Orientation of LamB signal peptides in bilayers: influence of lipid probes on peptide binding and interpretation of fluorescence quenching data.

The orientation in lipid bilayers of the signal sequence of the bacterial protein LamB was studied using binding, circular dichroism, and fluorescence quenching experiments. Measurements were made of binding modifications caused by the incorporation of lipid probes (brominated or nitroxide-labeled phospholipids) used in the parallax fluorescence quenching method of determining peptide penetration depth [Abrams, F. S., and London, E. (1992) Biochemistry 31, 5312-5322]. The signal peptide bound to a similar extent to neutral bilayers composed of either egg phosphatidylcholine (EPC) or phosphatidylcholines brominated at various positions on their acyl chains. The fluorescence of a tryptophan in either the 18 or 24 position of the peptide was quenched more by bromines in the 6 and 7 than in the 9 and 10 positions on the lipid hydrocarbon chain. Parallax calculations showed that tryptophan-18 was located only 4 A from the hydrocarbon-water interface, consistent with the peptide adopting a "hammock" configuration in the bilayer, with both termini exposed to the aqueous phase and the central alpha-helix located near the hydrocarbon-water interface. In contrast, the incorporation of 10% nitroxide-labeled lipids into EPC bilayers modified peptide binding in a manner dependent on the position of the nitroxide on the hydrocarbon chain; 7-Doxyl PC reduced the percent peptide bound by about one-half, whereas 12-Doxyl PC had little effect on binding. These binding differences modified tryptophan quenching by these probes, making parallax analysis problematical. In the presence of the positively charged LamB peptide, the incorporation of negatively charged phospholipids into EPC bilayers increased the level of peptide binding and modified tryptophan quenching by nitroxide probes. These results suggest that the nitroxide probe could be partially excluded from negatively charged lipid domains where the peptide preferentially bound. Quite different binding and quenching results were obtained with a negatively charged peptide analogue, showing that the charge on both the peptide and bilayer affects peptide-nitroxide probe interactions.     

Diffusional dynamics of an active rhodamine-labeled 1,4-dihydropyridine in sarcolemmal lipid multibilayers.

A "membrane bilayer pathway" model, involving ligand partition into the bilayer, lateral diffusion, and receptor binding has been invoked to describe the 1,4-dihydropyridine (DHP) calcium channel antagonist receptor binding mechanism. In an earlier study (Chester et al. 1987. Biophys. J. 52:1021-1030), the diffusional component of this model was examined using an active fluorescence labeled DHP calcium channel antagonist, nisoldipine-lissamine rhodamine B (Ns-R), in purified cardiac sarcolemmal (CSL) lipid multibilayers. Diffusion coefficient measurements on membrane-bound drug and phospholipid at maximum bilayer hydration yielded similar values (3.8 x 10(-8) cm2/s). However, decreases in bilayer hydration resulted in dramatically reduced diffusion coefficient values for both probes with substantially greater impact on Ns-R diffusion. These data suggested that hydration dependent diffusional differences could be a function of relative probe location along the bilayer normal. In this communication, we have addressed the relative effect of the rhodamine substituent on Ns-R diffusion complex by examining the diffusional dynamics of free rhodamine B under the same conditions used to evaluate Ns-R complex and phospholipid diffusion. X-ray diffraction studies were performed to determine the Ns-R location in the membrane and model the CSL lipid bilayer profile structure to give a rationale for the differences in probe diffusional dynamics as a function of interbilayer water space.     

Evaluation of bilayer disks as plant cell membrane models in partition studies

We have studied the partitioning of a set of phenolic compounds used as lignin precursor models into lipid bilayer disks and liposomes. The bilayer disks are open bilayer structures stabilized by polyethylene glycol-conjugated lipids. Our results indicate that disks generate more accurate partition data than do liposomes. Furthermore, we show that the partitioning into the membrane phase is reduced slightly if disks composed of 1,2-distearoyl-sn-glycero-3-phosphocholine and cholesterol are exchanged for disks with a lipid composition mimicking that of the root tissue of Zea mays L.     

