Rapid lateral diffusion of phospholipids in rabbit sarcoplasmic reticulum

Diffusive transport of nanoscale objects through cell membranes: a computational perspective

Diffusion is an essential and fundamental means of transport of substances on cell membranes, and the dynamics of biomembranes plays a crucial role in the regulation of numerous cellular processes. The understanding of the complex mechanisms and the nature of particle diffusion have a bearing on establishing guidelines for the design of efficient transport materials and unique therapeutic approaches. Herein, this review article highlights the most recent advances in investigating diffusion dynamics of nanoscale objects on biological membranes, focusing on the approaches of tailored computer simulations and theoretical analysis. Due to the presence of the complicated and heterogeneous environment on native cell membranes, the diffusive transport behaviors of nanoparticles exhibit unique and variable characteristics. The general aspects and basic theories of normal diffusion and anomalous diffusion have been introduced. In addition, the influence of a series of external and internal factors on the diffusion behaviors is discussed, including particle size, membrane curvature, particle-membrane interactions or particle-inclusion, and the crowding degree of membranes. Finally, we seek to identify open problems in the existing experimental, simulation, and theoretical research studies, and to propose challenges for future development.

Tunable dynamic hydrophobic attachment of guest molecules in amphiphilic core–shell polymers

In this study, synthesis and dynamic properties of amphiphilic core–shell polymers are reported as monitored through their interaction with small amphiphilic ligands. Our findings point to a most complex self-assembling nature of those host and guest molecules.

An update on post-ejaculatory remodeling of the sperm surface before mammalian fertilization

The fusion of a sperm with an oocyte to form new life is a highly regulated event. The activation-also termed capacitation-of the sperm cell is one of the key preparative steps required for this process. Ejaculated sperm has to make a journey through the female uterus and oviduct before it can approach the oocyte. The oocyte at that moment also has become prepared to facilitate monospermic fertilization and block immediately thereafter the chance for polyspermic fertilization. Interestingly, ejaculated sperm is not properly capacitated and consequently is not yet able to fertilize the oocyte. During the capacitation process, the formation of competent lipid-protein domains on the sperm head enables sperm-cumulus and zona pellucida interactions. This sperm binding allows the onset for a cascade reaction ultimately resulting in oocyte-sperm fusion. Many different lipids and proteins from the sperm surface are involved in this process. Sperm surface processing already starts when sperm are liberated from the seminiferous tubules and is followed by epididymal maturation where the sperm cell surface is modified and loaded with proteins to ensure it is prepared for its fertilization task. Although cauda epididymal sperm can fertilize the oocyte IVF, they are coated with so-called decapacitation factors during ejaculation. The seminal plasma-induced stabilization of the sperm surface permits the sperm transit through the cervix and uterus but prevents sperm capacitation and thus inhibits fertilization. For IVF purposes, sperm are washed out of seminal plasma and activated to get rid of decapacitation factors. Only after capacitation, the sperm can fertilize the oocyte. In recent years, IVF has become a widely used tool to achieve successful fertilization in both the veterinary field and human medicine. Although IVF procedures are very successful, scientific knowledge is still far from complete when identifying all the molecular players and processes during the first stages the fusion of two gametes into a new life. A concise overview in the current understanding of the process of capacitation and the sperm surface changes is provided. The gaps in knowledge of these prefertilization processes are critically discussed.

Fluorescence correlation spectroscopy measurements of the membrane protein TetA in Escherichia coli suggest rapid diffusion at short length scales

Structural inhomogeneities in biomembranes can lead to complex diffusive behavior of membrane proteins that depend on the length or time scales that are probed. This effect is well studied in eukaryotic cells, but has been explored only recently in bacteria. Here we used fluorescence recovery after photobleaching (FRAP) and fluorescence correlation spectroscopy (FCS) to study diffusion of the membrane protein TetA-YFP in E. coli. We find that the diffusion constant determined from FRAP is comparable to other reports of inner membrane protein diffusion constants in E. coli. However, FCS, which probes diffusion on shorter length scales, gives a value that is almost two orders of magnitude higher and is comparable to lipid diffusion constants. These results suggest there is a population of TetA-YFP molecules in the membrane that move rapidly over short length scales (~ 400 nm) but move significantly more slowly over the longer length scales probed by FRAP.

