Role of lipids in the permeabilization of membranes by class L amphipathic helical peptides

Lipid tails modulate antimicrobial peptide membrane incorporation and activity

Membrane disrupting antimicrobial peptides (AMPs) are often amphipathic peptides that interact directly with lipid bilayers. AMPs are generally thought to interact mostly with lipid head groups, but it is less clear how the lipid alkyl chain length and saturation modulate interactions with membranes. Here, we used native mass spectrometry to measure the stoichiometry of three different AMPs-LL-37, indolicidin, and magainin-2-in lipid nanodiscs. We also measured the activity of these AMPs in unilamellar vesicle leakage assays. We found that LL-37 formed specific hexamer complexes but with different intermediates and affinities that depended on the bilayer thickness. LL-37 was also most active in lipid bilayers containing longer, unsaturated lipids. In contrast, indolicidin incorporated to a higher degree into more fluid lipid bilayers but was more active with bilayers with thinner, less fluid lipids. Finally, magainin-2 incorporated to a higher degree into bilayers with longer, unsaturated alkyl chains and showed more activity in these same conditions. Together, these data show that higher amounts of peptide incorporation generally led to higher activity and that AMPs tend to incorporate more into longer unsaturated lipid bilayers. However, the activity of AMPs was not always directly related to amount of peptide incorporated.

Assembly of MreB filaments on liposome membranes: a synthetic biology approach

The physical interaction between the cytoskeleton and the cell membrane is essential in defining the morphology of living organisms. In this study, we use a synthetic approach to polymerize bacterial MreB filaments inside phospholipid vesicles. When the proteins MreB and MreC are expressed inside the liposomes, the MreB cytoskeleton structure develops at the inner membrane. Furthermore, when purified MreB is used inside the liposomes, MreB filaments form a 4-10 μm rigid bundle structure and deform the lipid vesicles in physical contact with the vesicle inner membrane. These results indicate that the fibrillation of MreB filaments can take place either in close proximity of deformable lipid membrane or in the presence of associated protein. Our finding might be relevant for the self-assembly of cytoskeleton filaments toward the construction of artificial cell systems.

On the role of anionic lipids in charged protein interactions with membranes

We investigate the role of anionic lipids in the binding to, and subsequent movement of charged protein groups in lipid membranes, to help understand the role of membrane composition in all membrane-active protein sequences. We demonstrate a small effect of phosphatidylglycerol (PG) lipids on the ability of an arginine (Arg) side chain to bind to, and cross a lipid membrane, despite possessing a neutralizing charge. We observe similar membrane deformations in lipid bilayers composed of phosphatidylcholine (PC) and PC/PG mixtures, with comparable numbers of water and lipid head groups pulled into the bilayer hydrocarbon core, and prohibitively large ~20 kcal/mol barriers for Arg transfer across each bilayer, dropping by just 2-3 kcal/mol due to the binding of PG lipids. We explore the causes of this small effect of introducing PG lipids and offer an explanation in terms of the limited membrane interaction for the choline groups of PC lipids bound to the translocating ion. Our calculations reveal a surprising lack of preference for Arg binding to PG lipids themselves, but a small increase in interfacial binding affinity for lipid bilayers containing PG lipids. These results help to explain the nature of competitive lipid binding to charged protein sequences, with implications for a wide range of membrane binding domains and cell perturbing peptides.

Amphipathic helices and membrane curvature

Numerous data have been collected on lipid-binding amphipathic helices involved in membrane-remodeling machineries and vesicular transport. Here we describe how, with regard to lipid composition, the physicochemical features of some amphipathic helices explain their ability to recognize membrane curvature or to participate in membrane remodeling. We propose that sensing highly-curved membranes requires that the polar and hydrophobic faces of the helix do not cooperate in lipid binding. A more detailed description of the interaction between amphipathic helices and lipids is however needed; notably to explain how new helices contribute to detection of modest changes in curvature or even negative curvature.