Orientation and dynamics of an antimicrobial peptide in the lipid bilayer by solid-state NMR spectroscopy

The orientation and dynamics of an 18-residue antimicrobial peptide, ovispirin, has been investigated using solid-state NMR spectroscopy. Ovispirin is a cathelicidin-like model peptide (NH(2)-KNLRRIIRKIIHIIKKYG-COOH) with potent, broad-spectrum bactericidal activity. (15)N NMR spectra of oriented ovispirin reconstituted into synthetic phospholipids show that the helical peptide is predominantly oriented in the plane of the lipid bilayer, except for a small portion of the helix, possibly at the C-terminus, which deviates from the surface orientation. This suggests differential insertion of the peptide backbone into the lipid bilayer. (15)N spectra of both oriented and unoriented peptides show a reduced (15)N chemical shift anisotropy at room temperature compared with that of rigid proteins, indicating that the peptide undergoes uniaxial rotational diffusion around the bilayer normal with correlation times shorter than 10(-4) s. This motion is frozen below the gel-to-liquid crystalline transition temperature of the lipids. Ovispirin interacts strongly with the lipid bilayer, as manifested by the significantly reduced (2)H quadrupolar splittings of perdeuterated palmitoyloleoylphosphatidylcholine acyl chains upon peptide binding. Therefore, ovispirin is a curved helix residing in the membrane-water interface that executes rapid uniaxial rotation. These structural and dynamic features are important for understanding the antimicrobial function of this peptide.     

Transmembrane helix structure, dynamics, and interactions: multi-nanosecond molecular dynamics simulations.

To probe the fundamentals of membrane/protein interactions, all-atom multi-nanosecond molecular dynamics simulations were conducted on a single transmembrane poly(32)alanine helix in a fully solvated dimyristoyphosphatidylcholine (DMPC) bilayer. The central 12 residues, which interact only with the lipid hydrocarbon chains, maintained a very stable helical structure. Helical regions extended beyond these central 12 residues, but interactions with the lipid fatty-acyl ester linkages, the lipid headgroups, and water molecules made the helix less stable in this region. The C and N termini, exposed largely to water, existed as random coils. As a whole, the helix tilted substantially, from perpendicular to the bilayer plane (0 degree) to a 30 degrees tilt. The helix experienced a bend at its middle, and the two halves of the helix at times assumed substantially different tilts. Frequent hydrogen bonding, of up to 0.7 ns in duration, occurred between peptide and lipid molecules. This resulted in correlated translational diffusion between the helix and a few lipid molecules. Because of the large variation in lipid conformation, the lipid environment of the peptide was not well defined in terms of "annular" lipids and on average consisted of 18 lipid molecules. When compared with a "neat" bilayer without peptide, no significant difference was seen in the bilayer thickness, lipid conformations or diffusion, or headgroup orientation. However, the lipid hydrocarbon chain order parameters showed a significant decrease in order, especially in those methylene groups closest to the headgroup.     

Electrochemical measurement of lateral diffusion coefficients of ubiquinones and plastoquinones of various isoprenoid chain lengths incorporated in model bilayers.

The long-range diffusion coefficients of isoprenoid quinones in a model of lipid bilayer were determined by a method avoiding fluorescent probe labeling of the molecules. The quinone electron carriers were incorporated in supported dimyristoylphosphatidylcholine layers at physiological molar fractions (<3 mol%). The elaborate bilayer template contained a built-in gold electrode at which the redox molecules solubilized in the bilayer were reduced or oxidized. The lateral diffusion coefficient of a natural quinone like UQ10 or PQ9 was 2.0 +/- 0.4 x 10(-8) cm2 s(-1) at 30 degrees C, two to three times smaller than the diffusion coefficient of a lipid analog in the same artificial bilayer. The lateral mobilities of the oxidized or reduced forms could be determined separately and were found to be identical in the 4-13 pH range. For a series of isoprenoid quinones, UQ2 or PQ2 to UQ10, the diffusion coefficient exhibited a marked dependence on the length of the isoprenoid chain. The data fit very well the quantitative behavior predicted by a continuum fluid model in which the isoprenoid chains are taken as rigid particles moving in the less viscous part of the bilayer and rubbing against the more viscous layers of lipid heads. The present study supports the concept of a homogeneous pool of quinone located in the less viscous region of the bilayer.     

