Wasp mastoparans follow the same mechanism as the cell-penetrating peptide transportan 10

We have been examining the mechanism and kinetics of the interactions of a selected set of peptides with phospholipid membranes in a quantitative manner. This set was chosen to cover a broad range of physical-chemical properties and cell specificities. Mastoparan (masL) and mastoparan X (masX) are two similar peptides from the venoms of the wasps Vespula lewisii and Vespa xanthoptera, respectively, and were chosen to complete the set. The rate constants for masX association with and dissociation from membranes are reported here for the first time. The kinetics of dye efflux induced by both mastoparans from phospholipid vesicles were also examined and quantitatively analyzed. We find that masL and masX follow the same graded kinetic model that we previously proposed for the cell-penetrating peptide transportan 10 (tp10), but with different parameters. This comparison is relevant because tp10 is derived from masL by addition of a mostly nonpolar segment of seven residues at the N-terminus. Tp10 is more active than the mastoparans toward phosphatidylcholine vesicles, but the mastoparans are more sensitive to the effect of anionic lipids. Furthermore, the Gibbs free energies of binding and insertion of the peptides calculated using the Wimley-White transfer scales are in good agreement with the values derived from our experimental data and are useful for understanding peptide behavior.

Thermodynamics of lipid interactions in complex bilayers

The mutual interactions between lipids in bilayers are reviewed, including mixtures of phospholipids, and mixtures of phospholipids and cholesterol (Chol). Binary mixtures and ternary mixtures are considered, with special emphasis on membranes containing Chol, an ordered phospholipid, and a disordered phospholipid. Typically the ordered phospholipid is a sphingomyelin (SM) or a long-chain saturated phosphatidylcholine (PC), both of which have high phase transitions temperatures; the disordered phospholipid is 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) or dioleoylphosphatidylcholine (DOPC). The unlike nearest-neighbor interaction free energies (omega(AB)) between lipids (including Chol), obtained by an variety of unrelated methods, are typically in the range of 0-400 cal/mol in absolute value. Most are positive, meaning that the interaction is unfavorable, but some are negative, meaning it is favorable. It is of special interest that favorable interactions occur mainly between ordered phospholipids and Chol. The interpretation of domain formation in complex mixtures of Chol and phospholipids in terms of phase separation or condensed complexes is discussed in the light of the values of lipid mutual interactions.

Small changes in the primary structure of transportan 10 alter the thermodynamics and kinetics of its interaction with phospholipid vesicles

The kinetics and thermodynamics of binding of transportan 10 (tp10) and four of its variants to phospholipid vesicles, and the kinetics of peptide-induced dye efflux, were compared. Tp10 is a 21-residue, amphipathic, cationic, cell-penetrating peptide similar to helical antimicrobial peptides. The tp10 variants examined include amidated and free peptides, and replacements of tyrosine by tryptophan. Carboxy-terminal amidation or substitution of tryptophan for tyrosine enhance binding and activity. The Gibbs energies of peptide binding to membranes determined experimentally and calculated from the interfacial hydrophobicity scale are in good agreement. The Gibbs energy for insertion into the bilayer core was calculated using hydrophobicity scales of residue transfer from water to octanol and to the membrane/water interface. Peptide-induced efflux becomes faster as the Gibbs energies for binding and insertion of the tp10 variants decrease. If anionic lipids are included, binding and efflux rate increase, as expected because all tp10 variants are cationic and an electrostatic component is added. Whether the most important effect of peptide amidation is the change in charge or an enhancement of helical structure, however, still needs to be established. Nevertheless, it is clear that the changes in efflux rate reflect the differences in the thermodynamics of binding and insertion of the free and amidated peptide groups.

Mechanism of the cell-penetrating peptide transportan 10 permeation of lipid bilayers

The mechanism of the interaction between the cell-penetrating peptide transportan 10 (tp10) and phospholipid membranes was investigated. Tp10 induces graded release of the contents of phospholipid vesicles. The kinetics of peptide association with vesicles and peptide-induced dye efflux from the vesicle lumen were examined experimentally by stopped-flow fluorescence. The experimental kinetics were analyzed by directly fitting to the data the numerical solution of mathematical kinetic models. A very good global fit was obtained using a model in which tp10 binds to the membrane surface and perturbs it because of the mass imbalance thus created across the bilayer. The perturbed bilayer state allows peptide monomers to insert transiently into its hydrophobic core and cross the membrane, until the peptide mass imbalance is dissipated. In that transient state tp10 "catalyzes" dye efflux from the vesicle lumen. These conclusions are consistent with recent reports that used molecular dynamics simulations to study the interactions between peptide antimicrobials and phospholipid bilayers. A thermodynamic analysis of tp10 binding and insertion in the bilayer using water-membrane transfer hydrophobicity scales is entirely consistent with the model proposed. A small bilayer perturbation is both necessary and sufficient to achieve very good agreement with the model, indicating that the role of the lipids must be included to understand the mechanism of cell-penetrating and antimicrobial peptides.

