Studies of the conductance changes induced in bimolecular lipid membranes by alamethicin

The addition of alamethicin to lecithin bilayers results in both voltage-dependent and voltage-independent conductance changes. In the voltage-dependent region, the slope of the conductance-voltage curve varies with the charge of the cation present in the aqueous phase. It may be shown that these effects may be accounted for by a kinetic model which incorporates the following suppositions: (1) alamethicin molecules are adsorbed at the membrane-water interface; (2) the effect of the potential is to redistribute alamethicin-cation complexes between the two surfaces of the bilayer; (3) conduction through the bilayer follows the surface interaction of approximately six alamethicin molecules; and (4) there is an assymetry in the rate constants for corresponding transitions on opposite sides of the bilayer.The effects of alamethicin are found to be approximately the same at neutral and low pH and are unchanged when bilayers are formed from phosphatidyl serine rather than lecithin. These findings are discussed in relation to current hypotheses of the molecular nature of the conduction mechanism.

Co-localization of cell surface receptors at high spatial resolution by single-particle fluorescence imaging

Dual-wavelength single-particle fluorescence imaging has been used to quantify the co-localization of receptors and/or ligands on cells by widefield microscopy. Methods for correction of chromatic aberration and identification of submicroscopic artefacts are presented, with data for the lipopolysaccharide/CD14 and MHC class II/CD74 systems.

Measurements of associations of cell-surface receptors by single-particle fluorescence imaging

SPFI (single-particle fluorescence imaging) uses the high sensitivity of fluorescence to visualize individual molecules that have been selectively labelled with small fluorescent particles. The images of particles are diffraction-limited spots that are analysed by fitting with a two-dimensional Gaussian function. The spot intensities depend on whether they arise from one or more particles; this provides the basis for determining self-association of cell-surface receptors. We have used this approach to determine dimerization of MHC class II molecules and its disruption by interface peptides. We have also exploited the positional information obtained from SPFI to detect co-localization of cell-surface molecules. This involves labelling two different molecules with different coloured fluorophores and determining their positions separately by dual wavelength imaging. The images are analysed to quantify the overlap of the particle images and hence determine the extent of co-localization of the labelled molecules. The technique provides quantification of the extent of co-localization and can detect whether co-localized molecules occur singly or in clusters. We have obtained preliminary data for co-localization of lipopolysaccharide and CD14 on intact cells. We also show that HLA-DR (human leukocyte antigen-DR) and CD74 are partially co-localized and that interaction between these molecules involves the peptide-binding groove of HLA-DR.

Carpal tunnel syndrome despite negative neurophysiological studies

The purpose of the present study was to compare the results of conservative and operative treatment for patients with carpal tunnel syndrome having normal neurophysiological studies. We studied 125 patients with normal neurophysiological studies and analysed eight symptoms and signs as "prognostic factors". Ninety-six patients were treated conservatively (splintage, steroid injection, antiinflammatory medications, activity modification) and 29 were treated surgically (open decompression). One year after initiation of treatment we assessed the outcome and statistically analysed (chi-square test) the differences between the two groups. We did not find any statistically significant correlation between "prognostic factors" and outcome. Twenty four percent of the group treated non-operatively had a good or excellent outcome, whereas 90% of the group treated operatively had a good or excellent outcome. This difference was statistically significant (p < 0.0001). Our study supports the view that the diagnosis of carpal tunnel syndrome is clinical and not neurophysiological. We now recommend operative treatment for these patients.

Investigations of spectrin-lipid interactions using fluoresceinphosphatidylethanolamine as a membrane probe

The binding of human erythrocyte spectrin to large unilamellar vesicles (LUVET) formed by the extrusion technique has been studied using fluoresceinphosphatidylethanolamine (FPE) as a reporter of electrostatic membrane potential. Spectrin aliquots were added to a suspension of FPE-labelled LUVETs to elucidate both the type of charge involved and the dissociation constants for spectrin binding to various lipids. All binding experiments showed serial increases in FPE fluorescence intensity upon serial additions of spectrin, indicative of increasing positive charge at the membrane surface. This proves for the first time that although exhibiting an overall net negative charge, spectrin binds to lipid surfaces by presenting positive charges to the lipid surface. Binding curves were obtained from the change in fluorescence intensity upon each spectrin addition and analysed to determine dissociation constants. A K(d) of 0.14+/-0.12 microM was found for spectrin binding to FPE-labelled phosphatidylcholine/phosphatidylserine (PC/PS) LUVETs at 22 degrees C in high salt conditions. A similar K(d) of 0.17+/-0.11 microM was obtained for spectrin binding to neutral LUVETs composed of PC. However, binding was found to be much weaker for PC/PS LUVETs under low salt conditions with a K(d) of 1.22+/-0.48 microM.