The anticancer activity of lytic peptides is inhibited by heparan sulfate on the surface of the tumor cells

BACKGROUND

Cationic antimicrobial peptides (CAPs) with antitumor activity constitute a promising group of novel anticancer agents. These peptides induce lysis of cancer cells through interactions with the plasma membrane. It is not known which cancer cell membrane components influence their susceptibility to CAPs. We have previously shown that CAPs interact with the two glycosaminoglycans (GAGs), heparan sulfate (HS) and chondroitin sulfate (CS), which are present on the surface of most cells. The purpose of this study was to investigate the role of the two GAGs in the cytotoxic activity of CAPs.

METHODS

Various cell lines, expressing different levels of cell surface GAGs, were exposed to bovine lactoferricin (LfcinB) and the designer peptide, KW5. The cytotoxic effect of the peptides was investigated by use of the colorimetric MTT viability assay. The cytotoxic effect on wild type CHO cells, expressing normal amounts of GAGs on the cell surface, and the mutant pgsA-745, that has no expression of GAGs on the cell surface, was also investigated.

RESULTS

We show that cells not expressing HS were more susceptible to CAPs than cells expressing HS at the cell surface. Further, exogenously added heparin inhibited the cytotoxic effect of the peptides. Chondroitin sulfate had no effect on the cytotoxic activity of KW5 and only minor effects on LfcinB cytotoxicity.

CONCLUSION

Our results show for the first time that negatively charged molecules at the surface of cancer cells inhibit the cytotoxic activity of CAPs. Our results indicate that HS at the surface of cancer cells sequesters CAPs away from the phospholipid bilayer and thereby impede their ability to induce cytolysis.

Antimicrobial peptides: New candidates in the fight against bacterial infections

Antimicrobial peptides (AMPs) of innate origin are agents of the most ancient form of defense systems. They can be found in a wide variety of species ranging from bacteria through insects to humans. Through the course of evolution, host organisms developed arsenals of AMPs that protect them against a large variety of invading pathogens including both Gram-negative and Gram-positive bacteria. At a time of increasing bacterial resistance, AMPs have been the focus of investigation in a number of laboratories worldwide. Although recent studies show that some of the peptides are likely to have intracellular targets, the vast majority of AMPs appear to act by permeabilization of the bacterial cell membrane. Their activity and selectivity are governed by the physicochemical parameters of the peptide chains as well as the properties of the membrane system itself. In this review, we will summarize some of the recent developments that provide us with a better understanding of the mode of action of this unique family of antibacterial agents. Particular attention will be given to the determinants of AMP-lipid bilayer interactions as well as to the different pore formation mechanisms. The emphasis will be on linear AMPs but representatives of cysteine-bridged AMPs will also be discussed.

Nanotubules formed by highly hydrophobic amphiphilic alpha-helical peptides and natural phospholipids

We previously reported that the 18-mer amphiphilic alpha-helical peptide, Hel 13-5, consisting of 13 hydrophobic residues and five hydrophilic amino acid residues, can induce neutral liposomes (egg yolk phosphatidylcholine) to adopt long nanotubular structures and that the interaction of specific peptides with specific phospholipid mixtures induces the formation of membrane structures resembling cellular organelles such as the Golgi apparatus. In the present study we focused our attention on the effects of peptide sequence and chain length on the nanotubule formation occurring in mixture systems of Hel 13-5 and various neutral and acidic lipid species by means of turbidity measurements, dynamic light scattering measurements, and electron microscopy. We designed and synthesized two sets of Hel 13-5 related peptides: 1) Five peptides to examine the role of hydrophobic or hydrophilic residues in amphiphilic alpha-helical structures, and 2) Six peptides to examine the role of peptide length, having even number residues from 12 to 24. Conformational, solution, and morphological studies showed that the amphiphilic alpha-helical structure and the peptide chain length (especially 18 amino acid residues) are critical determinants of very long tubular structures. A mixture of alpha-helix and beta-structures determines the tubular shapes and assemblies. However, we found that the charged Lys residues comprising the hydrophilic regions of amphiphilic structures can be replaced by Arg or Glu residues without a loss of tubular structures. This suggests that the mechanism of microtubule formation does not involve the charge interaction. The immersion of the hydrophobic part of the amphiphilic peptides into liposomes initially forms elliptic-like structures due to the fusion of small liposomes, which is followed by a transformation into tubular structures of various sizes and shapes.

