Rapid lateral diffusion of phospholipids in rabbit sarcoplasmic reticulum

Intra-axonal diffusion of [3H]acetylcholine and [3H]gamma-aminobutyric acid in a neurone of Aplysia

1. [3H]acetylcholine (ACh) or [3H]gamma-aminobutyric acid (GABA) was injected into the cell body of a cholinergic neurone of Aplysia kurodai. 2. [3H]ACh moved down the axon at a maximum speed of 2.5 mm/sec at 20 degrees C. 3. 20 mM-colchicine suppressed this movement, but some passive movement of radioactivity was noted along the axon. 4. Profiles of the passive movement coincided with theoretically obtained diffusion profiles. 5. The diffusion coefficient of ACh in the axoplasm was estimated. It was 3 x 10(-6) cm2/sec at 5 degrees C, 4 X 10(-6) cm2/sec at 15 degrees C and 6.5 x 10(-6) cm2/sec at 30 degrees C. The Q10 was 1.35, the activation energy was about 5 kcal/degrees C. These diffusion coefficients were lower than that of free diffusion of ACh (8 x 10(-6) cm2/sec at 18 degrees C, Fatt, 1954), and assumed to be reasonable, if one considers frictional resistivity of axoplasm in the diffusion of ACh molecules. 6. [3H]GABA diffused similarly to ACh, and the diffusion coefficients agreed with the estimated value when the molecular size differences were taken into account. 7. Both ACh and GABA seemed to diffuse in soluble form or as single molecules in the axoplasm. 8. Intra-axonal diffusion is very effective for short distances.

On two-dimensional passive random walk in lipid bilayers and fluid pathways in biomembranes

The lateral mobility of pyrene, pyrene decanoic acid, and 1-palmitoyl-2-pyrene decanoyl-phosphatidyl choline (pyrene lecithin) in lipid bilayers is determined by the excimer formation technique. This method is applied to vesicles of lecithins differing in chain length and in the degree of saturation of the hydrocarbon chains. These values are compared with results in cephalins of different chain length and in dipalmitoyl phosphatidic acid at variable pH. The influence of cholesterol is investigated. The results are analyzed in terms of the Montroll model of two-dimensional random walk. The jump frequency of the probe molecule within the lipid lattice is obtained. The advantage of this measure of transport in lipid layers is that it does not involve lipid lattice parameters. The main results of the present work are: (i) The lateral mobility of a given solute molecule in lamellae of saturated lecithins is independent of hydrocarbon chain length and rather a universal function of temperature. (ii) In unsaturated dioleyl lecithin the amphiphatic molecules have lateral mobilities of the same size as in saturated lipids. The jump frequency of pyrene, however, is by a factor of two larger in the unsaturated lecithin. (iii) The jump frequencies in phosphatidyl ethanolamines are about equal to those in lecithins. (iv) In phosphatidic acid layers the hopping frequencies depend on the charges of the head groups of both the lipids and the probes. (v) Cholesterol strongly reduces the jump frequency in fluid layers. (vi) The lateral mobility in biological membranes is comparable to that in artificial lipid bilayers. The experimental results are discussed in terms of the free volume model of diffusion in fluids. Good agreement with the predictions made from this model is found. A striking result is the observation of a tilt in dioleyl-lecithin bilayer membranes from the hopping frequencies of pyrene and pyrene lecithin. A tilt angle of phi = 17 degrees is estimated.

Specificity of memory cells raised against trinitrophenyl-conjugated syngeneic cells

Exogenous lipid analogues can be incorporated into purified plasma membranes of EL4 tumor cells. When EL4 membranes are incubated with sonicated lipid vesicles containing spin-labeled phosphatidylcholine and then purified, they exhibit paramagnetic resonance spectra characteristic of spin labels dilutely dispersed in the lipid bilayer. Also, when EL4 membranes are incubated with vesicles of fluorescent lipid (N-4-nitrobenzo-2-oxa-1,3-diazole phosphatidylethanolamine), the fluorescence intensity of the mixture gradually increases and the labeled membranes are strongly fluorescent under a microscope, whereas the original lipid vesicles are much less fluorescent because of self-quenching at the high relative concentration of the fluorophore in the vesicles (33%, wt/wt). Thus, lipid analogues can be integrated into the bilayers of EL4 plasma membranes. The ability of the EL4 membranes containing trinitrophenyl phosphatidylethanolamine to induce a secondary proliferation of C57BL/6 mouse spleen lymphocytes primarily stimulated by trinitrophenyl-modified autologous cells was investigated. Although trinitrophenyl-modified plasma membranes as well as trinitrophenyl-modified lymphocytes or EL4 cells caused a specific, H-2 restricted response, membranes containing trinitrophenyl phosphatidylethanolamine did not induce a secondary proliferative response. Because of the strong likelihood that all three lipid analogues mentioned above are similar to one another in having a high lateral mobility and in not being strongly associated with H-2 cell-surface molecules, these results can be viewed, with caution, as being consistent with a "modified self" model rather than a dual recognition model for H-2 restriction in the cell-mediated immune response to chemically modified syngeneic cells.