Digital fluorescence imaging of trafficking of endosomes containing low-density lipoprotein in brain astroglial cells

We have used digital fluorescence microscopy to examine transport of LDL-containing endosomes in rat brain astroglial cells to show that individual middle endosomes undergo rapid transitions between forward/backward movements and immobile states over short distances. The population of rapidly moving endosomes (>0.04 microm/sec) was 35. 9%, and the remaining endosomes were slowly moving or temporarily immobile (<0.04 microm/sec). The averaged motion was, however, a very slow perinuclear motion with a velocity of 3.25 microm/h. This small velocity is mainly due to frequent changing of directions in movements, requiring 6 h for a significant concentration around the circumference of the cell nuclei. The application of both anti-dynein antibodies and vanadate in permeabilized cells resulted in peripherally concentrated distribution of endosomes, probably due to inhibition of perinuclear motion by dynein-like motor proteins. These results imply that both dynein-like and kinesin-like proteins bind to the same endosome resulting in both perinuclear and peripherally directed movements.

Intracellular and cell surface heterotypic associations of human leukocyte antigen-DR and human invariant chain

The intracellular and cell-surface heterotypic associations of HLA-DR in the presence and absence of the invariant chain were investigated. Simultaneous confocal microscopy imaging of the Golgi apparatus and HLA-DR molecules revealed that cells transfected only with HLA-DR and not the invariant chain or HLA-DM, accumulate class II molecules mostly in the Golgi apparatus, proximal to the cell nucleus. In contrast, in cells transfected with both HLA-DR and the invariant chain, or HLA-DR, the invariant chain and HLA-DM, the class II molecules are more evenly distributed in intracellular compartments. Confocal microscopy and flow cytometry revealed that in the absence of the invariant chain, a greater number of HLA-DR molecules are transported to the cell surface. Biochemical experiments and nonequilibrium pH gradient electrophoresis revealed that HLA-DR associates with surface invariant chain in the presence of HLA-DM. In cells that lack HLA-DM, no cell-surface association of HLA-DR and Ii was observed. Taken together, these results reveal two separate and distinct functions for surface and intracellular invariant chain subsets. The intracellular invariant chain "arrests" the class II molecules in the endocytic pathway. In contrast, cell-surface invariant chain associates with class II molecules at the cell surface, possibly playing a role in recycling empty class II molecules or as an accessory molecule.

Rapid diffusion of spectrin bound to a lipid surface

Human erythrocyte spectrin was labelled with the probe 5, 5'-disulfato-1-(6-hexanoic acid N-hydroxysuccinimide ester)-1'-ethyl-3,3,3',3'-tetramethylindocarbocyanine (Cy3). Cy3-spectrin was bound to the outer surface of dimyristoylphosphatidylcholine (DMPC) multilamellar vesicles and its diffusion measured by fluorescence recovery after photobleaching (FRAP). It was found that at 30 degrees C, above the lipid gel to liquid-crystalline phase transition of the lipids, Cy3-spectrin had an unexpectedly high diffusion coefficient D=(2.1+/-0.6)x10(-7)) cm2/s. At the phase transition, diffusion of Cy3-spectrin was only slightly lower; D=(1.3+/-0.3)x10(-7) cm2/s, whereas at 14 degrees C, well below the lipid phase transition, diffusion was found to be much slower with D=(3.1+/-0.12)x10(-9) cm2/s. The fast diffusion of Cy3-spectrin on the lipid surface implies that the individual bonds which bind spectrin to the lipid surface must rapidly be made and broken. In the light of these results, spectrin-lipid interactions alone appear unlikely to have any significant role in supporting the cell membrane. Probably, the interactions serve only to localise the spectrin at the inner lipid surface in order to facilitate formation of the cytoskeleton.

Anomalous diffusion of major histocompatibility complex class I molecules on HeLa cells determined by single particle tracking

Single-particle tracking (SPT) was used to determine the mobility characteristics of MHC (major histocompatibility complex) class I molecules at the surface of HeLa cells at 22 degrees C and on different time scales. MHC class I was labeled using the Fab fragment of a monoclonal antibody (W6/32), covalently bound to either R-phycoerythrin or fluorescent microspheres, and the particles were tracked using high-sensitivity fluorescence imaging. Analysis of the data for a fixed time interval suggests a reasonable fit to a random diffusion model. The best fit values of the diffusion coefficient D decreased markedly, however, with increasing time interval, demonstrating the existence of anomalous diffusion. Further analysis of the data shows that the diffusion is anomalous over the complete time range investigated, 4-300 s. Fitting the results obtained with the R-phycoerythrin probe to D = D0talpha-1, where Do is a constant and t is the time, gave D0 = (6.7 +/- 4.5) x 10(-11) cm2 s-1 and alpha = 0.49 +/- 0.16. Experiments with fluorescent microspheres were less reproducible and gave slower anomalous diffusion. The R-phycoerythrin probe is considered more reliable for fluorescent SPT because it is small (11 x 8 nm) and monovalent. The type of motion exhibited by the class I molecules will greatly affect their ability to migrate in the plane of the membrane. Anomalous diffusion, in particular, greatly reduces the distance a class I molecule can travel on the time scale of minutes. The present data are discussed in relation to the possible role of diffusion and clustering in T-cell activation.