Unrestricted diffusion of exogenous and endogenous PIP(2 )in baby hamster kidney and Chinese hamster ovary cell plasmalemma

We used two approaches to characterize the lateral mobility of phosphatidylinositol 4,5-bisphosphate (PIP(2)) in the plasmalemma of baby hamster kidney and Chinese hamster ovary fibroblasts. First, nitrobenzoxadiazole-labeled C6-phosphatidylcholine and C16-PIP(2) were incorporated into plasma membrane "lawns" ( approximately 20 x 30 microm) from these cells and into the outer monolayer of intact cells. Diffusion coefficients determined by fluorescence recovery after photobleaching were similar for the two lipids and were higher in lawns, approximately 0.3 microm(2)/s, than on the cell surface, approximately 0.1 microm(2)/s. For membrane lawns, the fractional recoveries (75-90%) were close to those expected from the fraction of total membrane bleached, and labeling by the probes was several times greater than for intact cells. Second, we analyzed cells expressing M1 muscarinic receptors and green fluorescent protein fused with PIP(2)-binding pleckstrin-homology domains, Tubby domains or diacylglycerol (DAG)-binding C1 domains. On-cell gigaseal patches were formed with pipette tips >5 microm in diameter. When the agonist carbachol (0.3 mM: ) was applied either within or outside of the pipette, lipid signals crossed the pipette barrier rapidly in both directions and membrane blebbing occurred on both membrane sides. Accurate simulations of lipid gradients required diffusion coefficients >1 microm(2)/s. Exogenous DAG also crossed the pipette barrier rapidly. In summary, we found no evidence for restricted diffusion of signaling lipids in these cells. The lower mobility and incorporation of phospholipid at the extracellular leaflet may reflect a more ordered and condensed extracellular monolayer, as expected from previous studies.

All-trans retinol in rod photoreceptor outer segments moves unrestrictedly by passive diffusion

The visual pigment protein of vertebrate rod photoreceptors, rhodopsin, contains an 11-cis retinyl moiety that is isomerized to all-trans upon light absorption. Subsequently, all-trans retinal is released from the protein and reduced to all-trans retinol, the first step in the recycling of rhodopsin's chromophore group through the series of reactions that constitute the visual cycle. The concentration of all-trans retinol in photoreceptor outer segments can be monitored from its fluorescence. We have used two-photon excitation (720 nm) of retinol fluorescence and fluorescence recovery after photobleaching to characterize the mobility of all-trans retinol in frog photoreceptor outer segments. Retinol produced after rhodopsin bleaching moved laterally in the disk membrane bilayer with an apparent diffusion coefficient of 2.5 +/- 0.3 micro m(2) s(-1). The diffusion coefficient of exogenously added retinol was 3.2 +/- 0.5 micro m(2) s(-1). These diffusion coefficients are in close agreement with those reported for lipids, suggesting that retinol is not tightly bound to protein sites that would be diffusing much more slowly in the plane of the membrane. In agreement with this interpretation, a fluorescent-labeled C-16 fatty acid diffused laterally with a similar diffusion coefficient, 2.2 +/- 0.2 micro m(2) s(-1). Retinol also moved along the length of the rod outer segment, with an apparent diffusion coefficient of 0.07 +/- 0.01 micro m(2) s(-1), again suggesting that it is not tightly bound to proteins that would confine it to the disks. The axial diffusion coefficient of exogenously added retinol was 0.05 +/- 0.01 micro m(2) s(-1). In agreement with passive diffusion, the rate of axial movement was inversely proportional to the square of the length of the rod outer segment. Diffusion of retinol on the plasma membrane of the outer segment can readily account for the measured value of the axial diffusion coefficient, as the plasma membrane comprises approximately 1% of the total outer-segment membrane. The values of both the lateral and axial diffusion coefficients are consistent with most of the all-trans retinol in the outer segments moving unrestricted and not being bound to carrier proteins. Therefore, and in contrast to other steps of the visual cycle, there does not appear to be any specialized processing for all-trans retinol within the rod outer segment.

Mobility of proteins associated with the plasma membrane by interaction with inositol lipids

Translocation of a protein to the plasma membrane in response to the generation of polyphosphoinositol lipids is believed to be an important component of cellular regulation, in part because it increases the effective concentration of that protein relative to other proteins in the same membrane by restricting it to a two-dimensional space. However, such a concept assumes that, once translocated, a protein retains the free mobility it had in the cytoplasm, and also that the possible existence of partitioned pools of inositol lipids does not restrict its sphere of influence. We have explored by fluorescence recovery after photobleaching (FRAP) the mobility of four green-fluorescent-protein-tagged proteins, GAP1(IP4BP) and GAP1(m), when they are either cytoplasmic or attached to the plasma membrane, and the PH domain of PI-PLCdelta(1) and ICAM as representative of, respectively, another inositol-lipid-anchored protein and a single-transmembrane-span-domain protein. The data from GAP1(m) and the PI-PLCdelta(1) PH domain show that, when proteins associate with inositol lipids in the plasma membrane, they retain a mobility similar to that in the cytoplasm, and probably also similar to the inositol lipid to which they are attached, suggesting a free diffusion within the plane of the membrane. Moreover, this free diffusion is similar whether they are bound to PtdIns(3,4,5)P(3) or to PtdIns(4,5)P(2), and no evidence was found by these criteria for restricted pools of PtdIns(4,5)P(2). The mobility of GAP1(IP4BP), which has been reported to associate with PtdIns(4,5)P(2) in the plasma membrane, is much lower, suggesting that it might interact with other cellular components. Moreover, the mobility of GAP1(IP4BP) is not detectably altered by the generation of either of its two potential regulators, Ins(1,3,4,5)P(4) or PtdIns(3,4,5)P(3).