The apoptotic protein tBid promotes leakage by altering membrane curvature

The apoptotic protein tBid is effective in promoting both leakage and lipid mixing in liposomes composed of cardiolipin and phosphatidylcholine at a molar ratio of 1:2 in the presence of calcium. When half of the phosphatidylcholine component of these liposomes is replaced with phosphatidylethanolamine, a lipid that promotes negative membrane curvature, the rates of both leakage and lipid mixing caused by tBid are substantially increased. Replacement of cardiolipin with phosphatidylglycerol, a lipid that is structurally similar to cardiolipin but does not promote negative membrane curvature in the presence of calcium, prevents the tBid from promoting leakage. The promotion of leakage by tBid is also inhibited by several substances that promote positive membrane curvature, including lysophosphatidylcholine, tritrpticin, a potent antimicrobial peptide, and cyclosporin A, a known inhibitor of cytochrome c release from mitochondria. We directly measured the effect of tBid on membrane curvature by (31)P NMR. We found that tBid promotes the formation of highly curved non-lamellar phases. All of these data are consistent with the hypothesis that tBid promotes negative curvature, and as a result it destabilizes bilayer membranes. Bcl-X(L) inhibits leakage and lipid mixing induced by tBid. Bcl-X(L) is anti-apoptotic. It reduces the promotion of non-bilayer phases by tBid, although by itself Bcl-X(L) is capable of promoting their formation. Bcl-X(L) has little effect on liposomal integrity. Our results suggest that the anti-apoptotic activity of Bcl-X(L) is not a consequence of its interaction with membranes, but rather with other proteins, such as tBid.

Osmotically induced membrane tension modulates membrane permeabilization by class L amphipathic helical peptides: nucleation model of defect formation

The mechanism of action of lytic peptides on membranes is widely studied and is important in view of potential medical applications. Previously (I. V. Polozov, A. I. Polozova, E. M. Tytler, G. M. Anantharamaiah, J. P. Segrest, G. A. Woolley, and R. M., Biochemistry, 36:9237--9245) we analyzed the mechanism of membrane permeabilization by 18L, the archetype lytic peptide featuring the class L amphipathic alpha-helix, according to the classification of Segrest et al. (J. P. Segrest, G. de Loof, J. G. Dohlman, C. G. Brouillette, and G. M. Anantharamaiah, 1990, Proteins, 8:103--117). We concluded that the 18L peptide destabilizes membranes, leading to a transient formation of large defects that result in contents leakage and, in the presence of bilayer-bilayer contact, could lead to vesicle fusion. Here we report that this defect formation is strongly enhanced by the membrane tension induced by osmotic swelling of vesicles. Even below standard leakage-inducing peptide/lipid ratios, membrane resistance to osmotic tension drops from hundreds to tens of milliosmoles. The actual decrease is dependent on the peptide/lipid ratio and on the type of lipid. We propose that under membrane tension a peptidic pore serves as a nucleation site for the transient formation of a lipidic pore. The tension is released upon pore expansion with inclusion of more peptides and lipids into the pore lining. This tension modulation of leakage was observed for other class L peptides (mastoparan, K18L) and thus may be of general applicability for the action of membrane active lytic peptides.

Effect of magainin, class L, and class A amphipathic peptides on fatty acid spin labels in lipid bilayers

Magainins and other antimicrobial peptides increase ion flux across the membrane. They may do this by forming some type of pore or by perturbing lipid organization due to peptide lying on the bilayer surface. In order to determine if magainins perturb the lipid sufficiently to permeabilize the bilayer, their effect on the motion of fatty acid and lipid spin labels in phosphatidylcholine/phosphatidylglycerol (PC/PG) lipid vesicles was determined. Their effect was compared to two synthetic peptides, 18L and Ac-18A-NH(2), designed to mimic the naturally occurring classes of lytic (class L) and apolipoprotein (class A) amphipathic helices, respectively. We show that although magainins and 18L both had significant effects on lipid chain order, much greater than Ac-18A-NH(2), there was no correlation between these effects and the relative ability of these three peptide classes to permeabilize PC/PG vesicles in the order magainins=Ac-18A-NH(2) >> 18L. This suggests that the perturbing effects of magainins on lipid chain order at permeabilizing concentrations are not directly responsible for the increased leakage of vesicle contents. The greater ability of the magainins to permeabilize PC/PG vesicles relative to 18L is thus more likely due to formation of some type of pore by magainins. The greater ability of Ac-18A-NH(2) relative to 18L to permeabilize PC/PG vesicles despite its lack of disordering effect must be due to its ability to cause membrane fragmentation. Effects of these peptides on other lipids indicated that the mechanism by which they permeabilize lipid bilayers depends both on the peptide and on the lipid composition of the vesicles.