The eosin-5-maleimide binding site on human erythrocyte band 3: investigation of membrane sidedness and location of charged residues by triplet state quenching

The triplet probe eosin-5-maleimide (EMA) is a specific inhibitor of anion transport mediated by the erythrocyte membrane protein, band 3. It was previously shown that the eosin moiety is located close to the anion binding site when EMA is covalently bound to band 3 [Pan, R.-j., and Cherry, R. J. (1995) Biochemistry 34, 4880-4888]. In the present study the electrostatic properties and membrane sidedness of the EMA binding site of band 3 were further investigated by triplet state quenching. A series of stable nitroxyl free radicals, which are characterized by different charges, and I- were used as the quenchers. Time-resolved laser spectroscopy was employed to measure the triplet lifetime of EMA. It was found that the quenching reaction between the quenchers and band 3-bound EMA follows a linear Stern-Volmer plot. The quenching rate constants (Kq) of the quenchers are in the order of NH3+-TEMPO (Kq = 6.34 x 10(6) M-1 s-1) > TEMPO-Choline+ (Kq = 2.18 x 10(6) M-1 s-1) > TEMPO (Kq = 1.13 x 10(6) M-1 s-1) > I- (Kq = 2.46 x 10(5) M-1 s-1) > pyrroline-COO- (Kq = 2.18 x 10(4) M-1 s-1). Experiments with resealed ghosts and inside-out vesicles revealed that negatively charged quenchers can only access the EMA binding site from the extracellular side of the membrane while the positively charged quenchers acted from the cytoplasmic side. The ionic strength dependence of the quenching rate constants and the effects of pH on the quenching reaction were also studied. For both TEMPO-Choline+ and I-, the Kq values decreased as the ionic strength increased, but quenching by TEMPO was independent of the ionic strength variation over the same range. It was also found that at lower pH, the I- quenching rate constant increases but the TEMPO-choline+ quenching rate constant decreases. In both cases, the dependence of quenching on pH exhibited an apparent pKa of about 6.5, which suggests the involvement of one or more histidine residues. This notion gained further support from the finding that modification of His residues of band 3 by DEPC reduced I- quenching at pH 6. On the basis of these results, it is proposed that eosin is located in the anion transport channel such that it is accessible from both sides of the membrane. Histidine residues, which have previously been proposed to lie in the anion channel, probably are located on either side of the eosin probe where they contribute to electrostatic interactions which determine the Kq values for the charged quenchers.

Mobility of cell surface receptors: a re-evaluation

It has long been known from fluorescence recovery after photobleaching experiments that the mobility of most cell surface receptors is much smaller than expected for free diffusion of proteins in a fluid lipid bilayer. Single-particle tracking experiments are currently revealing the complexity of the constraints to free diffusion. Evidence has been obtained for several different processes: domain-limited diffusion, temporary confinement and anomalous diffusion. The type of motion exhibited by a given receptor will profoundly influence the rate of any functional process which requires movement in the plane of the membrane. In particular, anomalous diffusion greatly reduces the distance travelled by a receptor on a time scale of minutes.

Detection of dimers of dimers of human leukocyte antigen (HLA)-DR on the surface of living cells by single-particle fluorescence imaging

The technique of single-particle fluorescence imaging was used to investigate the oligomeric state of MHC class II molecules on the surface of living cells. Cells transfected with human leukocyte antigen (HLA)-DR A and B genes were labeled at saturation with a univalent probe consisting of Fab coupled to R-phycoerythrin. Analysis of the intensities of fluorescent spots on the cell surface revealed the presence of single and double particles consistent with the simultaneous presence of HLA-DR heterodimers and dimers of dimers. The proportion of double particles was lower at 37 degrees C than at 22 degrees C, suggesting that the heterodimers and dimers of dimers exist in a temperature-dependent equilibrium. These results are discussed in the context of a possible role for HLA-DR dimers of dimers in T cell receptor-MHC interactions. The technique is validated by demonstrating that fluorescence imaging can distinguish between dimers and tetramers of human erythrocyte spectrin deposited from solution onto a solid substrate. The methodology will have broad applicability to investigation of the oligomeric state of immunological and other membrane-bound receptors in living cells.