Determination of Site−Site Distance and Site Concentration within Polymer Beads: A Combined Swelling-Electron Paramagnetic Resonance Study

This work proposes a combined swelling-electron paramagnetic resonance (EPR) approach aiming at determining some unusual polymer solvation parameters relevant for chemical processes occurring inside beads. Batches of benzhydrylamine-resin (BHAR), a copolymer of styrene-1% divinylbenzene containing phenylmethylamine groups were, labeled with the paramagnetic amino acid 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amine-4-carboxylic acid (TOAC), and their swelling properties and EPR spectra were examined in DCM and DMF. By taking into account the BHARs labeling degrees, the corresponding swelling values, and some polymer structural characteristics, it was possible to calculate polymer swelling parameters, among them, the volume and the number of sites per bead, site-site distances and site concentration. The latter values ranged from 17 to 170 A and from 0.4 to 550 mM, respectively. EPR spectroscopy was applied to validate the multistep calculation strategy of these swelling parameters. Spin-spin interaction was detected in the labeled resins at site-site distances less than approximately 60 A or probe concentrations higher than approximately 1 x 10(-2) M, in close agreement with the values obtained for the spin probe free in solution. Complementarily, the yield of coupling reactions in different resins indicated that the greater the inter-site distance or the lower the site concentration, the faster the reaction. The results suggested that the model and the experimental measurements developed for the determination of solvation parameters represent a relevant step forward for the deeper understanding and improvement of polymer-related processes.

Dynamic behavior of rod photoreceptor disks

Eukaryotic cells use membrane organelles, like the endoplasmic reticulum or the Golgi, to carry out different functions. Vertebrate rod photoreceptors use hundreds of membrane sacs (the disks) for the detection of light. We have used fluorescent tracers and single cell imaging to study the properties of rod photoreceptor disks. Labeling of intact rod photoreceptors with membrane markers and polar tracers revealed communication between intradiskal and extracellular space. Internalized tracers moved along the length of the rod outer segment, indicating communication between the disks as well. This communication involved the exchange of both membrane and aqueous phase and had a time constant in the order of minutes. The communication pathway uses approximately 2% of the available membrane disk area and does not allow the passage of molecules larger than 10 kDa. It was possible to load the intradiskal space with fluorescent Ca(2+) and pH dyes, which reported an intradiskal Ca(2+) concentration in the order of 1 microM and an acidic pH 6.5, both of them significantly different than intracellular and extracellular Ca(2+) concentrations and pH. The results suggest that the rod photoreceptor disks are not discrete, passive sacs but rather comprise an active cellular organelle. The communication between disks may be important for membrane remodeling as well as for providing access to the intradiskal space of the whole outer segment.

Water transport across biological membranes

The rate of the lateral diffusion of straight-chain phospholipids predicts the rate of water diffusion through bilayers. A new model of lipid dynamics integrates these processes. Substances such as cholesterol that reduce water diffusion proportionally reduce lateral diffusion. The model yields a number of predictions about the dynamics of the lipids at the Tm and suggests different mechanisms for how water diffuses across bilayers of other-than-straight-chain lipids, and how proteins bind to membranes. A second recent development in water transport across biological membranes is the discovery of a ubiquitous family of water transport proteins that facilitate large-volume water translocation. Like water diffusion through lipid bilayers, water transport by these proteins is directed by osmosis and is therefore under the control of ATP and ion pumps. The presence of water transport proteins in membranes is often regulated by hormones.

