critical value, the disk lipids spontaneously convert to a nonbilayer phase. This suggests that the structure of the ROS is stabilized by an internal framework that acts to keep the disks apart from one another and from the plasmalemma. Thus, the necessity of avoiding phase separations may provide a rationale for the peculiar morphology of the ROS.

Effect of glycophorin on lipid polymorphism. A 31P-NMR study

(1) The effect of glycophorin, a major intrinsic glycoprotein of the human erythrocyte membrane, on lipid polymorphism has been investigated by 31P-NRM (at 36.4 MHz) and be freeze-fracture electron microscopy. (2) Incorporation of glycophorin into vesicles of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) results in the formation of unilamellar vesicles ((1 000-5 000 A diameter) which exhibit 31P-NRM bilayer spectra over a wide range of temperature. A reduction in the chemical shift anisotropy ( delta sigma eff csa) and an increase in spectral linewidth in comparison to dioleoylphosphatidylcholine liposomes may suggest a decrease in phospholipid headgroup order. (3) 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE), in the presence of excess water, undergoes a bilayer to hexagonal (HII) phospholipid arrangement as the temperature is increased above 0 degrees C. Incorporation of glycophorin into this system stabilizes the bilayer configuration, prohibiting the formation of the HII phase. (4) Cosonication of glycophorin with DOPE in aqueous solution (pH 7.4) produced small, stable unilamellar vesicles (300-1 000 A diameter), unlike DOPE alone which is unstable and precipitates from solution. (5) The current study demonstrates the bilayer stabilizing capacity of an intrinsic membrane protein, glycophorin, most likely by means of a strong hydrophobic interaction between the membrane spanning portion of glycophorin and the hydrophobic region of the phospholipid.