Restriction by ankyrin of band 3 rotational mobility in human erythrocyte membranes and reconstituted lipid vesicles

Rotational diffusion of eosin-5-maleimide-labeled band 3 was measured in erythrocyte membranes at pH 9.4-10.4. Band 3 was found to be more mobile in this pH range than at pH 7.5. Similar results were obtained with spectrin-actin-depleted membranes, where it was further shown that ankyrin is the only detectable protein released from the membrane at pH 10. Further experiments were performed at pH 7.5 to investigate the effects of rebinding purified ankyrin and/or band 4.1 to ghosts stripped of skeletal proteins. Ankyrin was found to reduce band 3 rotational mobility, but band 4.1 had no effect. A fluorescence binding assay revealed that fluorescein isothiocyanate-labeled ankyrin had similar binding parameters to those reported previously using 125I labeling. Finally, the rotational mobility of purified band 3 reconstituted into lipid bilayers was determined before and after ankyrin binding. The results of these reconstitution experiments were globally analyzed, assuming the existence of two populations of band 3 with different correlation times. The faster correlation time is consistent with that expected for either dimers or compact tetramers of band 3. Ankyrin binding reduces the proportion of band 3 contributing to the faster component. This result demonstrates that ankyrin promotes the association of band 3 into more slowly rotating complexes independently of any other components of the erythrocyte membrane. It has been reported that ankyrin contains two binding sites for band 3 [Michaely, P., & Bennett, V. (1995) J. Biol. Chem. 270, 22050-22057]. The results of the present study are thus explained by the ability of ankyrin to cross-link band 3 into larger diameter complexes. Cross-linking by ankyrin in part accounts for the slow components in the anisotropy decays of band 3 in the erythrocyte membrane. Other factors which probably influence band 3 aggregation include the membrane "fluidity" and protein concentration.

Single particle tracking of cell-surface HLA-DR molecules using R-phycoerythrin labeled monoclonal antibodies and fluorescence digital imaging

The mobility of cell surface MHC molecules and their ability to form dynamic associations may be related to the physiological status of the cell and to the potential to bind effector T lymphocytes. To investigate these properties, we have prepared HLA DR specific monoclonal antibodies coupled in a 1:1 mole ratio to the fluorescent phycobiliprotein, R-phycoerythrin (PE). We show that these small particles can be sequentially imaged using a cooled slow-scan charge coupled device camera and hence can be used for single particle tracking experiments. We have applied this technique to investigate the movements of HLA DR molecules on fibroblasts transfected with human DR alpha and DR beta genes. PE-IgG was bound to the transfected fibroblasts and particle tracks were obtained by sequential imaging over a period of typically 30 minutes. Analysis of particle tracks revealed the presence of directed motion and domain-limited diffusion in addition to random diffusion. The contributions of these three types of motion showed cell to cell variability. Velocities of directed motion were of the order of 2 nm second-1 whilst domain diameters were in the range 200–800 nm. Diffusion coefficients for random diffusion were in the range 1 × 10(−13)-5 × 10(−12) cm2 second-1. The higher mobilities were observed for the lower intensity fluorescent spots, which possibly correspond to images of single particles. Much lower mobility was observed with a cell where the spot intensities were approximately double that of the lower intensity spots. These spots could be images of double particles implying the association of at least two HLA DR alpha beta dimers. These data are relevant to the study of MHC class II cell surface redistribution and antigen presentation in specific immunity.

Loss of rotational mobility of band 3 proteins in human erythrocyte membranes induced by antibodies to glycophorin A

The effect of antibodies to glycophorin A on the rotational diffusion of band 3 in human erythrocyte membranes was investigated by transient dichrosim. Three antibodies that recognize different epitopes on the exofacial domain of glycophorin A all strongly reduce the rotational mobility of band 3. The effect is at most only weakly dependent on the distance of the epitope from the membrane surface. The degree of immobilization obtained with two of the antibodies, BRIC14 and R18, is very similar to that produced by antibodies to band 3 itself. Similar results were obtained with membranes stripped of skeletal proteins. Fab fragments and an antibody to glycophorin C had no effect on band 3 rotational mobility. These results rule out a mechanism whereby band 3 rotational immobilization results from enhanced interactions with the membrane skeleton that are mediated by a conformational change in glycophorin A. Rather, they strongly indicate that the antibodies to glycophorin A cross-link existing band 3-glycophorin A complexes that have lifetimes that are long compared with the millisecond time scale of the transient dichroism measurements.

Evidence that eosin-5-maleimide binds close to the anion transport site of human erythrocyte band 3: a fluorescence quenching study

The interaction between eosin-5-maleimide (EMA), an inhibitor of the anion transport protein, band 3, and I-, a transportable substrate, was investigated by fluorescence quenching. The Stern-Volmer plot for the quenching reaction between EMA-labeled band 3 and I- exhibits downward curvature both in human erythrocyte ghosts and in purified band 3. The quenching reaction is insensitive to the viscosity of the bulk phase. The shape of the Stern-Volmer plot becomes more linear with increasing temperature. Following the approach of Blatt et al. [(1986) Biophys. J. 50, 349-356], we have developed a binding-diffusion model which is in good agreement with the quenching data. The model supposes that EMA is located in a compartment or "pocket" in band 3 which is separate from the bulk phase and contains a binding site or sites for the quencher. Quenching of band 3-bound EMA fluorescence by I- is inhibited by DIDS and by the transportable anions Cl-, HCO3-, and Br-. Analysis of these experiments yields dissociation constants for the anions which are in reasonable agreement with those determined from transport kinetics and by NMR. We thus deduce that the quencher binding site is the anion binding/transport site on band 3. We propose that EMA is located in the wall of the anion access channel such that it does not inhibit anion binding. The methods described in this report should facilitate detailed studies of anion binding to the transport site on band 3 under a variety of experimental conditions.