Two-dimensional diffusion of amphiphiles in phospholipid monolayers at the air-water interface

Steady-state and time-resolved fluorescence spectroscopy has been used to examine lateral diffusion in dipalmitoyl-L-alpha-phosphatidylcholine (DPPC) and dimyristoyl-L-alpha-phosphatidylcholine (DMPC) monolayers at the air-water interface, by studying the fluorescence quenching of a pyrene-labeled phospholipid (pyrene-DPPE) by two amphiphilic quenchers. Steady-state fluorescence measurements revealed pyrene-DPPE to be homogeneously distributed in the DMPC lipid matrix for all measured surface pressures and only in the liquid-expanded (LE) phase of the DPPC monolayer. Time-resolved fluorescence decays for pyrene-DPPE in DMPC and DPPC (LE phase) in the absence of quencher were best described by a single-exponential function, also suggesting a homogeneous distribution of pyrene-DPPE within the monolayer films. Addition of quencher to the monolayer film produced nonexponential decay behavior, which is adequately described by the continuum theory of diffusion-controlled quenching in a two-dimensional environment. Steady-state fluorescence measurements yielded lateral diffusion coefficients significantly larger than those obtained from time-resolved data. The difference in these values was ascribed to the influence of static quenching in the case of the steady-state measurements. The lateral diffusion coefficients obtained in the DMPC monolayers were found to decrease with increasing surface pressure, reflecting a decrease in monolayer fluidity with compression.

Dynamics and aggregation of the peptide ion channel alamethicin. Measurements using spin-labeled peptides

Two spin-labeled derivatives of the ion conductive peptide alamethicin were synthesized and used to examine its binding and state of aggregation. One derivative was spin labeled at the C-terminus and the other, a leucine analogue, was spin labeled at the N-terminus. In methanol, both the C and N terminal labeled peptides were monomeric. In aqueous solution, the C-terminal derivative was monomeric at low concentrations, but aggregated at higher concentrations with a critical concentration of 23 microM. In the membrane, the C-terminal label was localized to the membrane-aqueous interface using 13C-NMR, and could assume more than one orientation. The membrane binding of the C-terminal derivative was examined using EPR, and it exhibited a cooperativity seen previously for native alamethicin. However, this cooperativity was not the result of an aggregation of the peptide in the membrane. When the spectra of either the C or N-terminal labeled peptide were examined over a wide range of membrane lipid to peptide ratios, no evidence for aggregation could be found and the peptides remained monomeric under all conditions examined. Because electrical measurements on this peptide provide strong evidence for an ion-conductive aggregate, the ion-conductive form of alamethicin likely represents a minor fraction of the total membrane bound peptide.

Lateral diffusion of phospholipids in the lipid surface of human low-density lipoprotein measured with a pyrenyl phospholipid probe

Human low-density lipoprotein (LDL) was labelled with the excimeric fluorescent phospholipid analogue 1-palmitoyl-2-(1'-pyreneoctanoyl)-sn-glycero-3-phosphocholine by using phosphatidylcholine-specific transfer protein for the probe insertion. The lateral diffusivity of the probe in the phospholipid/cholesterol surface monolayer of LDL was determined from the measured dependence of the pyrene monomer fluorescence yield on probe concentration. The data were analyzed by the milling-crowd model (J. Eisinger et al. (1986) Biophys. J. 49, 987-1001] to obtain the short-range lateral diffusivity of the probe. The lateral mobility of the probe in LDL was compared to that in model lipid systems, i.e. in protein-free LDL-like lipid particles and in small unilamellar vesicles, with a phospholipid/cholesterol composition characteristic of LDL. This analysis with the probability PE = 1 for excimer production between nearest-neighbour probes gives the lower limits for f, the frequency of translational lipid--lipid exchanges of the probe of 0.62 x 10(8), 0.19 x 10(8) and 0.19 x 10(8)s-1 in LDL, LDL-like lipid particles, and small unilamellar vesicles, respectively. The lower limits for the corresponding lateral diffusion constants are 16, 5 and 5 microns 2 s-1. The results suggest that the translational mobility of phospholipid molecules in the lipid--protein surface of LDL is not constrained by the apolipoprotein B-100 moiety or the neutral lipid core of the lipoprotein. Instead, the protein moiety may perturb the lipid order with the lipid--associating peptide domains and thus fluidize the amphiphilic surface monolayer of LDL relative to the protein-free model systems. In general, lateral diffusivity of the pyrenyl phospholipid probe in LDL and the model lipid systems is comparable to the lateral mobility of lipid analogue probes in a variety of model and biological membranes.

Fatty-acid spin probe interactions with erythrocyte ghosts and liposomes prepared from erythrocyte ghosts

A model for the binding of 5-nitroxide stearate, I(12.3), to human erythrocyte ghosts was developed by comparing spin probe interactions with ghosts and liposomes prepared from ghosts. At low probe/lipid (P/L less than 1/2500), I(12.3) binds to a similar class of high-affinity, noninteracting sites in both ghosts and liposomes, indicating that lipid moieties are responsible for probe uptake. Saturation occurs in both systems with increasing P/L, and, at higher loading (e.g., P/L = 1/360 for ghosts and liposomes), the probe inserts itself at initially dilute sites to form a class of low-affinity sites consisting of clusters of variable size. At still higher P/L ranges (greater than 1/100), much increased probe uptake was observed in ghosts than in liposomes, which was attributed to another class of low-affinity sites, representing nonspecific interactions of I(12.3) with membrane proteins. The nature of the spectral components and ultrafiltration experiments with ghosts labeled at high P/L indicate that both 'dilute' and 'clustered' I(12.3) are due to membrane-incorporated probe.