Cholesterol levels and plasma membrane fluidity in 3T3 and SV101-3T3 cells

Polyene antibiotics such as filipin selectively inhibit wheat germ agglutinin-induced agglutination of transformed and malignant cells compared to normal cells (Hatten ME, Burger MM: Biochemistry 18: 739, 1979). Since filipin binds specifically to cholesterol, we measured cholesterol levels in 3T3 cells and SV101-3T3 cells. SV101-3T3 cells contained 50-100% more cholesterol per cell than 3T3 cells. Both cell types were starved for cholesterol by growth in lipid-depleted medium plus 25-hydroxycholesterol. The cholesterol level of SV101-3T3 cells decreased by 30-50%, while the level in 3T3 cells remained constant. Filipin-stained SV101-3T3 cells revealed bright patches of filipin under fluorescence microscopy. These patches were absent in 3T3 cells and in SV101-3T3 and 3T3 cells starved for cholesterol. We selectively labeled plasma membranes of these cells with a spin label analog of phosphatidylcholine. The spin label indicated differences in plasma membrane fluidity that may be related to the different cholesterol levels in 3T3 and SV101-3T3 cells.

Lipid-protein interactions in membranes: effect of lipid composition on mobility of spin-labeled cysteine residues in yeast plasma membrane

In order to gain direct evidence for lipid-dependent protein conformation in membrane, effects of modification of lipid composition on mobility of spin-labeled cysteine residues were investigated in the plasma membrane of the yeast Saccharomyces cerevisiae. Conversion of the bulk of phospholipids to diglycerides by treatment of the membrane with phospholipase C substantially enhanced spectral anisotropy. However, alterations of the viscosity of the lipid-bilayer by enriching the membrane with palmitelaidic or oleic acid had no effect on mobility of spin-labeled cysteine residues. These observations indicate that while the spin-labeled residues are not in direct contact with the lipid core of the membrane, there are lipid-protein interactions to the extent that removal of the polar portion of the bulk of phospholipids induces conformational changes in proteins, which in turn restrict mobility of these residues. It is concluded that conformation of membrane proteins on lipid structure and that phospholipids have a role in preserving the native conformation of proteins.

Introduction of antigenic phospholipids into the plasma membrane of mammalian cells: organization and antibody-induced lipid redistribution

Phosphatidylethanolamine bearing the 2,4,6-trinitrophenyl hapten was introduced into the surface membrane of mammalian fibroblasts by incubating the cells with small unilamellar vesicles containing this hapten-conjugated lipid. Consistent with integration of the antigen into the plasma membrane lipid bilayer, the exogenously supplied lipid was observed by immunofluorescence to diffuse rapidly (D greater than or equal to 0.6 X 10(-8) cm2/sec) over the surface of polykaryons formed between vesicle- and non-vesicle-treated cells. Association of the exogenous lipids with cells via adsorption of vesicles to the plasma membrane was rigorously excluded by a combination of ultrastructural and immunofluorescence studies. The distribution of the integrated antigenic lipid in the plasma membranes of vesicle-treated cells was followed by immunofluorescence microscopy. The exogenously supplied hapten-conjugated phospholipid was observed to be uniformly distributed and remained so for up to 1 hr at 37 degrees C. However, upon the addition of bivalent, but not monovalent, antihapten antibodies, the phospholipid underwent a rapid temperature-dependent redistribution, forming small patches that eventually coalesced into one or more large aggregates. This unexpected finding is discussed in terms of the mode of insertion of the lipid into the cell surface and the possible mechanisms by which bivalent ligands might alter the mobility and distribution of cell surface phospholipids.

Fluorotyrosine M13 coat protein: fluorine-19 nuclear magnetic resonance study of the motional properties of an integral membrane protein in phospholipid vesicles

We have prepared in vivo a fluorotyrosyl derivative of M13 coat protein and have incorporated it at high levels in small phospholipid vesicles, using a urea-cholate dialysis procedure. 19F nuclear magnetic resonance experiments at 254 MHz with this system indicate a T1 of 0.32 s and line width of 300 Hz. The observed line width increases dramatically below the gel to liquid-crystalline transition temperature for the lipid, indicating that the probe is sensitive to the phase state of the bilayer. Neclear Overhauser enhancement and field dependence of line width were used to establish the relative contributions of dipolar interactions and chemical-shift anisotropy to the observed T1 and line width. From this relaxation data, we have constructeda model for the motional properties of the protein in the lipid bilayer. This model is characterized by correlation times for rotation about the alphabeta and betagamma bonds of the two tyrosyl residues of 2 x 10(-8) and k x 10(-9) s, respectively. Rapid intermolecular dipole-dipole interactions are required to account for theestimated dipolar contribution to T1. A reasonable model for these interactions is that lipid methylene protons are involved in relaxation of the fluorine probes (which reside in the hydrophobic region of this integral membrane protein). We estimate a minimum translational diffusion coefficient for such lipids of D greater than 3 x 10(-9) cm2/s.