The cooperative response of synaptotagmin I C2A. A hypothesis for a Ca2+-driven molecular hammer

In the current understanding of exocytosis at the nerve terminal, the C2 domain of synaptotagmin (C2A) is presumed to bind Ca2+ and the membrane in a stepwise fashion: cation then membrane as cation increases the affinity of protein for membrane. Fluorescence spectroscopy data were gathered over a variety of lipid and Ca2+ concentrations, enabling the rigorous application of microscopic binding models derived from partition functions to differentiate between Ca2+ and phosphatidylserine contributions to binding. The data presented here are in variance with previously published models, which were based on the Hill approximation. Rather, the data are consistent with two forms of cooperativity that modulate the responsiveness of C2A: in Ca2+ binding to a network of three cation sites and in interaction with the membrane surface. We suggest synaptotagmin I C2A is preassociated with the synaptic vesicle membrane or nerve terminal. In this state, upon Ca2+ influx the protein will bind the three Ca2+ ions immediately and with high cooperativity. Thus, membrane association creates a high-affinity Ca2+ switch that is the basis for the role of synaptotagmin I in Ca2+-regulated exocytosis. Based on this model, we discuss the implications of protein-induced phosphatidylserine demixing to the exocytotic process.

Investigation of domain formation in sphingomyelin/cholesterol/POPC mixtures by fluorescence resonance energy transfer and Monte Carlo simulations

We have recently proposed a phase diagram for mixtures of porcine brain sphingomyelin (BSM), cholesterol (Chol), and 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) on the basis of kinetics of carboxyfluorescein efflux induced by the amphipathic peptide δ-lysin. Although that study indicated the existence of domains, phase separations in the micrometer scale have not been observed by fluorescence microscopy in BSM/Chol/POPC mixtures, though they have for some other sphingomyelins (SM). Here we examine the same BSM/Chol/POPC system by a combination of fluorescence resonance energy transfer (FRET) and Monte Carlo simulations. The results clearly demonstrate that domains are formed in this system. Comparison of the FRET experimental data with the computer simulations allows the estimate of lipid-lipid interaction Gibbs energies between SM/Chol, SM/POPC, and Chol/POPC. The latter two interactions are weakly repulsive, but the interaction between SM and Chol is favorable. Furthermore, those three unlike lipid interaction parameters between the three possible lipid pairs are sufficient for the existence of a closed loop in the ternary phase diagram, without the need to involve multibody interactions. The calculations also indicate that the largest POPC domains contain several thousand lipids, corresponding to linear sizes of the order of a few hundred nanometers.

Temperature and composition dependence of the interaction of delta-lysin with ternary mixtures of sphingomyelin/cholesterol/POPC

The kinetics of carboxyfluorescein efflux induced by the amphipathic peptide delta-lysin from vesicles of porcine brain sphingomyelin (BSM), 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC), and cholesterol (Chol) were investigated as a function of temperature and composition. Sphingomyelin (SM)/Chol mixtures form a liquid-ordered (L(o)) phase whereas POPC exists in the liquid-disordered (L(d)) phase at ambient temperature. delta-Lysin binds strongly to L(d) and poorly to L(o) phase. In BSM/Chol/POPC vesicles the rate of carboxyfluorescein efflux induced by delta-lysin increases as the POPC content decreases. This is explained by the increase of delta-lysin concentration in L(d) domains, which enhances membrane perturbation by the peptide. Phase separations in the micrometer scale have been observed by fluorescence microscopy in SM/Chol/POPC mixtures for some SM, though not for BSM. Thus, delta-lysin must detect heterogeneities (domains) in BSM/Chol/POPC on a much smaller scale. Advantage was taken of the inverse variation of the efflux rate with the L(d) content of BSM/Chol/POPC vesicles to estimate the L(d) fraction in those mixtures. These results were combined with differential scanning calorimetry to obtain the BSM/Chol/POPC phase diagram as a function of temperature.

A simple theory of peptide interactions on a membrane surface: Excluded volume and entropic order

A simple theory of the interactions of peptides bound onto a lipid membrane is developed, modeling the peptides as rods on a surface. At low peptide surface-concentration, excluded volume dominates the peptide-peptide interactions and the orientation of the peptides is random, resulting in an isotropic configuration. However, at high peptide density on the membrane, the peptides become orientationally ordered, resulting in an anisotropic configuration. This effect is entirely entropic in origin, and simply reflects the fact that peptides can be exchanged more easily on the surface if they are equally aligned, resulting in a larger number of possible configurations. In three dimensions, this phenomenon corresponds to the well-known isotropic-nematic phase transition. In two dimensions, the problem is not as well understood. The theoretical treatment presented here yields a simple, manageable expression which can be compared with experimental data. Two-dimensional ordering results in an increase in the apparent binding constant of peptides to membranes at high concentration of peptides relative to what is expected from the effect of excluded volume alone. The possible implications of side-by-side alignment for several biological processes, such as peptide translocation across membranes and plaque formation in Alzheimer's disease, are discussed.