Interactions between biliary lipid micelles and intestinal brush border membranes investigated by 1H and 31P nuclear magnetic resonance

The effect of taurocholate and lecithin-cholesterol-taurocholate mixed micelles on the structure of isolated intestinal brush border membranes was investigated by nuclear magnetic resonance (NMR). Rabbit brush border membranes isolated by a Mg2+ precipitation step were chosen for this study because of their stability and integrity as revealed by 31P NMR. Incubation of taurocholate with the brush border membranes does not induce significant solubilization of these membranes even when the taurocholate/phospholipid ratio reaches 3.0. 1H NMR studies indicate that taurocholate is included in the membrane bilayer at low concentration (3 mM). However this biliary salt produces a size diminution of the vesicles when its concentration increases. Incorporation of lecithin or lecithin-cholesterol in micelles of taurocholate and subsequent incubation with brush border membranes lead simultaneously to a decrease in the 31P NMR isotropic/bilayer line ratio, and to an increase in delta sigma. These results indicate a protective effect of these compounds against lytic damage of taurocholate. Furthermore the equilibrium distribution of lecithin between mixed micelles and the membrane bilayer is strongly in favour of complete integration of micellar components in the bilayer. These data suggest that uptake of lipids from the micellar phase by isolated brush border membranes involves an interaction of the micelles with membranes followed by a fusion process.

Dynamic properties of the haptenic site of lipid haptens in phosphatidylcholine membranes. Their relation to the phase transition of the host lattice

The relation between the dynamic properties of the haptenic site of lipid haptens and the phase transition of the host lattice was investigated using head group spin-labeled phosphatidylethanolamines, that is, spin-label lipid haptens (Brûlet, P., and H. M. McConnell, 1976, Proc. Natl. Acad. Sci. USA., 73:2977-2981; Brûlet, P., and H. M. McConnell, 1977, Biochemistry, 16:1209-1217). The electron spin resonance (ESR) spectra of the lipid haptens in liposomal membranes showed three narrow resonance lines, whose widths and hyperfine splitting values suggested that the haptenic site, i.e., the spin-label moiety, should be exposed in the water phase. The line width of each peak depended on the host lipid species and on the incubation temperature. A temperature study using dipalmitoylphosphatidylcholine (DPPC) liposomes showed that the dynamic properties of the haptenic site were related to the main phase transition and the subphase transition of the host lattice but not to the prephase transition. The angular amplitudes of the tumbling motion of the haptenic site were estimated using oriented multibilayer systems. The angular amplitude of dipalmitoyl-phosphatidyl-N-[[N-(1-oxyl-2,2,6, 6-tetramethyl-4-piperidinyl)-carbamoyl]-methyl]-ethanolamine in DPPC membranes was 63 degrees at 2 degrees C, and it increased slightly with an increase in temperature regardless of the phase transition of the host lattice. The value for egg phosphatidylcholine (PC) at 25 degrees C was the same as for DPPC above its main phase transition temperature. Rotational correlation time analysis showed that the axial rotation of the haptenic site was preferable to the tumbling motion of the rotational axis, and the predominance depended on the phase transition, Lc----L beta' and P beta'----L alpha. Elongation of the spacer arm between the haptenic site and phosphate increased the angular amplitude of the tumbling motion but reduced the effect of the host lattice. Spin-label lipid haptens with unsaturated fatty acyl chains were distributed heterogeneously in DPPC membranes, whereas those with the same fatty acyl chain as the host lattice were distributed randomly. The ESR spectrum of a lipid hapten under its prephase transition temperature showed two components, broad and narrow. This suggests that at least two different domains, a hapten-rich domain and a hapten-poor one, may coexist in membranes. ESR measurements at various temperatures suggested that the haptenic site fraction in the hapten-rich domain decreased in part during the phase transition from L beta' to P beta', and disappeared completely in the La phase. The spatial mobility and lateral diffusion of lipid haptens will be discussed in greater detail.