Diffusion of dihydropyridine calcium channel antagonists in cardiac sarcolemmal lipid multibilayers.

A membrane bilayer pathway model has been proposed for the interaction of dihydropyridine (DHP) calcium channel antagonists with receptors in cardiac sarcolemma (Rhodes, D.G., J.G. Sarmiento, and L.G. Herbette. 1985. Mol. Pharmacol. 27:612-623) involving drug partition into the bilayer with subsequent receptor binding mediated (though probably not rate-limited) by diffusion within the bilayer. Recently, we have characterized the partition step, demonstrating that DHPs reside, on a time-average basis, near the bilayer hydrocarbon core/water interface. Drug distribution about this interface may define a plane of local concentration for lateral diffusion within the membrane. The studies presented herein examine the diffusional dynamics of an active rhodamine-labeled DHP and a fluorescent phospholipid analogue (DiIC16) in pure cardiac sarcolemmal lipid multibilayer preparations as a function of bilayer hydration. At maximal bilayer hydration, the drug diffuses over macroscopic distances within the bilayer at a rate identical to that of DiI (D = 3.8 X 10(-8) cm2/s), demonstrating the overall feasibility of the membrane diffusion model. The diffusion coefficients for both drug and lipid decreased substantially as the bilayers were dehydrated. While identical at maximal hydration, drug diffusion was significantly slower than that of DiIC16 in partially dehydrated bilayers, probably reflecting differences in mass distribution of these probes in the bilayer.     

Using fluorine nuclear magnetic resonance to probe changes in the structure and dynamics of membrane-active peptides interacting with lipid bilayers

The antimicrobial peptide MSI-78 serves as a model system for studying interactions of bioactive peptides with membranes. Using a series of MSI-78 peptides that incorporate l-4,4,4-trifluoroethylglycine, a small and sensitive (19)F nuclear magnetic resonance probe, we investigated how the local structure and dynamics of the peptide change when it binds to the lipid bilayer. The fluorinated MSI-78 analogues exhibited position-specific changes in (19)F chemical shift ranging from 1.28 to -1.35 ppm upon binding to lipid bicelles. The largest upfield shifts are associated with the most hydrophobic positions in the peptide. Changes in solvent isotope effects (H(2)O/D(2)O) on (19)F chemical shifts were observed for the peptides that are consistent with the MSI-78 solvent-inaccessible hydrophobic core upon binding bicelles. Transverse relaxation measurements of the (19)F nucleus, using the Carr-Purcell-Meiboom-Gill pulse sequence, were used to examine changes in the local mobility of MSI-78 that occur upon binding to the lipid bilayer. Positions in the hydrophobic core of peptide-membrane complex show the greatest decrease in mobility upon binding of the lipid bilayer, whereas residues that interact with lipid headgroups are more mobile. The most mobile positions are at the N- and C-termini of the peptide. These results provide support for the proposed mechanism of membrane disruption by MSI-78 and reveal new details about the dynamic changes that accompany membrane binding.     

Synthetic analogues of alamethicin: effect of C-terminal residue substitutions and chain length on the ion channel lifetimes.