Thermodynamics of membrane domains

The concept of lipid rafts and the intense work toward their characterization in biological membranes has spurred a renewed interest in the understanding of domain formation, particularly in the case of cholesterol-containing membranes. The thermodynamic principles underlying formation of domains, rafts, or cholesterol/phospholipid complexes are reviewed here, along with recent work in model and biological membranes. A major motivation for this review was to present those concepts in a way appropriate for the broad readership that has been drawn to the field. Evidence from a number of different techniques points to the conclusion that lipid-lipid interactions are generally weak; therefore, in most cases, massive phase separations are not to be expected in membranes. On the contrary, small, dynamic lipid domains, possibly stabilized by proteins are the most likely outcome. The results on mixed lipid bilayers are used to discuss recent experiments in biological membranes. The clear indication is that proteins partition preferentially into fluid, disordered lipid domains, which is contrary to their localization in ordered, cholesterol/sphingomyelin rafts inferred from detergent extraction experiments on cell membranes. Globally, the evidence appears most consistent with a membrane model in which the majority of the lipid is in a liquid-ordered phase, with dispersed, small, liquid-disordered domains, where most proteins reside. Co-clustering of proteins and their concentration in some membrane areas may occur because of similar preferences for a particular domain but also because of simultaneous exclusion from other lipid phases. Specialized structures, such as caveolae, which contain high concentrations of cholesterol and caveolin are not necessarily similar to bulk liquid-ordered phase.

Permeabilization of raft-containing lipid vesicles by delta-lysin: a mechanism for cell sensitivity to cytotoxic peptides

Delta-lysin is a linear, 26-residue peptide that adopts an alpha-helical, amphipathic structure upon binding to membranes. Delta-lysin preferentially binds to mammalian cell membranes, the outer leaflets of which are enriched in sphingomyelin, cholesterol, and unsaturated phosphatidylcholine. Mixtures including these lipids have been shown to exhibit separation between liquid-disordered (l(d)) and liquid-ordered (l(o)) domains. When rich in sphingomyelin and cholesterol, these ordered domains have been called lipid "rafts". We found that delta-lysin binds poorly to the l(o) (raft) domains; therefore, in mixed-phase lipid vesicles, delta-lysin preferentially binds to the l(d) domains. This leads to the concentration of delta-lysin in l(d) domains, enhancing peptide aggregation and, consequently, the rate of peptide-induced dye efflux from lipid vesicles. The efficient lysis of eukaryotic cells by delta-lysin can thus be attributed not to specific delta-lysin-cholesterol or delta-lysin-sphingomyelin interactions but, rather, to the exclusion of delta-lysin from ordered rafts. The degree to which the kinetics of dye efflux are enhanced in mixed-phase vesicles over those observed in pure, unsaturated phosphatidylcholine vesicles directly reflects the amount of l(d) phase present in mixed-phase systems. This effect of lipid domains has broader consequences, beyond the hemolytic efficiency of delta-lysin. We discuss the hypothesis that bacterial sensitivity to antimicrobial peptides may be determined by a similar mechanism.

Allosterism in Membrane Binding: A Common Motif of the Annexins?

Annexins are a family of proteins generally described as Ca(2+)-dependent for phospholipid binding. Yet, annexins have a wide variety of binding behaviors and conformational states, some of which are lipid-dependent and Ca(2+)-independent. We present a model that captures the cation and phospholipid binding behavior of the highly conserved core of the annexins. Experimental data for annexins A4 and A5, which have short N-termini, were globally modeled to gain an understanding of how the lipid-binding affinity of the conserved protein core is modulated. Analysis of the binding behavior was achieved through use of the lanthanide Tb(3+) as a Ca(2+) analogue. Binding isotherms were determined experimentally from the quenching of the intrinsic fluorescence of annexins A4 and A5 by Tb(3+) in the presence or absence of membranes. In the presence of lipid, the affinity of annexin for cation increases, and the binding isotherms change from hyperbolic to weakly sigmoidal. This behavior was modeled by isotherms derived from microscopic binding partition functions. The change from hyperbolic to sigmoidal binding occurs because of an allosteric transition from the annexin solution state to its membrane-associated state. Protein binding to lipid bilayers renders cation binding by annexins cooperative. The two annexin states denote two affinities of the protein for cation, one in the absence and another in the presence of membrane. In the framework of this model, we discuss membrane binding as well as the influence of the N-terminus in modifying the annexin cation-binding affinity by changing the probability of the protein to undergo the postulated two-state transition.