Immobilized lipid in acetylcholine receptor-rich membranes from Torpedo marmorata

The lipid environment of acetylcholine receptor-rich membranes from Torpedo marmorata has been studied with spin labels. The electron spin resonance spectra of both stearic acid and steroid probes in the membranes revealed an immobilized lipid component, in addition to the fluid component which is found in aqueous bilayer dispersions of the extracted lipids. The spin labels also cause a differential paramagnetic quenching of the intrinsic protein fluorescence of the membranes, which is sensitive to the action of cholinergic ligands and follows a modified Stern-Volmer law. Electron spin resonance difference spectroscopy shows that the protein-associated lipid is immobilized with respect to rotation both around and perpendicular to the long molecular axis, with correlation times : formula: (see text) approximately 50-70 ns. The proportion of lipid in the immobilized component is greater than calculated for a single boundary layer around the protein and corresponds more closely to the total interstitial lipid occupying the area between densely packed protein units in acetylcholine receptor-rich membranes.

Lateral diffusion of the phospholipid molecule in dipalmitoylphosphatidylcholine bilayers. An investigation using nuclear spin--lattice relaxation in the rotating frame

Measurements of the proton NMR spin--lattice relaxation time in the rotating frame (T 1rho) have permitted the explicit determination of the lateral diffusion coefficient of phospholipid molecules in the lamellar mesophase of dipalmitoylphosphatidylcholine at temperatures above the phase-transition temperature. The experimentally observed temperature and frequency dependence of T 1rho for the dipalmitoylphosphatidylcholine protons suggest that intermolecular dipole--dipole relaxation contributions are important. Proton T 1rho experiments involving dilution with deuterated dipalmitoylphosphatidylcholine support the premise that intermolecular dipolar interactions are significant and, concomitantly, that lateral diffusion is the motion modulating that interaction. The lateral diffusion coefficient is determined directly from the dependence of the rotating frame spin--lattice relaxation rate (1/T 1rho) on the strength of the applied radiofrequency field in the spin-locking experiment. A series of experiments with varying concentrations of dipalmitoylphosphatidylcholine in the lamellar mesophase indicates that the lateral diffusion coefficient varies as a function of phospholipid concentration.

A fluorescence approach of the determination of translational diffusion coefficients of lipids in phospholipid monolayer at the air-water interface

In the present work, it is shown that the photobleaching technique as well as experimentation based on fluorescence recovery after bleaching can be extended to monolayers spread at the air-water interface. A mathematical model is derived which allows the determination of translational diffusion coefficients of species diffusing in such a system. Using 12-(9-anthroyl)stearic acid (anthroylstearate) as a fluorescent probe, dispersed either in dipalmitoylphosphatidylcholine or in dipalmitoylphosphatidylglycerol in various conditions of subphase ionic composition and surface pressure of the monolayer, including phase transition domains, we are led to the following conclusions: 1. Anthroylstearate molecules seem to aggregate in 'microdomains' where their fluorescence properties remain unchanged regardless of the compression states of the host monolayer. 2. In any case, a break in the diffusion constants appears on compressing films of both dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylglycerol. In particular, this break coincides with the liquid expanded to gel phase transition of these lipids when it occurs. 3. Diffusion of anthroylstearate in dipalmitoylphosphatidylglycerol depends strongly on the subphase ionic strength and on the nature of cations: Na+, Mg2+, Ca2+.

Ganglioside structures and distribution: are they localized at the nerve ending?

Gangliosides generally provide a small portion of the complex carbohydrate content of cell surfaces. An exception is the central nervous system where they comprise up to 5--10% of the total lipid of some membranes. This tissue is unique in that the quantity of lipid-bound sialic acid exceeds that of the protein-bound fraction. Over 30 different molecular species have been characterized to date. These range in complexity from sialosylgalactosyl ceramide with 2 sugars to the pentasialoganglioside of fish brain with 9 carbohydrate units. Virtually all cellular and subcellular fractions of brain that have been carefully examined contain gangliosides to one degree or another, but the majority of brain ganglioside is located in the neurons. Their mode of distribution within the neuron has not been entirely clarified by subcellular studies. Calculations based on reported values for axon terminal density and synaptosomal ganglioside concentration in the rat reveal that nerve endings contribute less than 12% of total cerebral cortical ganglioside. It is concluded that the plasma membranes of neuronal processes contain most of the neuronal ganglioside. These and other considerations suggest the possibility that gangliosides may be distributed over the entire neuronal surface.