Circular-dichroism and fluorescence studies on melittin: effects of C-terminal modifications on tetramer formation and binding to phospholipid vesicles

Studies were performed on a series of melittin analogues with selective alterations to the positively charged amino acid sequence at the C-terminus. Fluorescence studies were undertaken using the sole tryptophan residue in the analogues as an intrinsic fluorescence probe for indications of tetramer formation in free solution, and binding and insertion of the melittins into phospholipid bilayers. Studies were performed under conditions of low-salt buffer with increasing concentrations of phosphate added to promote self-association of the melittin monomers, and also in the presence of phospholipid vesicles. C.d. studies were also performed under conditions of increasing phosphate concentrations and in the presence of lipid vesicles to monitor the alpha-helical content of the melittins. It was found that selective replacement of the C-terminal basic amino acids by glutamine has different effects on self-association, alpha-helix formation and lipid binding of melittin.

Analysis of receptor clustering on cell surfaces by imaging fluorescent particles

Fluorescently labeled low density lipoproteins (LDL) and influenza virus particles were bound to the surface of human fibroblasts and imaged with a cooled slow-scan CCD camera attached to a fluorescence microscope. Particles were also imaged after attachment to polylysine-coated microscope slides. The digital images were analyzed by fitting data points in the region of fluorescent spots by a two-dimensional Gaussian function, thus obtaining a measure of spot intensity with correction for local background. The intensity distributions for particles bound to polylysine slides were mainly accounted for by particle size distributions as determined by electron microscopy. In the case of LDL, the intensity distributions for particles bound to fibroblasts were considerably broadened, indicative of clustering. The on-cell intensity distributions were deconvolved into 1-particle, 2-particle, 3-particle, etc. components using the data obtained with LDL bound to polylysine-coated slides as an empirical measure of the single particle intensity distribution. This procedure yielded a reasonably accurate measure of the proportion of single particles, but large errors were encountered in the proportions of larger cluster sizes. The possibility of studying the dynamics of clustering was investigated by binding LDL to cells at 4 degrees C and observing changes in the intensity distribution with time after warming to 20 degrees C.

Measurement of the rate of uptake and subcellular localization of porphyrins in cells using fluorescence digital imaging microscopy

A fluorescence imaging system incorporating a cooled slow-scan charge-coupled device camera was used to study the rate of uptake and subcellular localization of prophyrins in living cells. Measurements were carried out on human dermal fibroblasts (D532) using two different prophyrins meso-tetra(4-N-methylpyridyl)porphine (TMPP) and meso-tetra(4-N-hexylpyridyl)porphine (THPP). It was observed that TMPP was rapidly taken up by cells and principally located in the nucleus. The THPP, on the other hand, internalized more slowly and exhibited a particulate distribution in the cytoplasm.

Retention can be improved!

A unique program has been developed to encourage retention of nurses who prefer to remain in direct patient care roles. Both recognition and monetary reward are available to nurses who join the "Professional Excellence in Nursing" (PEN) program. The underlying philosophy is that stable employment, clearly defined and attainable goals and recognition for a higher level of performance are keys to job satisfaction--and thus to retention. Evaluative data on this young program are not available but informal surveys indicate a positive outcome.

Aggregation of band 3 in hereditary ovalocytic red blood cell membranes. Electron microscopy and protein rotational diffusion studies

Microaggregation of band 3 proteins in hereditary ovalocytic membranes was investigated by rotational diffusion measurements and by electron microscopy. It was previously shown that band 3 in ovalocytic membranes has decreased rotational mobility compared with band 3 in normal cells (Tilley, L., Nash, G.B., Jones, G.L. and Sawyer, W.L. (1991) J. Membr. Biol. 121, 59–66). This result could arise from either altered interactions with cytoskeletal proteins or from band 3 microaggregation. In the present study it was found that removal of spectrin and actin from the membrane had no effect on the rotational mobility of ovalocytic band 3. Additional removal of ankyrin and band 4.1, as well as cleavage of the cytoplasmic domain of band 3 with trypsin, did enhance band 3 mobility, as is the case in the membranes from normal cells. However, the rotational mobility of ovalocytic band 3 was always considerably less than that of normal band 3 under the same conditions. Scanning electron microscopy and low power electron micrographs of freeze-fracture replicas revealed that the surfaces of ovalocytes were more irregular than those of normal erythrocytes. At higher magnification, numerous linearly arranged intramembranous particles were observed on the P-faces of freeze-fractured ovalocytes but not on normal cells. These clusters consist of straight or slightly curved lines of 10–15 particles in single rows. From these results it is deduced that the reduced rotational mobility of band 3 in ovalocytes is a consequence of the formation of microaggregates, which are very probably induced by the mutation in the membrane-bound domain of ovalocytic band 3.