In a previous study, a synthetic analogue of the peptaibol alamethicin, in the sequence of which all alpha-aminoisobutyric acid (Aib) were substituted by leucine residues and the C-terminal residue modified, was shown to display the same single-channel behaviour as alamethicin in planar lipid bilayer, except that the sublevel lifetimes were much reduced. New analogues differing in their C-terminal residue (Phe-NH2, Pheol, Trp-NH2) have now been tested for their single channel properties in neutral lipid bilayers. The conductance amplitudes and open channel lifetimes do not differ significantly from the previous analogue. Thus, the nature of the last residue, which may be located near the membrane interface, does not seem to play an important role in the destabilisation of the conducting aggregate observed after the Aib substitution by Leu. Since the deletion of one residue (Glu18) in the 14-20 moiety induces a slight decrease of the increment between the conductance levels, but has no effect upon the channel lifetimes, this residue and the length of this segment do not interfer much with the channel lifetime of peptaibols. In conclusion the factors influencing the aggregate stability may be sought in the helix-helix interactions.     

Weak Glycolipid Binding of a Microdomain-Tracer Peptide Correlates with Aggregation and Slow Diffusion on Cell Membranes

Organized assembly or aggregation of sphingolipid-binding ligands, such as certain toxins and pathogens, has been suggested to increase binding affinity of the ligand to the cell membrane and cause membrane reorganization or distortion. Here we show that the diffusion behavior of the fluorescently tagged sphingolipid-interacting peptide probe SBD (Sphingolipid Binding Domain) is altered by modifications in the construction of the peptide sequence that both result in a reduction in binding to ganglioside-containing supported lipid membranes, and at the same time increase aggregation on the cell plasma membrane, but that do not change relative amounts of secondary structural features. We tested the effects of modifying the overall charge and construction of the SBD probe on its binding and diffusion behavior, by Surface Plasmon Resonance (SPR; Biacore) analysis on lipid surfaces, and by Fluorescence Correlation Spectroscopy (FCS) on live cells, respectively. SBD binds preferentially to membranes containing the highly sialylated gangliosides GT1b and GD1a. However, simple charge interactions of the peptide with the negative ganglioside do not appear to be a critical determinant of binding. Rather, an aggregation-suppressing amino acid composition and linker between the fluorophore and the peptide are required for optimum binding of the SBD to ganglioside-containing supported lipid bilayer surfaces, as well as for interaction with the membrane. Interestingly, the strength of interactions with ganglioside-containing artificial membranes is mirrored in the diffusion behavior by FCS on cell membranes, with stronger binders displaying similar characteristic diffusion profiles. Our findings indicate that for aggregation-prone peptides, aggregation occurs upon contact with the cell membrane, and rather than giving a stronger interaction with the membrane, aggregation is accompanied by weaker binding and complex diffusion profiles indicative of heterogeneous diffusion behavior in the probe population.     

Alamethicin channels incorporated into frog node of ranvier: calcium-induced inactivation and membrane surface charges

Alamethicin, a peptide antibiotic, partitions into artificial lipid bilayer membranes and into frog myelinated nerve membranes, inducing a voltage-dependent conductance. Discrete changes in conductance representing single-channel events with multiple open states can be detected in either frog node or lipid bilayer membranes. In 120 mM salt solution, the average conductance of a single channel is approximately 600 pS. The channel lifetimes are roughly two times longer in the node membrane than in a phosphatidylethanolamine bilayer at the same membrane potential. With 2 or 20 mM external Ca and internal CsCl, the alamethicin-induced conductance of frog nodal membrane inactivates. Inactivation is abolished by internal EGTA, suggesting that internal accumulation of calcium ions is responsible for the inactivation, through binding of Ca to negative internal surface charges. As a probe for both external and internal surface charges, alamethicin indicates a surface potential difference of approximately -20 to -30 mV, with the inner surface more negative. This surface charge asymmetry is opposite to the surface potential distribution near sodium channels.     