Temperature-induced protein conformational changes in barley root plasma membrane-enriched microsomes: I. Effect of temperature on membrane protein and lipid mobility

A protein spin label and lipid spin probes were used to study the temperature-dependent motion of protein and lipid, respectively, in barley (Hordeum vulgare L. cv Conquest) root plasma membrane-enriched microsomes. Using membranes from seedlings grown at 20 degrees C, the temperature-dependence of the relative motion of membrane surface spin probes and a spin label covalently attached to membrane proteins suggested abrupt changes in the lipid and protein mobilities at about 12 degrees C. Spin probe spin-spin exchange broadening and fluorescent probe eximer formation indicated apparent temperature-induced alterations in probe lateral diffusion within the membrane at about 12 to 14 degrees C. The results suggest the presence of temperature-induced quasicrystalline lipid clusters which may influence the activity of membrane-bound enzymes.

Binding interaction studies of selected receptor subpopulations after partial cross-linking receptor-ligand complexes with a photoactivated heterobifunctional reagent

Certain hormonal and nonhormonal binding systems such as the leukoagglutinin-lymphocyte model exhibit complex receptor-ligand interactions that result in nonlinear Scatchard plots. Such plots are interpreted as indicating either homogeneous negatively interacting binding sites or heterogeneous sites with different and fixed affinity. We assessed the validity of these interpretations in our system by conjugating the ligand to a photoactivated heterobifunctional agent and cross-linking the conjugate to a subset of receptors before studying the binding interactions of non-cross-linked sites. Conjugation did not qualitatively or quantitatively affect the binding properties of the ligand. Cross-linking was specific, efficient, and stable and had no effect on irrelevant surface receptors. Cross-linking of only 3% of the total receptors resulted in 50% decreased ligand binding to high affinity sites consistent with a calculated inactivation of 85% and 2% of high and low affinity sites, respectively. Such preferential inactivation of high affinity sites in an unequivocal demonstration of binding sites heterogeneity in this system and shows a clear rejection of the homogeneous cooperation model.

New biophysical techniques and their application to the study of membranes

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Spin probe clustering in human erythrocyte ghosts

A model has been developed for 5-nitroxide stearate, I(12,3), distribution in human erythrocyte ghosts which accurately predicts ESR spectral alterations observed with increased probe/total lipid (P/L) at 37 degrees C. This spin probe occupies a class of high-affinity, noninteracting sites at low loading. Saturation occurs with increasing probe concentration, and, at higher loading, the probe inserts itself at initially dilute sites to form membrane-bound clusters of variable size. No 'low' probe remains at high P/L where all I(12,3) clusters in a 'concentrated' phase. This model allows determination of the dilute/clustered probe ratio, and shows that I(12,3) segregates in erythrocytes at what might otherwise be considered low P/L (e.g., 1/359). These findings validate the earlier use of empirical parameters to estimate probe sequestration in biological membranes.

Components of the plasma membrane of growing axons. II. Diffusion of membrane protein complexes

Intramembrane particles (IMPs) of the plasmalemma of mature, synapsing neurons are evenly distributed along the axon shaft. In contrast, IMPs of growing olfactory axons form density gradients: IMP density decreases with increasing distance from the perikarya, with a slope that depends upon IMP size (Small, R., and K. H. Pfenninger, 1984, J. Cell Biol., 98: 1422-1433). These IMP density gradients resemble Gaussian tails, but they are much more accurately described by the equations formulated for diffusion in a system with a moving boundary (a Stefan Problem), using constants that are dependent upon IMP size. The resulting model predicts a shallow, nearly linear IMP density profile at early stages of growth. Later, this profile becomes gradually transformed into a steep nonlinear gradient as axon elongation proceeds. This prediction is borne out by the experimental evidence. The diffusion coefficients calculated from this model range from 0.5 to 1.8 X 10(-7) cm2/s for IMPs between 14.8 and 3.6 nm, respectively. These diffusion coefficients are linearly dependent upon the inverse IMP diameter in accordance with the Stokes-Einstein relationship. The measured viscosity is approximately 7 centipoise. Our findings indicate (a) that most IMPs in growing axons reach distal locations by lateral diffusion in the plasma membrane, (b) that IMPs--or complexes of integral membrane proteins--can diffuse at considerably higher rates than previously reported for iso-concentration systems, and (c) that the laws of diffusion determined for macroscopic systems are applicable to the submicroscopic membrane system.