Effects of the antibiotic peptide microcin J25 on liposomes: role of acyl chain length and negatively charged phospholipid

This paper reports the effects of microcin J25 (MccJ25) on the microviscosity and permeability of phospholipid vesicles of different compositions. The results obtained indicate that MccJ25 interacts with egg L-alpha-phosphatidylcholine (PC) vesicles as demonstrated by peptide intrinsic fluorescence determinations. The interaction depends on the lipid composition of the vesicles. MccJ25 interaction induces a significant fluidity increase of egg PC vesicles. This effect is time and concentration dependent. Both trimethyl ammonium 1,6-diphenyl-1,3,5-hexatriene and 1,6-diphenyl-1, 3,5-hexatriene gave the same results. The microviscosity of L-alpha-phosphatidylcholine dipalmitoyl small unilamellar vesicles (SUVs) was affected while that of L-alpha-phosphatidylcholine dimyristoyl vesicles was not, indicating that the effect was strongly dependent on the chain length of fatty acids. On the other hand, negatively charged L-alpha-phosphatidyl-DL-glycerol (PG) vesicles remarkably inhibited the peptide effect. Nevertheless vesicles composed of L-alpha-phosphatidylethanolamine:PG:cardiolipin (7:2:1), a composition resembling bacterial membrane, were sensitive to the MccJ25 effect. MccJ25 effectively dissipated the valinomycin-induced membrane potential, but induced only a modest leakage (5%) of the trapped Tb(+3)-dipicolinic acid complex. These results indicate that the peptides interact and perturb the bilayer of SUVs. The relationships between this effect and bactericidal action remain to be elucidated.

Design of a novel membrane-destabilizing peptide selectively acting on acidic liposomes

The design of amphipathic peptides resulted in a novel peptide with a selective ability to destabilize lipid bilayers of acidic liposomes. The newly synthesized peptide, termed mast 21, is a 21-residue long amino acid chain and can only act effectively on acidic liposomes lacking cholesterol. Moreover, mast 21 killed gram-positive and gram-negative bacteria, and it had no hemolytic activity. The antimicrobial and hemolytic activities paralleled the results of membrane destabilizing activity using liposomes. Circular dichroism and Trp-fluorescence emission spectra showed changes in the peptide conformation and circumstances around the peptide during interaction with liposomes. These changes were consistent with an increased alpha-helical content and a less polar environment for the tryptophan residue of the peptide. Mast 21 was observed under dark-field microscopy in real time attacking liposomes. Acidic liposomes were attacked, which resulted in peeling of the lipid bilayer with its subsequent destruction.

Location of tryptophan residues in free and membrane bound Escherichia coli alpha-hemolysin and their role on the lytic membrane properties

alpha-hemolysin (HlyA) is an extracellular protein toxin secreted by Escherichia coli that acts at the level of plasma cell membranes of target eukaryotic cells. Previous studies showed that toxin binding to the bilayers occurs in at least two ways, a reversible adsorption and an irreversible insertion. Studies of HlyA insertion into bilayers formed from phosphatidylcholine show that insertion is accompanied by an increase in the protein intrinsic fluorescence. In order to better define structural parameters of the membrane-bound form, the location of tryptophan residues was studied by means of quenchers of their intrinsic fluorescence located at 7, 12 and 16 positions of the acyl chain of phosphatidylcholine. The quenching was progressively weaker suggesting an interfacial location of the Trp. In parallel, HlyA was subjected to oxidation with N-bromosuccinimide to study the role of Trp residues exposed to aqueous media in its structure-function relationship. In the folded toxin molecule, a single residue was susceptible to oxidation with NBS, whereas incubation with LUV of the toxin prior modification prevents its oxidation, suggesting that Trp residue(s) are directly involved in toxin binding and insertion. Finally, the modification of residues exposed to solvent resulted in a complete impairment of the lytic activity. It was concluded that the modification-sensitive Trp residues are essential for the structure and function of native HlyA. These results are consistent with the model proposed by Soloaga et al. (Mol. Microbiol. 31 (1999) 1013-1024) according to which HlyA is bound to a single monolayer through a number of amphipathic instead of inserted transmembrane helices.