Spin label studies on rat liver and heart plasma membranes: effects of temperature, calcium, and lanthanum on membrane fluidity

The structures of rat liver and heart plasma membranes were studied with the 5-nitroxide stearic acid spin probe, I(12,3). The polarity-corrected order parameters (S) of liver and heart plasma membranes were independent of probe concentration only if experimentally determined low I(12,3)/lipid ratios were employed. At higher probe/lipid ratios, the order parameters of both membrane systems decreased with increasing probe concentration, and these effects were attributed to enhanced nitroxide radical interactions. Examination of the temperature dependence of approximate and polarity-corrected order parameters indicated that lipid phase separations occur in liver (between 19 degrees and 28 degrees C) and heart (between 21 degrees and 32 degrees C) plasma membranes. The possibility that a wide variety of membrane-associated functions may be influenced by these thermotropic phase separations is considered. Addition of 3.9 mM CaCl2 to I(12,3)-labeled liver plasma membrane decreased the fluidity as indicated by a 5% increase in S at 37 degrees C. Similarly, titrating I(12,3)-labeled heart plasma membranes with either CaCl2 or LaCl3 decreased the lipid fluidity at 37 degrees C, although the magnitude of the La3+ effect was larger and occurred at lower concentrations than that induced by Ca2+; addition of 0.2 mM La3+ or 3.2 mM Ca2+ increased S by approximately 7% and 5%, respectively. The above cation effects reflected only alterations in the membrane fluidity and were not due to changes in probe--probe interactions. Ca2+ and La3+ at these concentrations decrease the activities of such plasma membrane enzymes as Na+, K+-ATPase and adenylyl cyclase, and it is suggested that the inhibition of these enzymes may be due in part to cation-mediated decreases in the lipid fluidity.

Direct modification of the glycocalyx of a cultured muscle cell line by incorporation of foreign gangliosides and an integral membrane glycoprotein

As part of a program to better understand the cause-or-effect nature of the relationship between cell surface carbohydrate and cell properties and behaviour, experiments have been carried out on direct modification of the glycocalyx of cultured cells. Modification was by incorporation of gangliosides and an integral membrane glycoprotein chosen to be dissimilar to species occurring naturally in the cell line. Two methods of incorporation were investigated: simple addition of the new components to the culture medium for various times, or assembly of the components into the walls of lipid vesicles which were subsequently fused with cells. Gangliosides from beef brain and glycophorin, the major human erythrocyte sialoglycoprotein, were successfully added to the surface of myoblasts in quantities sufficient to represent a significant perturbation. Changes in cell adhesion, morphology, and viability were observed which seem to be a direct result of glycocalyx modification.

Factors restricting diffusion of water-soluble spin labels

Line broadening of spin label signals is treated in terms of concentration, viscosity, charge and temperature dependencies. Line broadening of spin label signals may be caused either by spin label interactions or by the interaction between a spin label and a second paramagnetic species. Line broadening has been related to collision frequency in the literature and is treated in that way here. Collision frequency is related to diffusion processes in a way that allows information to be obtained about the diffusion environment. Several potential spin label line-broadening agents are compared as to their effectiveness. Small polymer beads with graduated pore sizes are used to show that collisional broadening has a marked dependence on the long-range structure of the diffusion environment. Application of these results to biological diffusion processes is considered.

Fluorescence polarization studies of rat intestinal microvillus membranes

Rat intestinal microvillus membranes and lipid extracts prepared from them have been studied by fluorescence polarization with three lipid-soluble fluorophores: diphenylhexatriene, retinol, and anthroyl-stearate. The degree of fluorescence polarization of diphenylhexatriene, which provides an index of the "microviscosity" of the lipid regions of the membrane, is exceptionally high in microvillus membranes, the highest yet reported in normal biological membranes. Both the membrane proteins and lipids were found to contribute to the high values. With each of the three probes the polarization values are higher in ileal microvillus membranes as compared to membranes from proximal intestinal segments. Temperature-dependence studies of the fluorescence polarization of diphenylhexatriene and anthroylstearate demonstrate a phase transition in microvillus membranes and in liposomes prepared from their lipid extracts at approximately 26+/-2 degrees C. Ambient pH influences markedly the diphenylhexatriene fluorescence polarization in microvillus membranes but has little effect on that of human erythrocyte ghost membranes. The "microviscosity" of jejunal microvillus membranes is maximal at pH 6.5-7.0 and decreases as much as 50% at pH 3.0, an effect which depends largely upon the membrane proteins. Addition of calcium ions to suspensions of microvillus membranes increases the fluorescence polarization of retinol and anthroyl-stearate, but not that of diphenyl-hexatriene. This confirms the localization of the last compound to the hydrophobic interior of the membrane, relatively distant from the hydrophilic head groups of the polar lipids. Microvillus membrane proteins solubilized with Triton X-100 give relatively high fluorescence polarization and intensity values with retinol, suggesting the presence of binding proteins which could play a role in the normal absorptive mechanism for the vitamin.

Mobility of ribosomes bound to microsomal membranes. A freeze-etch and thin-section electron microscope study of the structure and fluidity of the rough endoplasmic reticulum