Spatial differentiation in photosynthetic and non-photosynthetic membranes of Rhodopseudomonas palustris

The cytoplasmic membrane and the photosynthetic intracytoplasmic membranes of Rhodopseudomonas palustris are spatially differentiated into regions of extremely high intramembrane-particle density (4,400 to 9,800/micron 2) and areas of lower intramembrane-particle density (2,700 to 5,900/micron 2). The high intramembrane-particle-density areas were always seen in association with photosynthetic membrane stacks. This differentiation was also seen in those areas of the cytoplasmic membrane which adhere to the underlying intracytoplasmic membranes, implying that the cytoplasmic membrane too is differentiated for photosynthesis in these regions. Changes in intramembrane-particle size distribution in response to changes in light intensity during growth were measured. We found that, as light levels were decreased from 8,500 to 100 lx, the average particle diameter in the protoplasmic face of stacked intracytoplasmic and cytoplasmic membranes increased from 8.6 to 10.3 nm. We also observed a distinct periodicity in the sizes of the intramembrane particles found in the stacked regions--7.5, 10.0, 12.5, and 15.0 nm--with the larger-size peaks becoming more pronounced as light intensity decreased. This suggests that, as light levels decrease, subunits of discrete size are being added to a core particle. A comparison of propane jet-frozen cells versus fixed, glycerinated, and then frozen cells indicated that ultrarapid freezing leads to a higher quality of fine-structure preservation than does chemical fixation followed by glycerination and conventional freezing in Freon-12 or propane. The intramembrane particles appeared to be more regular in size, lacking the deformed or jagged appearance displayed in fixed preparations.

Regulation of citrate efflux from mitochondria of oleaginous and non-oleaginous yeasts by long-chain fatty acyl-CoA esters

1. Citrate efflux from a wide range of yeast mitochondria was inhibited by long-chain fatty-acyl-CoA esters. 2. Fatty-acyl-CoA esters with chain lengths of between 14 and 18 carbons were the most potent inhibitors of citrate efflux which was unaffected by the fatty acids themselves. 3. 50% inhibition of citrate transport was observed using palmitoyl-CoA and oleoyl-CoA at approx. 4-5 microM when the tricarboxylate carrier was saturated with L-malate as counter-anion. 4. The inhibition with palmitoyl-CoA and oleoyl-CoA was competitive with L-malate. 5. The possibility that the fatty-acyl-CoA esters were exerting their effect by acting as detergent was eliminated because of the low concentrations used and appropriate comparisons being made with non-specific detergents. Although detergents inhibited citrate efflux they also released citrate by causing membrane damage. 6. The effect of fatty-acyl-CoA esters on citrate efflux could be decreased by using higher mitochondrial protein levels and by adding bovine serum albumin. 7. The possibility is discussed that this inhibition represents a genuine feedback inhibition which could regulate the amount of lipid being synthesized by an oleaginous yeast.

Chemical crosslinking of cell membranes

The complexity of cell membranes makes the resolution of their macromolecular topology one of the more challenging problems in modern molecular and cellular biochemistry. Despite the difficulties inherent in any such analysis, a surprisingly simple yet powerful approach exists that has consistently yielded valuable results. This method is chemical crosslinking, in which cell membranes are treated with crosslinking reagents (usually bifunctional) which produce covalent linkages between membrane components. The resultant complexes are usually then separated and identified by electrophoresis. This review is intended to provide a guide to the investigator who is unfamiliar with this approach. The overall strategy of crosslinking is discussed including selection of reagents, conditions to optimize crosslinking and the cleavage of crosslinked complexes to regenerate the original target for identification purposes. The crosslinking of biological membranes is then reviewed with special emphasis on recent advances including macromolecular photoaffinity labeling, kinetic analysis to probe symmetry properties and potential artifacts that may complicate interpretation of results. Examples of specific applications of crosslinking to membranes are presented in tabular form. The final portion of the review discusses the synthesis and properties of the most widely employed crosslinking reagents. Available reagents are summarized in a series of comprehensive tables. It is hoped that our discussion will provide the uninitiated investigator with sufficient information to ascertain the applicability of chemical crosslinking to particular areas of interest.

Preparation and properties of spin-labeled lecithin-cholesterol liposomes

Lecithin-cholesterol vesicles of various compositions containing membrane-bound spin-labeled cholestane can be prepared by appropriate choice of initial concentrations of components during sonication. Increasing incorporation of spin label increases incorporation of cholesterol and decreases incorporation of lecithin, with the result that liposomes with cholesterol-lecithin molar ratios larger than 2 can be obtained. Besides associating with cholesterol-lecithin complexes in the liposome, the spin label seems to associate with cholesterol. Changes of the paramagnetic resonance spectrum of the liposome-bound spin label due to changes in liposomal cholesterol and spin label mole fractions - assessed by three parameters - can be used in cell-liposome interaction studies.