Dynamic properties of gramicidin A in phospholipid membranes

The flexibility of the tryptophan side chains of gramicidin A and the rotational diffusion of the peptide in methanolic solution and in three membrane systems were studied with deuterium nuclear magnetic resonance (NMR). Gramicidin A was selectively deuterated at the aromatic ring systems of its four tryptophan side chains. In methanolic solution, the tryptophan residues remained immobile and served as a probe for the overall rotation of the peptide. The experimentally determined rotational correlation time of tau c = 0.6 X 10(-9) s was consistent with the formation of gramicidin A dimers. For gramicidin A incorporated into bilayer membranes, quite different results were obtained depending on the chemical and physical nature of the lipids employed. When mixed with 1-palmitoyl-sn-glycero-3-phosphocholine (LPPC) at a stoichiometric lipid:peptide ratio of 4:1, gramicidin A induced the formation of stable bilayer membranes in which the lipids were highly fluid. In contrast, the gramicidin A molecules of this membrane remained completely static over a large temperature interval, suggesting strong protein-protein interactions. The peptide molecules appeared to form a rigid two-dimensional lattice in which the interstitial spaces were filled with fluidlike lipids. When gramicidin A was incorporated into bilayers of 1,2-dioleoyl-sn-glycero-3-phosphocholine or 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) above the lipid phase transition, the deuterium NMR spectra were motionally narrowed, indicating large-amplitude rotational fluctuations. From the measurement of the quadrupole echo relaxation time, a rotational correlation time of 2 X 10(-7) s was estimated, leading to a membrane viscosity of 1-2 P if the rotational unit was assumed to be a gramicidin A dimer. (ABSTRACT TRUNCATED AT 250 WORDS)     

Characterization of octapeptin-membrane interactions using spin-labeled octapeptin

Octapeptin is a membrane-active peptide antibiotic that contains a C10 fatty acid covalently attached to the peptide through an amide bond. Interactions of octapeptin with bacterial membranes and phospholipids were characterized by using spin-labeling techniques and octapeptin derivatives containing fatty acids of varying chain length. Acyl modification of octapeptin demonstrated that the fatty acid of the antibiotic contributed to the antimicrobial activity of octapeptin and its affinity for membranes. The influence of octapeptin and C2 acyloctapeptin on the rates of ascorbate reduction of several membrane-bound doxyl stearates was also examined. These studies demonstrated that octapeptin increaed the rate of diffusion of ascorbate into the lipid bilayer and suggested that the acyl chain contributed to this activity. In addition, an acyl spin-labeled analogue of octapeptin was prepared and shown to retain biological activity. Spectral analysis showed that octapeptin does not aggregate in solution over a wide concentration range. However, the isotropic splitting constant indicated that the acyl chain of octapeptin is not completely exposed to water. It is proposed that the acyl chain of octapeptin in solution interacts with hydrophobic amino acids in the peptide, which partially shields the acyl chain from water. Spectral features of the spin-labeled antibiotic bound to phospholipid dispersions were consistent with directional binding of octapeptin to lipid bilayers with insertion of the fatty acid into the hydrocarbon domain.     

Interaction of a pseudosubstrate peptide of protein kinase C and its myristoylated form with lipid vesicles: Only the myristoylated form translocates into the lipid bilayer

Lipopeptides derived from protein kinase C (PKC) pseudosubstrates have the ability to cross the plasma membrane in cells and modulate the activity of PKC in the cytoplasm. Myristoylation or palmitoylation appears to promote translocation across membranes, as the non-acylated peptides are membrane impermeant. We have investigated, by fluorescence spectroscopy, how myristoylation modulates the interaction of the PKC pseudosubstrate peptide KSIYRRGARRWRKL with lipid vesicles and translocation across the lipid bilayer. Our results indicate that myristoylated peptides are intimately associated with lipid vesicles and are not peripherally bound. When visualized under a microscope, myristoylation does appear to facilitate translocation across the lipid bilayer in multilamellar lipid vesicles. Translocation does not involve large-scale destabilization of the bilayer structure. Myristoylation promotes translocation into the hydrophobic interior of the lipid bilayer even when the non-acylated peptide has only weak affinity for membranes and is also only peripherally associated with lipid vesicles.     