The lateral mobility of ribosomes bound to rough endoplasmic reticulum (RER) membranes was demonstrated under experimental conditions. High-salt-washed rough microsomes were treated with pancreatic ribonuclease (RNase) to cleave the mRNA of bound polyribosomes and allow the movement of individual bound ribosomesmfreeze-etch and thin-section electron microscopy demonstrated that, when rough microsomes were treated with RNase at 4 degrees C and then maintained at this temperature until fixation, the bound ribosomes retained their homogeneous distribution on the microsomal surface. However, when RNase-treated rough microsomes were brought to 24 degrees C, a temperature above the thermotropic phase transition of the microsomal phospholipids, bound ribosomes were no longer distributed homogeneously but, instead, formed large, tightly packed aggregates on the microsomal surface. Bound polyribosomes could also be aggregated by treating rough microsomes with antibodies raised against large ribosomal subunit proteins. In these experiments, extensive cross-linking of ribosomes from adjacent microsomes also occurred, and large ribosome-free membrane areas were produced. Sedimentation analysis in sucrose density gradients demonstrated that the RNase treatment did not release bound ribosomes from the membranes; however, the aggregated ribosomes remain capable of peptide bond synthesis and were released by puromycin. It is proposed that the formation of ribosomal aggregates on the microsomal surface results from the lateral displacement of ribosomes along with their attached binding sites, nascent polypeptide chains, and other associated membrane proteins; The inhibition of ribosome mobility after maintaining rough microsomes at 4 degrees C after RNase, or antibody, treatment suggests that the ribosome binding sites are integral membrane proteins and that their mobility is controlled by the fluidity of the RER membrane. Examination of the hydrophobic interior of microsomal membranes by the freeze-fracture technique revealed the presence of homogeneously distributed 105-A intramembrane particles in control rough microsomes. However, aggregation of ribosomes by RNase, or their removal by treatment with puromycin, led to a redistribution of the particles into large aggregates on the cytoplasmic fracture face, leaving large particle-free regions.

Spin-label studies on rat liver and heart plasma membranes: do probe-probe interactions interfere with the measurement of membrane properties?

The structures of purified rat liver and heart plasma membranes were studied with the 5-nitroxide stearic acid spin probe, I(12,3). ESR spectra were recorded with a 50 gauss field sweep, and also with a new technique which "expands" the spectrum by (1) recording pairs of adjoining peaks with a smaller field sweep and (2) superposing the common peaks. The hyperfine splittings measured from the "expanded" spectra were significantly more precise than those obtained from the "unexpanded" spectra. Both procedures were used to study the effects of various I(12,3) probe concentrations on the spectra of liver and heart membranes, as well as the effects of temperature and CaCl2 additions on the spectra of liver membranes, and revealed the following: The polarity-corrected order parameters of liver (31 degrees) and heart (22 degrees) membranes were found to be independent of the probe concentration, if experimentally-determined low I(12,3)/lipid ratios were employed. The absence of obvious radical-interaction broadening in the unexpanded spectra indicated that "intrinsic" membrane properties may be measured at these low probe/lipid ratios. Here, "intrinsic" properties are defined as those which are measured when probe-probe interactions are negligible, and do not refer to membrane behavior in the absence of a perturbing spin label. At higher I(12,3)/lipid ratios, the order parameters of liver and heart membranes were found to substantially decrease with increasing probe concentration. The increase in the "apparent" fluidity of both membrane systems is attributed to enhanced radical interactions; however, an examination of these spectra (without reference to "low" probe concentration spectra) might incorrectly suggest that radical interactions were absent. For the membrane concentrations employed in these studies, the presence of "liquid-lines" (or "fluid components") in the unexpanded ESR spectra was a convenient marker of high probe concentrations. A thermotropic phase separation was observed in liver membranes between 19 degrees and 28 degrees. Addition of CaCl2 to liver plasma membrane [labelled with "low" I(12,3) concentrations] increased the rigidity of the membrane at 31 degrees and 37 degrees, without inducing a segregation of the probe in the bilayer. Previously reported data are discussed in relation to these results, and suggested minimal criteria for performing membrane spin label studies are included.

Amphiphile diffusion in model membrane systems studied by pulsed NMR

The translational diffusion of the amphiphilic molecules in a number of lyotropic liquid crystalline phases has been measured with the pulsed NMR pulsed magnetic field gradient method. The amphiphiles studied were soaps, monoglycerids and lecithins. Measurements were performed both for oriented lamellar and for cubic phases. The order of magnitude of the diffusion coefficients was found to be the same as in neat liquids of analogous compounds. It was also found that the difussion coefficient depend markedly on the amphiphile end group in a way that parallels the area per polar head group as determined in X-ray studies. When corrections for geometrical factors has been made the diffusion rate is approximately equal in cubic and lamellar phases containing the same amphiphile.

Biophysics with nitroxyl radicals

The present status of the spin labeling method as applied to Biophysics is examined. After an outline of the chemical and physical properties of NO radicals, the analysis of linear and non-linear ESR spectra of spin labels and the information it yields is described. The possibilities of the method are critically discussed in the light of recent experiments.