Binding and incorporation of lecithin-cholesterol vesicles to lymphocytes: a spin-label study

When lecithin-cholesterol vesicles, containing the membrane-bound spin probe 3-doxyl-cholestane, were set in contact with mouse lymphocytes, the vesicles adsorbed to the cell and vesicle-membrane components were transferred to it. The spin probe was enzymatically reduced at the inside of the cell membrane. The spin-label method provided a means to determine quantitatively the extent of vesicles adsorption and vesicle-cell fusion by measuring the transfer of vesicles membrane material to the cell. This method, together with the reduction of spin label by the cell, allowed also a quantitative estimate to the extent of endocytosis during cell-liposome interaction.

Calorimetric evidence for phase transitions in spin-label lipid bilayers

Dispersions of pure, spin-label phosphatidylcholines in aqueous buffer have been investigated with the Privalov high-sensitivity differential scanning calorimeter. The lipids studied are mixed-chain ones in which C-2 of glycerol bears a spin-label derivative of stearic acid and the fatty acid group at C-1 is palmitate. A well-defined phase transition is observed at 30.3-30.7 degrees C for the phosphatidylcholine labeled near the polar end of the stearate chain (label at C-5). A sharp transition (32-34 degrees C) is also observed for the lipid spin-labeled near the terminal methyl of stearate (label at C-16), but the thermodynamic parameters for this lipid depend strongly on the history of the sample. Calorimetric evidence for hysteresis in the phase transition of the C-16-labeled lipid is presented. In contrast to the above spin-label lipids, the lipid labeled at C-12 does not show a sharp transition in the region 5-35 degrees C. In general, therefore, the thermal behavior of the spin-label phosphatidylcholines resembles that of phosphatidylcholines bearing double bonds or branched methyl groups at similar locations on acyl chains. During synthesis of mixed-chain lipids, migration of acyl chains occurs. Methyl esterification procedures which are compatible with the acid-labile spin-label group are described. Gas chromatographic analysis of methyl esters shows that chain migration during synthesis gives 15-20% of the spin-label fatty acid at the glycerol C-1 position.

Lateral mobility in reconstituted membranes--comparisons with diffusion in polymers

The diffusion coefficients (D) of lipopolysaccharide, phospholipid, and Escherichia coli matrix protein were determined in reconstituted multibilayer membranes. Over a range of protein concentration of 0--60% by weight, D for lipopolysaccharide decreased 10-fold, whereas D for phospholipid remained essentially constant. The diffusion coefficient of matrix protein at a concentration of 50% was less than or equal to 10(-12) cm2 s-1. These results are discussed in terms of a model for diffusion in polymeric networks.

Relationships between bilayer lipid, motional freedom of oxidoreductase components, and electron transfer in the mitochondrial inner membrane

The relationships between bilayer lipid, diffusional and conformational activities of oxidoreduction components, and electron transfer activity in the mitochondrial inner membrane are considered. Using a new, low pH method to fuse liposome phospholipid (asolectin) with the isolated mitochondrial inner membrane, the membrane bilayer is enriched up to 700% with exogenous phospholipid. During such enrichment, ultrastructural analysis reveals that integral proteins diffuse freely and randomly into the expanding bilayer. Kinetic analysis reveals that a diffusion limited step occurs between succinate- and NADH dehydrogenase and cytochromes bc1, and that the dehydrogenases, ubiquinone, and cytochromes bc1 are free to diffuse independently of one another in the membrane plane. Whether cytochromes bc1 and cytochrome c oxidase codiffuse in the membrane plane, or diffuse independently of one another remains unclear. The specific activities of succinate- and NADH-dehydrogenase as well as cytochrome c oxidase are affected by bilayer enrichment. This most likely occurs through the direct modulation by the newly incorporated phospholipid on conformational activity required in the oxidoreductases for electron transfer.

The dynamics of membrane structure

The membranes of living organisms are involved in many aspects of the life, growth and development of all cells. The predominant structural elements of these membranes are lipids and proteins and the basic strucvture of these molecules has been reviewed. The physical properties of the lipid constituents particularly their behavior in aqueous systems has led to the concepts of thermotropic and lyotropic mesomorphism; the interaction between different types of lipid molecules modulate this behavior. Interaction of phospholipids in aqueous systems with cholesterol, ions and drugs have been examined in this context. In addition a variety of model lipid-protein systems have been investigated and the implications of interactions between lipids and different proteins in biological membranes has been evaluated. This leads to a detailed consideration of the way lipids and proteins ae organized in cell membranes and contains an appraisal of the evidence supporting contemporary views of membrane structure. Particular attention has been devoted to the question of how mobile the components are within the structure. Particular attention has been devoted to the question of how mobile the components are within the structure. Finally the biosynthesis, turnover and modulation of the properties of interacting membrane constituents is critically reviewed and possible ways of controlling the behavior of cells and organisms by altering the structural parameters of different membranes has been considered.