The effect of the presence of integral membrane protein (human band 3) on the membrane lytic properties of melittin in reconstituted systems

Reconstitution of the anion exchange protein from human erythrocytes (band 3) into phospholipid vesicles was shown to have a protective effect on melittin lysis of the vesicles when compared to pure lipid vesicles. Low salt buffer was found to cause an inhibition of lysis in both proteoliposomes and pure lipid vesicles compared to salt buffer. High phosphate concentration did not seem to cause inhibition of lysis in the reconstituted system. However, an inhibition is observed in pure lipid vesicle control, which is contradictory to previous reports.

Effect of membrane potential on band 3 conformation in the human erythrocyte membrane detected by triplet state quenching experiments

The triplet lifetime and absorption anisotropy decay of eosin-labeled band 3 was measured in resealed erythrocyte ghosts. Membrane potentials were generated by the addition of valinomycin in the presence of a K+ gradient. Neither negative nor positive membrane potentials had any detectable effect on the rotational diffusion of band 3 nor on the eosin triplet lifetime. The membrane potential did, however, affect quenching of the eosin triplet state by I- and TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl). Quenching was enhanced by a negative membrane potential (negative inside) and reduced by a positive membrane potential. In addition, it was found that a negative membrane potential enhanced the efficiency of eosin labeling of band 3 in intact erythrocytes. A positive membrane potential had the opposite effect. These results indicate that the eosin binding site on band 3 becomes more accessible to the extracellular aqueous phase in the presence of a negative membrane potential and less accessible in the presence of a positive membrane potential. Quenching by I- and TEMPO of the triplet state of eosin-labeled band 3 was further investigated as a function of pH. Quenching by TEMPO and its dependence on membrane potential were relatively insensitive to pH. In contrast, the rate of quenching by I- showed a marked decrease over the range pH 5.5-9.5. Moreover, the effect of a negative membrane potential on I- quenching also varied with pH. These results are discussed on the supposition that the eosin probe is located in the anion access channel of band 3.(ABSTRACT TRUNCATED AT 250 WORDS)

Tracking of cell surface receptors by fluorescence digital imaging microscopy using a charge-coupled device camera. Low-density lipoprotein and influenza virus receptor mobility at 4 degrees C

A fluorescence imaging system, based on using a cooled slow-scan CCD camera, has been developed for tracking receptors on the surfaces of living cells. The technique is applicable to receptors for particles such as lipoproteins and viruses that can be labeled with a few tens of fluorophores. The positions of single particles in each image are determined to within 25 nm by fitting the fluorescence distribution to a two-dimensional Gaussian function. This procedure also provides an accurate measure of intensity, which is used as a tag for automated tracking of particles from frame to frame. The method is applied to an investigation of the mobility of receptors for LDL and influenza virus particles on human dermal fibroblasts at 4 degrees C. In contrast to previous studies by FRAP (fluorescence recovery after photo-bleaching), it is found that receptors have a low but measurable mobility at 4 degrees C. Analysis of individual particle tracks indicates that whilst some receptors undergo random diffusion, others undergo directed motion (flow) or diffusion restricted to a domain. A procedure is proposed for subdividing receptors according to their different types of motion and hence determining their motional parameters. The finding that receptors are not completely immobilised at 4 degrees C is significant for studies of receptor distributions performed at this temperature.

Both ankyrin and band 4.1 are required to restrict the rotational mobility of band 3 in the human erythrocyte membrane

A population of band 3 proteins in the human erythrocyte membrane is known to have restricted rotational mobility due to interaction with cytoskeletal proteins. We have further investigated the cause of this restriction by measuring the effects on band 3 rotational mobility of rebinding ankyrin and band 4.1 to ghosts stripped of these proteins as well as spectrin and actin. Rebinding either ankyrin or 4.1 alone has no detectable effect on band 3 mobility. Rebinding both these proteins together does, however, reimpose a restriction on band 3 rotation. The effect on band 3 rotational mobility of rebinding ankyrin and 4.1 are similar irrespective of whether or not band 4.2 is removed from the membrane. We suggest that ankyrin and 4.1 together promote the formation of slowly rotating clusters of band 3.