Interaction of a pseudosubstrate peptide of protein kinase C and its myristoylated form with lipid vesicles: only the myristoylated form translocates into the lipid bilayer

Lipopeptides derived from protein kinase C (PKC) pseudosubstrates have the ability to cross the plasma membrane in cells and modulate the activity of PKC in the cytoplasm. Myristoylation or palmitoylation appears to promote translocation across membranes, as the non-acylated peptides are membrane impermeant. We have investigated, by fluorescence spectroscopy, how myristoylation modulates the interaction of the PKC pseudosubstrate peptide KSIYRRGARRWRKL with lipid vesicles and translocation across the lipid bilayer. Our results indicate that myristoylated peptides are intimately associated with lipid vesicles and are not peripherally bound. When visualized under a microscope, myristoylation does appear to facilitate translocation across the lipid bilayer in multilamellar lipid vesicles. Translocation does not involve large-scale destabilization of the bilayer structure. Myristoylation promotes translocation into the hydrophobic interior of the lipid bilayer even when the non-acylated peptide has only weak affinity for membranes and is also only peripherally associated with lipid vesicles.     

Identification of a voltage-responsive segment of the potential-gated colicin E1 ion channel.

The voltage dependence of channel activity of the bactericidal protein colicin E1 was found to be correlated with insertion into the membrane bilayer of a specific segment of the 178-residue COOH-terminal thermolytic colicin channel peptide. The insertion into the bilayer was detected by an increase in labeling by one of two different lipophilic photoaffinity probes or by a decrease in iodination of peptide tyrosines from the external solution. Imposition of a potassium diffusion potential of -100 mV resulted in an increase of 35-60% in the labeling of the peptide by the lipophilic probe in the bilayer and a concomitant decrease in labeling of Tyr residues in the peptide by the iodination reagent in the external solution. The change in labeling decreased upon dissipation of the membrane potential with a half-time of about 1 min. The labeling change was localized to a 36-residue peptide segment bounded by alanine-425 and by tryptophan-460. This segment containing seven positively charged residues at low pH is a voltage-sensitive region that inserts into the membrane bilayer when the channel is turned on by the potential and is extruded from it when the voltage is removed and the channel is turned off.     

Coupling optical and electrical measurements in artificial membranes: Lateral diffusion of lipids and channel forming peptides in planar bilayers

Planar lipid bilayers (PLB) were prepared by the Montal-Mueller technique in a FRAP system designed to simultaneously measure conductivity across, and lateral diffusion of, the bilayer. In the first stage of the project the FRAP system was used to characterise the lateral dynamics of bilayer lipids with regards to phospholipid composition (headgroup, chain unsaturation etc.), presence of cholesterol and the effect of divalent cations on negatively-charged bilayers. In the second stage of the project, lateral diffusion of two fluorescently-labelled voltage-dependent pore-forming peptides (alamethicin and S4s from Shaker K⁺ channel) was determined at rest and in the conducting state. This study demonstrates the feasibility of such experiments with PLBs, amenable to physical constraints, and thus offers new opportunities for systematic studies of structure-function relationships in membrane-associating molecules.     

Mixtures of a series of homologous hydrophobic peptides with lipid bilayers: a simple model system for examining the protein-lipid interface