Diversity of antimicrobial peptides and their mechanisms of action

Antimicrobial peptides encompass a wide variety of structural motifs. Many peptides have alpha-helical structures. The majority of these peptides are cationic and amphipathic but there are also hydrophobic alpha-helical peptides which possess antimicrobial activity. In addition, some beta-sheet peptides have antimicrobial activity and even antimicrobial alpha-helical peptides which have been modified to possess a beta-structure retain part of their antimicrobial activity. There are also antimicrobial peptides which are rich in a certain specific amino acid such as Trp or His. In addition, antimicrobial peptides exist with thio-ether rings, which are lipopeptides or which have macrocyclic Cys knots. In spite of the structural diversity, a common feature of the cationic antimicrobial peptides is that they all have an amphipathic structure which allows them to bind to the membrane interface. Indeed, most antimicrobial peptides interact with membranes and may be cytotoxic as a result of disturbance of the bacterial inner or outer membranes. Alternatively, a necessary but not sufficient property of these peptides may be to be able to pass through the membrane to reach a target inside the cell. The interaction of these peptides with biological membranes is not just a function of the peptide but is also modulated by the lipid components of the membrane. It is not likely that this diverse group of peptides has a single mechanism of action, but interaction of the peptides with membranes is an important requirement for most, if not all, antimicrobial peptides.

Mechanism of the binding, insertion and destabilization of phospholipid bilayer membranes by alpha-helical antimicrobial and cell non-selective membrane-lytic peptides

Permeation of the cell membrane leading to cell death is a mechanism used by a large number of membrane-lytic peptides. Some are linear, mostly helical, and others contain one or more disulfide bonds forming beta-sheet or both beta-sheet and alpha-helix structures. They are all soluble in solution but when they reach the target membrane, conformational changes occur which let them associate with and lyse the membrane. Some lytic peptides are not cell-selective and lyse different microorganisms and normal mammalian cells, while others are specific to either type of cells. Despite extensive studies, the mode of action of membrane-lytic peptides is not fully understood and the basis for their selectivity towards specific target cells is not known. Many studies have shown that peptide-lipid interactions leading to membrane permeation play a major role in their activity. Membrane permeation by amphipathic alpha-helical peptides has been proposed to occur via one of two general mechanisms: (i) transmembrane pore formation via a 'barrel-stave' mechanism; and (ii) membrane destruction/solubilization via a 'carpet' mechanism. This review, which is focused on the different stages of membrane permeation induced by representatives of amphipathic alpha-helical antimicrobial and cell non-selective lytic peptides distinguishes between the 'carpet' mechanism, which holds for antimicrobial peptides versus the 'barrel-stave' mechanism, which holds for cell non-selective lytic peptides.

The structure of the antimicrobial active center of lactoferricin B bound to sodium dodecyl sulfate micelles

Lactoferricin B (LfcinB) is a 25-residue antimicrobial peptide released from bovine lactoferrin upon pepsin digestion. The antimicrobial center of LfcinB consists of six residues (RRWQWR-NH2), and it possesses similar bactericidal activity to LfcinB. The structure of the six-residue peptide bound to sodium dodecyl sulfate (SDS) micelles has been determined by NMR spectroscopy and molecular dynamics refinement. The peptide adopts a well defined amphipathic structure when bound to SDS micelles with the Trp sidechains separated from the Arg residues. Additional evidence demonstrates that the peptide is oriented in the micelle such that the Trp residues are more deeply buried in the micelle than the Arg and Gln residues.