Irreversible activation of adenylate cyclase of toad erythrocyte plasma membrane by 5'-guanylylimidodiphosphate

The irreversible activation of adenylate-cyclase by 5'guanylylimidodiphosphate, a phosphoramidate analog of 5'GTP, has been examined in toad (Bufus marinus) plasma membranes using the technique of preincubating the membranes with the nucleotide under various controlled conditions followed by washing and subsequent assay of enzyme activity. Activation of adenylate cyclase by Gpp(NH)p, but not GTP, is essentially permanent and persists following extensive washing, prolonged incubation at 30 degrees C in the absence of the nucleotide, and after dissolution of the membranes with Lubrol PX. (-)-Isoproterenol increases the activation observed with maximal concentrations of Gpp(NH)p from eight- to 10-fold (in the absence of hormone) to 50- to 100-fold; final activities as high as 10-15 nmoles of cyclic AMP per min per mg protein are achieved. The activated state obtained with isoproterenol and Gpp(NH)p is also permanent and is not inhibited by propranolol. The synergism between Gpp(NH)p and hormone requires the simultaneous presence of these compounds, and the time-dependent enhancement of activation with (-)-isoproterenol may be interrupted by addition of propranolol. The stimulation is slow, and may proceed for as long as 45 min at 30 degrees C in the presence of maximal concentrations of Gpp(NH)p and (-)-isoproterenol. Very little activation occurs at 0 degrees C. The time course of activation at 30 degrees C exhibits an accelerating phase lasting from 5 to 30 min when Gpp(NH)p is added directly during assay of cyclase activity or when the membranes are preincubated for various times and washed prior to assay for a fixed time. The lag period occurs in the presence and absence of (-)-isoproterenol, although the rate of increase in velocity is greater with hormone. The length of the accelerating phase decreases with increasing concentrations of Gpp(NH)p, although it is still evident with maximal levels of Gpp(NH)p and hormone. However, prewarming the membranes at 30 degrees C for 10 min in the absence of Gpp(NH)p or (-)-isoproterenol results in an immediate onset of linear activation at a rate which is achieved in untreated membranes only after about 10 min. The events occurring during prewarming at 30 degrees C are readily reversible since chilling the warmed membranes to 0 degrees C results in a time course of activation identical to that of membranes maintained at 0 degrees C until addition of Gpp(NH)p. Activation is proportional to the concentration of Gpp(NH)p within the range of 10(-8) to 10(-4) mM. The apparent affinity for Gpp(NH)p increases with increasing time of incubation. The primary effect of increasing the concentration of Gpp(NH)p is to decrease the time required to obtain a maximal rate of activation. The possible relevance of these findings to the mechanism of action of Gpp(NH)p, adenylate cyclase and hormones is discussed within the context of current views of biological membranes which recognize the lateral mobility of membrane molecules.

A correlation between membrane fluidity and the critical temperature for cell adhesion

BHK 21 cells can adhere to a protein-coated plastic dish in the presence of Ca2+ at temperatures above 12 degrees C. However, they cannot adhere below 8 degrees C. The ESR spectrum of cells spin-labeled with a stearic acid label indicated that the membrane fluidity changed characteristically at 10 degrees C, 20 degrees C, and 30 degrees C. The critical temperature for cell adhesion coincided well with one of the characteristic temperatures for the membrane fluidity change. In the case of adhesion in the presence of Mg2+, no such correlation was observed.

Lecithin translational diffusion studied by pulsed nuclear magnetic resonance

The translational diffusion coefficient of egg yolk and dilauroyl lecithin in optically isotropic phases containing sodium cholate has been measured using the pulsed NMR magnetic field gradient method. After a correction for geometrical factors the measured diffusion coefficient is found to agree well with previous determinations in phospholipid systems. The experimental data imply that the cubic mesophase of the lecithin-sodium cholate-water system contains continuous lipid aggregates. A possible model of the arrangement of the different amphiphile molecules in the cubic phase is discussed.

Transbilayer phospholipid asymmetry and its maintenance in the membrane of influenza virus

Two phospholipid exchange proteins and two phospholipases C have been employed to determine the phospholipid composition of the outer surface of the membrane of influenza virus. These four protein probes have defined the same accessible and inaccessible pool for each viral phospholipid. Phospholipids which are exchangeable or hydrolyzable are located on the outer surface, whereas the inaccessible pool is located at the inner surface of the viral bilayer. The two pools are unequal in size, with ca. 30% of the total phospholipid accessible to the four proteins, and ca. 70% inaccessible. The membrane is thus highly asymmetric with regard to the amount of phospholipid on each side of the membrane. There is also a marked asymmetry of phospholipid composition. Phosphatidylcholine and phosphatidylinositol are enriched in the outer surface, and sphingomyelim is enriched in the inner surface, whereas phosphatidylethanolamine and phosphatidylserine are present in similar proportions in each surface. This distribution is qualitatively different from that previously reported for the human erythrocyte. The close agreement between results obtained with excahnge proteins and phospholipases C demonstrates that the hydrolytic action of these enzymes does not alter phospholipid asymmetry. The nonperturbing nature of the exchange proteins has permitted the rate of transmembrane movement of phospholipids (flip-flop) in the intact virion to be studied. This process could not be detected after 2 days at 37 degrees C. It was estimated that the half-time for flip-flop is indeterminately in excess of 30 days for sphingomyelin and 10 days for phosphatidylcholine at 37 degrees C. These extremely long times provide a simple explanation for the maintenance of transbilayer asymmetry in influenza virions and possibly, other membranes. Since the viral membrane is acquired by budding through the host cell plasma membrane, the transbilayer distribution of phospholipids observed in the virions presumably reflects a similar asymmetric distribution of phospholipids in the host cell surface membrane. Because animal cells in culture do not incorporate extracellular phospholipid, our results demonstrate that individual cells have the capacity to generate asymmetric membranes.