The interactions of several members of a homologous series of peptides with the phospholipid bilayer have been examined by using fluorescence and deuterium NMR spectroscopy, differential scanning calorimetry, and measurements of water-to-bilayer partition coefficients. 1,2-Dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayers and tripeptides of the form Ala-X-Ala-O-tert-butyl are used as a model system to probe the influence of amino acid side-chain substitution on the insertion of peptides into membranes and the behavior of peptide/bilayer mixtures. Tripeptides with X = Gly, Ala, Phe, and Trp have been examined. All of the tripeptides are water soluble, and all partition into DMPC bilayer vesicles to some extent. The Gly-containing peptide is the least soluble and the Trp-containing peptide the most soluble in the bilayer. The extent of perturbation of the bilayer structure induced by the peptides parallels their bilayer solubility: the Gly and Ala peptides act as simple impurities while peptides containing bulky aromatic rings cause a phase separation. Changes in the fluorescence properties of the Trp analogue upon incorporation into the bilayer indicate that the Trp side chain is probably immersed in the hydrocarbon region of the bilayer. Peptides of this form should serve as easily modifiable model systems with which to examine details of how the bilayer environment affects peptide conformation, as well as how hydrophobic peptides affect the bilayer structure.     

Lateral diffusion and fluorescence microscope studies on a monoclonal antibody specifically bound to supported phospholipid bilayers

Supported phospholipid bilayers prepared by Langmuir-Blodgett techniques were introduced recently as a new model membrane system [Tamm, L.K., & McConnell, H.M. (1985) Biophys. J. 47, 105-113]. Here, supported bilayers are applied to study the lateral diffusion and lateral distribution of membrane-bound monoclonal antibodies. A monoclonal anti-trinitrophenol antibody was found to bind strongly and with high specificity to supported phospholipid bilayers containing the lipid hapten (trinitrophenyl)phosphatidylethanolamine at various mole fractions. The lateral distribution of the membrane-bound antibodies was studied by epifluorescence microscopy. The bound antibodies aggregated into patches on a host lipid bilayer of dimyristoylphosphatidylcholine below the lipid chain melting phase transition and redistributed uniformly on fluid-phase supported bilayers. Lateral diffusion coefficients and mobile fractions of fluorescent phospholipid analogues and fluorescein-labeled antibodies were measured by fluorescence recovery after pattern photobleaching. The lateral diffusion coefficients of the membrane-bound antibodies resembled those of the phospholipids but were reduced by a factor of 2 in the fluid phase. The lipid chain melting phase transition was also reflected in the lateral diffusion coefficient of the bound antibody but occurred at a temperature about 3 deg higher than the phase transition in supported bilayers of pure phospholipids. The antibody lateral diffusion coefficients decreased in titration experiments monotonically with increasing antibody surface concentrations by a factor of 2-3. Correspondingly, a relatively small decrease of the antibody lateral diffusion coefficient was observed with increasing mole fractions of lipid haptens in the supported bilayer.     

Membrane interaction of Escherichia coli hemolysin: flotation and insertion-dependent labeling by phospholipid vesicles

The 1,024-amino-acid acylated hemolysin of Escherichia coli subverts host cell functions and causes cell lysis. Both activities require insertion of the toxin into target mammalian cell membranes. To identify directly the principal toxin sequences dictating membrane binding and insertion, we assayed the lipid bilayer interaction of native protoxin, stably active toxin, and recombinant peptides. Binding was assessed by flotation of protein-liposome mixtures through density gradients, and insertion was assessed by labeling with a photoactivatable probe incorporated into the target lipid bilayer. Both the active acylated hemolysin and the inactive unacylated protoxin were able to bind and also insert. Ca(2+) binding, which is required for toxin activity, did not influence the in vitro interaction with liposomes. Three overlapping large peptides were expressed separately. A C-terminal peptide including residues 601 to 1024 did not interact in either assay. An internal peptide spanning residues 496 to 831, including the two acylation sites, bound to phospholipid vesicles and showed a low level of insertion-dependent labeling. In vitro acylation had no effect on the bilayer interaction of either this peptide or the full-length protoxin. An N-terminal peptide comprising residues 1 to 520 also bound to phospholipid vesicles and showed strong insertion-dependent labeling, ca. 5- to 25-fold that of the internal peptide. Generation of five smaller peptides from the N-terminal region identified the principal determinant of lipid insertion as the hydrophobic sequence encompassing residues 177 to 411, which is conserved among hemolysin-related toxins.