Leakage of internal markers from erythrocytes and lipid vesicles induced by melittin, gramicidin S and alamethicin: a comparative study

The membrane-disruptive capacities of melittin, derivatised melittins, alamethicin and gramicidin S have been compared for the human erythrocyte membrane and lipid vesicles of three different compositions (phosphatidylcholine, 85% phosphatidylcholine/15% phosphatidylserine, and a lipid analogue of the outer leaflet of the human erythrocyte membrane). The sensitivity to ionic strength, divalent metal ions and polylysine of release of fluorescent markers from liposomes and of haemoglobin from intact erythrocytes has been assayed. Acetyl melittin was found to he more effective than melittin in lysing phosphatidylcholine and phosphatidylcholine/phosphatidylserine vesicles, somewhat less effective in the lipid analogue and markedly less effective in lysing erythrocytes. Succinyl melittin was non-haemolytic, but was able to lyse lipid vesicles at a high concentration. Ca2+ inhibited melittin haemolysis at high ionic strength (150 mM NaCl), but produced a more complex response of stimulation followed by inhibition at low ionic strength. In lipid vesicles, Ca2+ either stimulated melittin lysis or was ineffective. Zn2+ exerted effects similar to Ca2+ with lipid vesicles at approx. 10-fold lower concentration except that a weak inhibition was observed for the erythrocyte membrane lipid analogue at high ionic strength. Polylysine strongly inhibited haemolysis by melittin at low ionic strength, but was ineffective or stimulatory in lipid vesicle lysis. High phosphate concentration also inhibited melittin haemolysis, but again no corresponding effect could he found in any of the lipid vesicle systems. These disparities between effects of melittin on erythrocytes and lipid vesicles support the proposal that melittin-protein interactions are of consequence to its haemolytic action. Similar experiments were performed with gramicidin S and alamethicin in order to compare their lytic properties with those of melittin. It was found that each lysin exhibited its own individual pattern of sensitivity to lipid composition, ionic strength and inhibition by cations. It thus appears likely that the detailed molecular interactions responsible for lysis are significantly different for each of these three agents.

Rotational diffusion studies of the lipoyl domain of 2-oxoacid dehydrogenase multienzyme complexes

Rotational mobility of the lipoyl domain of a number of 2-oxoacid dehydrogenase complexes was investigated by transient dichroism after the domain had been specifically labeled with the triplet probe eosin-5-maleimide. Complexes investigated included pyruvate dehydrogenase complexes from Bacillus stearothermophilus, ox heart, and Escherichia coli (in which the E2 component had been genetically engineered to contain one lipoyl domain) and 2-oxoglutarate dehydrogenase complexes from ox heart and E. coli. Measurements were also performed with ox heart pyruvate dehydrogenase and 2-oxoglutarate dehydrogenase complexes specifically labeled on E1. Anisotropy decays were recorded in glycerol-buffer solutions of varying viscosity and at different temperatures. For E2-labeled complexes, the decays were found to be multiexponential, and the fastest correlation time was considerably shorter than expected for tumbling of the whole complex. This fast correlation time was absent from E1-labeled complexes and was assigned to independent motion of the lipoyl domain. Plots of the fast correlation time against eta/T showed a surprisingly weak dependence on viscosity and extrapolated to a time of 30-40 microseconds at zero viscosity. To explain this result, a model is proposed in which the lipoyl domain is in equilibrium between "free" and bound states. The time of 30-40 microseconds is shown to correspond to 1/koff, where koff is the rate constant for dissociation of the domain from binding sites on the complex. This dissociation phenomenon only contributes to the anisotropy decay when the viscosity of the solution is sufficiently high to slow the tumbling of the whole complex to times that are long in comparison to 1/koff.

Electron microscopic observation of the aggregation of membrane proteins in human erythrocyte by melittin

Human erythrocytes and erythrocyte ghost membranes were treated with native and modified melittins, up to 250 nmol/mg membrane protein. Native melittin induced aggregation of intramembranous particles (IMPs, observed by freeze-fracture electron microscopy), and created large, smooth bilayer areas devoid of IMP. The degree of IMP aggregation increased with increasing concentration of melittin, corresponding to hemolysis results. Membrane ghosts were slightly more susceptible to IMP aggregation than membranes on intact cells. The potency of inducing IMP aggregation was ranked in the order of: native melittin greater than acetylated melittin greater than succinylated melittin = 0. The concentration range of melittin which caused IMP aggregation corresponded to that which caused the immobilization of band 3 proteins as detected by measurement of rotational mobility by transient dichroism (Dufton et al. (1984) Eur. J. Biophys. 11, 17-24). Because both IMP aggregation and band 3 protein immobilization decreased with decreasing positive charge of the melittins used, the nature of melittin-protein interaction is likely to be at least in part electrostatic in the case of human erythrocyte membranes. Possible roles of IMP aggregation and the consequent creation of 'exposed' bilayer areas in the cytotoxic reaction of melittins are discussed.