Adsorption of glyceraldehyde 3-phosphate dehydrogenase on condensed monolayers of phospholipid

The adsorption of [14C] alkylated glyceraldehyde 3-phosphate dehydrogenase from rabbit muscle to condensed monolayers of phosphatidic acid was investigated under a variety of conditions. 2. The rate constant for association at 20 degrees C depended on ionic strength. At I/2=60mM the rate constant was 0.39min-1. At I/2=260mM it decreased to 0.27min-1. 3. The apparent association constant (Kass.) for adsorption at I/2=60mM was 1.06 X 10(6)M-1 and was strongly influenced by subphase changes in pH and ionic strength. Measurements of Kass. at 20 degrees and 5 degrees C gave a value for the apparent enthalpy change on adsorption of -33kJ-mol-1. Calculations of the apparent change in free energy and apparent entropy change for the adsorption process gave values of -34kJ-mol-1 and +2J-K-1-mol-1 respectively. 4. Decreasing the amount of phosphatidic acid in the monolayer by replacement with phosphatidylcholine caused the shape of the adsorption isotherm to change from apparent hyperbolic to sigmoid. Subphase changes in pH or ionic strength did not affect the shape of the adsorption isotherm. However, adsorption of enzyme on monolayers of 100% phosphatidic acid in the presence of 1mM-CaCl2 was sigmoid in nature. 5. It is concluded that glyceraldehyde 3-phosphate dehydrogenase binds to condensed charged monolayers by multiple electrostatic interactions. At low concentrations of phosphatidic acid in the monolayer or in the presence of Ca2+, this occurs in a two-step process and depends on lateral diffusion of phosphatidic acid for strong binding to take place.

Distribution of transplantation antigens on cell surfaces

Significant advances have been made over the past few years in elucidating the genetics, the chemical composition, and, more recently, the in situ relation of the major histocompatibility antigens of the mouse and man. Attempts to map the arrangement of individual antigens on the surface of cells have revealed that some antigens specified by a given subregion of both the H-2 and HL-A systems are in close proximity on the cell membrane and that attachment of antibody to one site to a certain degree blocks or inhibits the binding of antibody to the adjacent site. Allelic antigens in the H-2 system tend to inhibit binding. H-2D and H-2K antigens show either inhibition or noninteraction, possibly reflecting a cis-trans effect. Unlike with the H-2, inhibition of binding occurs only between HL-A antigens specified by homologous chromosomes. Also, a number of instances have been noted where inhibition of binding is unidirectional, possibly reflecting a polymeric nature of antigen or stratification of moieties at cell surface. Inhibition of antibody attachment between several alloantigenic systems on thymocytes in mice and also a variation in the mobility of the histocompatibility antigens suggest that the moieties bearing histocompatibility antigens are comprised of several gene products. Further work is needed to establish the validity of this assumption and to fully define the composition of these units. Ample data have been obtained from both biological and biophysical experiments to support the suggestion that single or multiple complexes of glycoproteins can move in the plane of the membrane. Although the composition of these cell membrane components remains a question, direct visualization by fluorescence and electron microscopy indicates that these moieties are small and, under natural conditions, distributed uniformly over the cell surface. Direct and indirect labeling techniques have shown that the complexes have no fixed position in the cell membrane and can be displaced laterally in the plane of the membrane without affecting the distribution of other surface molecules, such as sIg and species-specific antigens. Additional evidence suggests that H-2D and H-2K antigen complexes, as well as their gene products specified by different parent chromosomes, may be displaced separately. These observations are especially interesting and must be reconciled and data obtained by proximity analysis which indicate an association of some allelic products and possibly certain combinations of D and K antigens. Whether the differences noted in reactivity of the various surface antigens following attachment of antibodies are attributable to difference in size or to differences in the manner of their intercalation in the cell membrane remains to be elucidated. The rapid advances in elucidation of the molecular structure of biological membranes suggest that experimental work should be done on the biophysics of the structure of the antigenic sites and the mechanism of migration...

Comparison of membrane organization in mitochondria from yeast and rat liver by nuclear magnetic resonance spectroscopy

Proton magnetic resonance (PMR) and carbon-13 magnetic resonance (CMR) spectra of intact, unsonicated yeast and rat liver motochondria show differences which may be correlated with the composition of the membranes. High resolution PMR and CMR signals in intact yeast mitochondria have been assigned to regions of fluid lipid-lipid interaction on the basis of spectra of extracted lipid and protein, and the temperature dependence of NMR signals from the intact membrane. PMR spectra suggest that about 20% of total yeast phospholipid is in regions where both intramolecular fatty acid chain mobility and lateral diffusion of entire phospholipid molecules are possible. No such regions apear to exist in rat liver mitochondria. For both yeast and rat liver mitochondria, comparison of PMR and CMR spectra suggests that about 50% of phospholipid appears to be in regions where intramolecular fatty acid chain motion is considerable, but lateral diffusion is restricted. The remaining phospholipid appears to have little inter- or intramolecular mobility. Since NMR observation of lipid extracts from membranes indicates that phospholipid-sterol interactions do not account for the spectra of intact mitochondria, these effects are interpreted in terms of extensive lipid-protein interactions.