Morphological behavior of acidic and neutral liposomes induced by basic amphiphilic alpha-helical peptides with systematically varied hydrophobic-hydrophilic balance

Lipid-peptide interaction has been investigated using cationic amphiphilic alpha-helical peptides and systematically varying their hydrophobic-hydrophilic balance (HHB). The influence of the peptides on neutral and acidic liposomes was examined by 1) Trp fluorescence quenched by brominated phospholipid, 2) membrane-clearing ability, 3) size determination of liposomes by dynamic light scattering, 4) morphological observation by electron microscopy, and 5) ability to form planar lipid bilayers from channels. The peptides examined consist of hydrophobic Leu and hydrophilic Lys residues with ratios 13:5, 11:7, 9:9, 7:11, and 5:13 (abbreviated as Hels 13-5, 11-7, 9-9, 7-11, and 5-13, respectively; Kiyota, T., S. Lee, and G. Sugihara. 1996. Biochemistry. 35:13196-13204). The most hydrophobic peptide (Hel 13-5) induced a twisted ribbon-like fibril structure for egg PC liposomes. In a 3/1 (egg PC/egg PG) lipid mixture, Hel 13-5 addition caused fusion of the liposomes. Hel 13-5 formed ion channels in neutral lipid bilayer (egg PE/egg PC = 7/3) at low peptide concentrations, but not in an acidic bilayer (egg PE/brain PS = 7/3). The peptides with hydrophobicity less than Hel 13-5 (Hels 11-7 and Hel 9-9) were able to partially immerse their hydrophobic part of the amphiphilic helix in lipid bilayers and fragment liposome to small bicelles or micelles, and then the bicelles aggregated to form a larger assembly. Peptides Hel 11-7 and Hel 9-9 each formed strong ion channels. Peptides (Hel 7-11 and Hel 5-13) with a more hydrophilic HHB interacted with an acidic lipid bilayer by charge interaction, in which the former immerses the hydrophobic part in lipid bilayer, and the latter did not immerse, and formed large assemblies by aggregation of original liposomes. The present study clearly showed that hydrophobic-hydrophilic balance of a peptide is a crucial factor in understanding lipid-peptide interactions.

Insertion of Escherichia coli alpha-haemolysin in lipid bilayers as a non-transmembrane integral protein: prediction and experiment

alpha-Haemolysin is an extracellular protein toxin (approximately 107 kDa) secreted by Escherichia coli that acts at the level of the plasma membranes of target eukaryotic cells. The nature of the toxin interaction with the membrane is not known at present, although it has been established that receptor-mediated binding is not essential. In this work, we have studied the perturbation produced by purified alpha-haemolysin on pure phosphatidylcholine bilayers in the form of large unilamellar vesicles, under conditions in which the toxin has been shown to induce vesicle leakage. The bilayer systems containing bound protein have been examined by differential scanning calorimetry, fluorescence spectroscopy, differential solubilization by Triton X-114, and freeze-fracture electron microscopy. All the data concur in indicating that alpha-haemolysin, under conditions leading to cell lysis, becomes inserted in the target membrane in the way of intrinsic or integral proteins. In addition, the experimental results support the idea that inserted alpha-haemolysin occupies only one of the membrane phospholipid monolayers, i.e. it is not a transmembrane protein. The experimental data are complemented by structure prediction studies according to which as many as ten amphipathic alpha-helices, appropriate for protein-lipid interaction, but no hydrophobic transmembrane helices are predicted in alpha-haemolysin. These observations and predictions have important consequences for the mechanism of cell lysis by alpha-haemolysin; in particular, a non-transmembrane arrangement of the toxin in the target membrane is not compatible with the concept of alpha-haemolysin as a pore-forming toxin.

The amphipathic helix concept: length effects on ideally amphipathic LiKj(i=2j) peptides to acquire optimal hemolytic activity

In a minimalist approach to modeling lytic toxins, amphipathic peptides of LiKj with i=2j composition and whose length varies from 5 to 22 residues were studied for their ability to induce hemolysis and lipid vesicle leakage. Their sequences were designed to generate ideally amphipathic alpha helices with a single K residue per putative turn. All the peptides were lytic, their activities varying by more than a factor of 103 from the shortest 5-residue-long peptide (5-mer) to the longest 22-mer. However, there was no monotonous increase versus length. The 15-mer was as active as the 22-mer and even more than melittin which is used as standard. Partition coefficients from the buffer to the membrane increased in relation to length up to 12 residues, then weakly decreased to reach a plateau, while they were expected to increase monotonously with peptide length and hydrophobicity as revealed from HPLC retention times. Fluorescence labeling by a dansyl group at the N-terminus, or by a W near the CO-terminus, show that up to 12 residues, the peptides were essentially monomeric while longer peptides strongly aggregated in the solution. Lipid affinity was then controlled by peptide length and was found to be limited by folding and self-association in buffer. The lytic activity resulted both from lipid affinity, which varied by a factor of 20-fold, and from efficiency in disturbing the membrane when bound, the latter steeply and monotonously increasing with length. The 15-residue-long peptide, KLLKLLLKLLLKLLK, had the optimal size for highest lytic activity. The shallow location of the fluorescent labels in the lipids is further evidence for a model of peptides remaining flat at the interface.