Transient dichroism studies of spectrin rotational diffusion in solution and bound to erythrocyte membranes

Spectrin was purified from human erythrocytes and labeled with the triplet probe eosin-5-maleimide. Rotational diffusion of spectrin was investigated by observing transient dichroism following flash excitation of the probe. Measurements were performed at 4 degrees C in solutions of varying viscosity and with spectrin rebound to spectrin/actin-depleted erythrocyte membranes. In solution, complex anisotropy decays were observed which could not be satisfactorily fitted by the equations for a rod-shaped molecule of appropriate dimensions. When spectrin was rebound to the erythrocyte membrane, a decay in the anisotropy was still present but was markedly less sensitive to solution viscosity and flatter at longer times. In order to overcome the objection that the cytoskeleton is only partially reconstituted when spectrin is rebound, a method was developed for labeling spectrin with eosin-5-maleimide in situ. Anisotropy decays for these labeled membranes exhibited features similar to those obtained for spectrin labeled in solution and subsequently rebound. Taken together, the results provide good evidence for segmental motion of spectrin when incorporated into the erythrocyte cytoskeleton. Upon increasing the temperature, the initial anisotropy ro for both rebound and in situ labeled spectrin decreases, and above 30 degrees C the measured anisotropies are small. Thus, at physiological temperature the probe is almost completely randomized by motions with correlation times less than 10 microseconds.

Digital fluorescence imaging of fusion of influenza virus with erythrocytes

Fusion of influenza virus with human erythrocytes at pH 5.2 was followed by fluorescence microscopy using a cooled slow-scan CCD camera. The high sensitivity of the CCD permits repetitive digital imaging of the same cells with minimal photobleaching. The experimental conditions were such that only a small number of virus particles were adsorbed per cell. Quantitative analysis of the data indicated that for most cells only a single fusion event took place. This was, however, sufficient to cause haemolysis within 30 min at 20-22 degrees C for about 60% of cells. There was a highly variable time lag between fusion and haemolysis. The lateral diffusion coefficient of virus particles on the cell surface when bound at pH 7.4 was less than 2 x 10(-13) cm2.s-1. The technique should be of value for more detailed studies of the dynamics of viral and other membrane fusion events.

Cytoskeletal restraints of band 3 rotational mobility in human erythrocyte membranes

The interaction of band 3 with cytoskeletal proteins was investigated in erythrocyte membranes by measuring the rotational mobility of band 3 using the method of transient dichroism. It was found that selective proteolysis of ankyrin, a protein known to link band 3 to the spectrin-actin network, had no significant effect on band 3 rotation. Incubating ghosts to 70 degrees C, at which temperature ankyrin is expected to be denatured, also had no effect. It thus appears probable that linkage of band 3 to the cytoskeleton via ankyrin does not act as a restraint on band 3 rotational motion. It is suggested that this is a consequence of flexibility in the cytoskeletal structure. In further investigations of the effect of heat treatment, a large enhancement of band 3 rotational mobility was found to result from incubation of intact cells for 1 h at 50 degrees C. This effect was not observed if ghosts were subjected to the same treatment, nor did it occur if the incubation of cells was performed at 47 degrees C. These findings, in combination with previous studies of band 3 rotational mobility, indicate that the interactions which restrain band 3 are likely to be more complex than commonly envisaged.

A comparative study of band 3 aggregation in erythrocyte membranes by melittin and other cationic agents

The technique of laser flash-induced transient dichroism has been used to measure the rotational diffusion of eosin-labelled band 3 proteins in erythrocyte ghosts. A retardation in the mobility of band 3, measured subsequent to the addition of a variety of polyvalent cationic species, has been interpreted to reflect aggregation or 'clustering' of the protein in the plane of the membrane. A comparative study is reported between three such aggregators: melittin, polylysine and Zn2+, wherein their respective abilities to induce aggregation have been measured under varying conditions. Unlike that for melittin, band 3 aggregation by polylysine and Zn2+ is shown to be sensitive to proteolytic degradation of the membrane and to the ionic strength of the surrounding medium. Studies with fragments of melittin derived from its chymotryptic cleavage show the hydrophilic C-terminal 20-26 section to possess independent aggregating ability, but also the requirement of the 1-19 hydrophobic section to be attached in order to prevent reversibility by high ionic strength buffers. Melittin is also shown to have a unique ability to aggregate bacteriorhodopsin reconstituted into DMPC vesicles, which is partially retained by its 1-19 but not by its 20-26 fragment.

Comparison of p25 presequence peptide and melittin. Red blood cell haemolysis and band 3 aggregation

The 25 residue presequence (p25) for subunit IV of yeast cytochrome oxidase had previously been shown to possess structural and behavioural characteristics in common with the bee venom polypeptide, melittin. The present study extends the results of leakage experiments on model-membrane systems to the haemolysis of human erythrocytes, which both peptides are shown to accomplish in a manner sensitive to membrane potential. In addition, the laser flash-induced transient dichroism technique for measuring protein rotational diffusion has been used to show that both peptides aggregate band 3, the major integral membrane protein of the erythrocyte. Aggregation cannot be reversed by high ionic strength; this serves to differentiate these peptides from other positively charged species such as polylysine that aggregate band 3 at low ionic strength. These results suggest that aggregation of membrane proteins may possibly prove to be a feature of the interaction of p25 signal peptide with mitochondrial membranes.