The application of fluorescent probes in membrane studies

Fluorescence has been used in biochemical studies for many years but only recently has the information content and the practical applicability of the fluorescence method been fully realized.

Following the early studies of Newton (1954) and Weber (11954) and after the initial utilization of fluorescent probes by Chance and coworkers (Azziet al.1969) and Tasakiet al.(1968), in the study of membranes, the use of fluorescence to provide structural information at microscopic or molecular levels in biological membranes has become widespread. widespread. The application of the fluorescence technique to biological systems has progressed parallel to the development of a theoretical basis for fluorescence data interpretation and the synthesis of a large number of fluorescent probes, organic molecules having fluorescence characteristics that are dependent on their environment.

Membrane structure and surface coat of Entamoeba histolytica. Topochemistry and dynamics of the cell surface: cap formation and microexudate

Treatment of living entamoeba histolytica cells with low concentrations of concanavalin A (con A) and peroxidase results in redistribution of the plasma membrane con A receptors to one pole of the cell where a morphologically distinct region--the uroid--is formed. Capping of con A receptors is not accompanied by parallel accumulation of ruthenium red-stainable components. In capped cells, the pattern of distribution of acidic sites ionized at pH 1.8 (labeled by colloidal iron) at the outer surface and of membrane particles (integral membrane components revealed by freeze-fracture) is not altered over the uroid region. Cytochemistry of substrate-attached microexudate located in regions adjacent to E. histolytica cells demonstrates the presence of con A binding sites and ruthenium red- and alcian blue-stainable components and the absent of colloidal iron binding sites. In a previous report we demonstrated that glycerol-induced aggregation of the plasma membrane particles is accompanied by a discontinuous distribution of colloidal iron binding sites, while con A receptors and acidic sites ionized at pH 4.0 remain uniformly distributed over the cell surface. Taken together, our experiments show that, in E. histolytica cells, peripheral membrane components may move independently of integral components and, also, that certain surface determinants may redistribute independently of others. These results point to the complexity of the membrane structure-cell surface relationship in E. histolytica plasma membranes relative to the membrane of the erythrocyte ghost where integral components (the membrane-intercalated particles) contain all antigens, receptors, and anionic sites labeled so far. We conclude that fluidity of integral membrane components (integral membrane fluidity) cannot be inferred from the demonstration of the mobility of surface components nor, conversely, can the fluidity of peripheral membrane components (peripheral membrane fluidity) be assumed from demonstration of the mobility of integral membrane components.

Studies on the biogenesis of smooth endoplasmic reticulum membranes in hepatocytes of phenobarbital-treated rats. II. The site of phospholipid synthesis in the initial phase of membrane proliferation

The specific activity of the acyltransferases of smooth microsomes of rat liver rose threefold by 12 h after injection of phenobarbital, while the activity of the acyltransferases of the rough microsomes rose slightly to peak at 3-4 h, and subsequently fell. The latter rise was abolished by treatment of the animal with actinomycin D or puromycin, while that of the smooth microsomes was unaffected. Incorporation of [(14)C]glycerol into phospholipid of smooth microsomes was elevated 100% by phenobarbital, while that of the rough microsomes was elevated 15%, and this could be accounted for by exchange between the microsomal phospholipids. The phospholipid/protein ratio of the smooth microsomes rose 1.5 times 3-4 h after injection of phenobarbital, while that of the rough microsomes fell slightly. The specific activity of NADPH cytochrome c reductase and NADPH diaphorase rose first in the rough microsomes, and subsequently in the smooth microsomes at a time coinciding with the return of the phospholipid/protein ratio to the control level. The rise in phospholipid/protein ratio was unaffected by actinomycin D or puromycin. These results indicate that the proliferating smooth membranes are the site of phospholipid synthesis, and that the phospholipid/protein ratio of these membranes may change independently.

Lateral diffusion of visual pigment in photorecptor disk membranes

Visual pigment molecules are found to move transversely, but not longitudinally, in both rod and cone outer segments of mud puppy and frog. This is consistent with the idea that they are immersed in a two-dimensional fluid disk membrane. The diffusion coefficient for the motion is about 5 x 10(-9) square centimeters per second at 20 degrees C, corresponding to a root-mean-square molecular displacement of 0.3 micrometer in 1 second.

Mobility and the restriction of mobility of plasma membrane lectin-binding components

Labeling by ferritin-conjugated agglutinins from Ricinus communis was used to demonstrate the relative mobilities of the agglutinin receptors located in specific regions on plasma membranes of rabbit spermatozoa. The relative mobility of lectin receptors was higher on postacrosomal regions of sperm than on acrosomal and tail regions. Lectin-induced clustering could not be demonstrated in the acrosomal and tail regions, an indication of the existence of localized restraints on the mobilities of lectin receptors. A system of transmembrane restraints may maintain the segregation of plasma membrane components into membrane domains on certain highly differentiated cells.