Kinetics of dye efflux and lipid flip-flop induced by delta-lysin in phosphatidylcholine vesicles and the mechanism of graded release by amphipathic, alpha-helical peptides

Delta-lysin is a 26-residue, amphipathic, alpha-helical peptide of bacterial origin. Its specificity is to some extent complementary to that of antimicrobial peptides. Therefore, understanding its mechanism is important for the more general goal of understanding the interaction of amphipathic peptides with membranes. In this article, we show that delta-lysin induces graded efflux of the contents of phosphatidylcholine vesicles. In view of this finding, carboxyfluorescein efflux kinetics were re-examined. In addition, peptide-induced lipid flip-flop was directly measured using fluorescence energy transfer between two lipid fluorophores initially placed on opposite leaflets of the bilayer. Carboxyfluorescein efflux and lipid flip-flop occur with essentially identical rate constants. On the basis of a detailed, quantitative analysis of the kinetics of peptide-vesicle interactions, we conclude that the peptide translocates across the bilayer as a small, transient aggregate, most likely a trimer. Dye efflux and lipid flip-flop occur concomitantly with the transient peptide-induced perturbation of the membrane. The experimental data are interpreted by comparing the predictions of the available models for the mechanism of action of amphipathic alpha-helical peptides. We demonstrate how the combination of the quantitative kinetic analysis, graded efflux, and reversibility of the peptide-vesicle interaction can be used to reject several models for this particular peptide. Two models are compatible with the data, the toroidal pore model and the sinking raft model. On the basis of the small aggregate size, a trimer, the latter appears to be more plausible. Some significant modifications are introduced in the sinking raft model to take into account the new finding of graded dye release. Furthermore, we present an explanation for the phenomenon of graded release in general, which, contrary to all-or-none efflux, has not been well-understood.

Lipid modulation of protein-induced membrane domains as a mechanism for controlling signal transduction

The reason for the enormous lipid variety present in eukaryotic membranes remains largely an enigma. We suggest that its role is to provide an on-off switch for a signaling event at the membrane level. This is achieved through lipid-lipid interactions that convert membrane protein binding and association events into very cooperative processes while maintaining reversibility. We have previously shown [Hinderliter, A., at al. (2001) Biochemistry 40, 4181-4191] that thermodynamic linkage between an intrinsic tendency for lipid demixing and a preferential interaction of a protein with a specific lipid within the mixture leads to dramatic changes in lipid and protein domain formation. Here, we tested the hypothesis that small alterations in lipid chemical structure alter the magnitude of the net interaction free energy (omega(AB)) between unlike lipids in a predictable manner, and that even very small changes in omega(AB) lead to dramatic changes in bilayer organization when coupled with protein binding. We systematically varied the chemical structure of phosphatidylcholine (PC), in mixtures with a fixed phosphatidylserine (PS), by changing the PC acyl chain length and the degree of unsaturation, and examined domain formation upon addition of a peripheral protein, the synaptotagmin I C2A motif. Experimental excimer/monomer ratios (E/M) of pyrene-substituted lipids mimicking the PS were interpreted using Monte Carlo computer simulations. E/M is larger if the PC melting temperature is lower, suggesting that domain formation is a thermodynamic consequence of weak interactions between PC and PS. Consistent with our hypothesis, only very small changes in omega(AB) were required for prediction of large changes in lipid and protein domain formation.

Mechanism and kinetics of delta-lysin interaction with phospholipid vesicles

Delta-lysin is a 26 amino acid, hemolytic peptide toxin secreted by Staphylococcus aureus. It has been reported to form an amphipathic helix upon binding to lipid bilayers and is often cited as a typical example of the barrel-stave model for pore formation in lipid bilayer membranes. However, the exact mechanism by which it lyses cells and the physical basis of its target specificity are still unknown. Moreover, the evidence for delta-lysin insertion and pore formation in the membrane stems largely from theoretical modeling of the toxin and lacks experimental confirmation. We investigated binding and insertion of delta-lysin into phospholipid bilayer vesicles. The kinetics of these processes were studied by stopped-flow fluorescence with two types of experiments: (a) carboxyfluorescein release from the vesicles upon peptide-vesicle interaction, with concomitant relief of dye self-quenching; (b) fluorescence energy transfer from the intrinsic tryptophan of the peptide to a membrane-bound lipid probe. We formulated a detailed kinetic mechanism with explicit molecular rate constants for peptide binding, association, and insertion, obtaining a quantitative description of the experimental results. delta-Lysin insertion is strongly dependent on the peptide-to-lipid ratio, suggesting that association of a critical number of monomers on the membrane is required for activity. However, we found no evidence for a stable membrane-inserted pore. Rather, the peptide appears to cross the membrane rapidly and reversibly and cause release of the lipid vesicle contents in this process.