Lipid polymorphism and protein-lipid interactions

Non-lamellar-forming lipids play an important role in determining the physical properties of membranes. They affect the activity of membrane proteins and peptides. In addition, peptides which lyse membranes as well as those which promote membrane fusion facilitate the formation of non-lamellar phases, either micelles, cubic or hexagonal phases. The relationship of these diverse effects on membrane curvature is discussed in relation to the function of certain peptides and proteins. Specific examples of ionophoric peptides, cytotoxic peptides and viral fusion peptides are given. In addition, we compare the modulation of the rate of photoisomerisation of an integral membrane protein, rhodopsin, by non-lamellar-forming lipids with the effects of these lipids on an amphitropic protein, protein kinase C. Among these diverse systems it is frequently observed that the modulation of biological activity can be described in terms of the effect of the peptide or protein on the relative stability of lamellar and non-lamellar structures.

E. coli alpha-hemolysin: a membrane-active protein toxin

alpha-Hemolysin is synthesized as a 1024-amino acid polypeptide, then intracellularly activated by specific fatty acylation. A second activation step takes place in the extracellular medium through binding of Ca2+ ions. Even in the absence of fatty acids and Ca2+ HlyA is an amphipathic protein, with a tendency to self-aggregation. However, Ca(2+)-binding appears to expose hydrophobic patches on the protein surface, facilitating both self-aggregation and irreversible insertion into membranes. The protein may somehow bind membranes in the absence of divalent cations, but only when Ca2+ (or Sr2+, or Ba2+) is bound to the toxin in aqueous suspensions, i.e., prior to its interaction with bilayers, can alpha-hemolysin bind irreversibly model or cell membranes in such a way that the integrity of the membrane barrier is lost, and cell or vesicle leakage ensues. Leakage is not due to the formation of proteinaceous pores, but rather to the transient disruption of the bilayer, due to the protein insertion into the outer membrane monolayer, and subsequent perturbations in the bilayer lateral tension. Protein or glycoprotein receptors for alpha-hemolysin may exist on the cell surface, but the toxin is also active on pure lipid bilayers.

Studies of kinetics and equilibrium membrane binding of class A and class L model amphipathic peptides

We studied the kinetics and equilibrium membrane binding of two amphipathic alpha-helical peptides: the 18L peptide, which belongs to the class L (lytic peptides), and the Ac-18A-NH2 peptide of the class A (apolipoprotein), according to classification of Segrest et al. ((1990) Proteins, 8, 103-117). Both for cationic 18L and zwitterionic Ac-18A-NH2, the presence of acidic lipids increased the membrane binding constants by two orders of magnitude. The free energy of peptide-membrane association was in the range of 8.5-12.8 kcal/mol. Binding isotherms corresponded to monomer partitioning with saturation at high peptide/lipid ratios. This was also supported by stopped flow studies of the kinetics of peptide-membrane association as measured by peptide tryptophan fluorescence or by energy transfer from the peptide to the lipid-anchored anthrylvinyl fluorophor. The apparent time required for peptide-membrane equilibration was in the millisecond range. At low peptide/lipid ratios it depended on lipid concentration and was independent of the peptide concentration. The rate of peptide-membrane association was found to be relatively close to the diffusion limit. Thus peptide-membrane affinity was mostly determined by the peptide dissociation rate, i.e. higher membrane affinity correlated with a decrease in dissociation rate and with a slower peptide exchange. We have shown that the dynamic character of the peptide membrane equilibrium can be used for selective peptide targeting and disruption of membranes with a specific lipid composition.