Model of interaction between a cardiotoxin and dimyristoylphosphatidic acid bilayers determined by solid-state 31P NMR spectroscopy

Specific Amino Acid Residues in the Three Loops of Snake Cytotoxins Determine Their Membrane Activity and Provide a Rationale for a New Classification of These Toxins

Cytotoxins (CTs) are three-finger membrane-active toxins present mainly in cobra venom. Our analysis of the available CT amino acid sequences, literature data on their membrane activity, and conformational equilibria in aqueous solution and detergent micelles allowed us to identify specific amino acid residues which interfere with CT incorporation into membranes. They include Pro9, Ser28, and Asn/Asp45 within the N-terminal, central, and C-terminal loops, respectively. There is a hierarchy in the effect of these residues on membrane activity: Pro9 > Ser28 > Asn/Asp45. Taking into account all the possible combinations of special residues, we propose to divide CTs into eight groups. Group 1 includes toxins containing all of the above residues. Their representatives demonstrated the lowest membrane activity. Group 8 combines CTs that lack these residues. For the toxins from this group, the greatest membrane activity was observed. We predict that when solely membrane activity determines the cytotoxic effects, the activity of CTs from a group with a higher number should exceed that of CTs from a group with a lower number. This classification is supported by the available data on the cytotoxicity and membranotropic properties of CTs. We hypothesize that the special amino acid residues within the loops of the CT molecule may indicate their involvement in the interaction with non-lipid targets.

Assessing Interactions Between a Polytopic Membrane Protein and Lipid Bilayers Using Differential Scanning Calorimetry and Solid-State NMR

It is known that the lipid composition within a cellular membrane can influence membrane protein structure and function. In this Article, we investigated how structural changes to a membrane protein upon substrate binding can impact the lipid bilayer. To carry out this study, we reconstituted the secondary active drug transporter EmrE into a variety of phospholipid bilayers varying in headgroup and chain length and carried out differential scanning calorimetry (DSC) and solid-state NMR experiments. The DSC results revealed a difference in cooperativity of the lipid phase transition for drug-free EmrE protonated at glutamic acid 14 (i.e., proton-loaded form) and the tetraphenylphosphonium (TPP⁺) bound form of the protein (i.e., drug-loaded form). To complement these findings, we acquired magic-angle-spinning (MAS) spectra in the presence and absence of TPP⁺ by directly probing the phospholipid headgroup using ³¹P NMR. These spectra showed a reduction in lipid line widths around the main phase transition for samples where EmrE was bound to TPP⁺ compared to the drug free form. Finally, we collected oriented solid-state NMR spectra on isotopically enriched EmrE that displayed chemical shift perturbations to both transmembrane and loop residues upon TPP⁺ binding. All of these results prompt us to propose a mechanism whereby substrate-induced changes to the structural dynamics of EmrE alters the surrounding lipids within the bilayer.

Membrane solid-state NMR in Canada: A historical perspective

This manuscript presents an overview of more than 40years of membrane solid-state nuclear magnetic resonance (NMR) research in Canada. This technique is a method of choice for the study of the structure and dynamics of lipid bilayers; bilayer interactions with a variety of molecules such as membrane peptides, membrane proteins and drugs; and to investigate membrane peptide and protein structure, dynamics, and topology. Canada has a long tradition in this field of research, starting with pioneering work on natural and model membranes in the 1970s in a context of emergence of biophysics in the country. The 1980s and 1990s saw an emphasis on studying lipid structures and dynamics, and peptide-lipid and protein-lipid interactions. The study of bicelles began in the 1990s, and in the 2000s there was a rise in the study of membrane protein structures. Novel perspectives include using dynamic nuclear polarization (DNP) for membrane studies and using NMR in live cells. This article is part of a Special Issue entitled: Biophysics in Canada, edited by Lewis Kay, John Baenziger, Albert Berghuis and Peter Tieleman.

Mechanisms of peptide-induced pore formation in lipid bilayers investigated by oriented 31P solid-state NMR spectroscopy

There is a considerable interest in understanding the function of antimicrobial peptides (AMPs), but the details of their mode of action is not fully understood. This motivates extensive efforts in determining structural and mechanistic parameters for AMP’s interaction with lipid membranes. In this study we show that oriented-sample ³¹P solid-state NMR spectroscopy can be used to probe the membrane perturbations and -disruption by AMPs. For two AMPs, alamethicin and novicidin, we observe that the majority of the lipids remain in a planar bilayer conformation but that a number of lipids are involved in the peptide anchoring. These lipids display reduced dynamics. Our study supports previous studies showing that alamethicin adopts a transmembrane arrangement without significant disturbance of the surrounding lipids, while novicidin forms toroidal pores at high concentrations leading to more extensive membrane disturbance.

Biochemistry of envenomation

Venoms and toxins are of significant interest due to their ability to cause a wide range of pathophysiological conditions that can potentially result in death. Despite their wide distribution among plants and animals, the biochemical pathways associated with these pathogenic agents remain largely unexplored. Impoverished and underdeveloped regions appear especially susceptible to increased incidence and severity due to poor socioeconomic conditions and lack of appropriate medical treatment infrastructure. To facilitate better management and treatment of envenomation victims, it is essential that the biochemical mechanisms of their action be elucidated. This review aims to characterize downstream envenomation mechanisms by addressing the major neuro-, cardio-, and hemotoxins as well as ion-channel toxins. Because of their use in folk and traditional medicine, the biochemistry behind venom therapy and possible implications on conventional medicine will also be addressed.

Biophysical studies of the interactions between the phage varphiKZ gp144 lytic transglycosylase and model membranes

The use of naturally occurring lytic bacteriophage proteins as specific antibacterial agents is a promising way to treat bacterial infections caused by antibiotic-resistant pathogens. The opportunity to develop bacterial resistance to these agents is minimized by their broad mechanism of action on bacterial membranes and peptidoglycan integrity. In the present study, we have investigated lipid interactions of the gp144 lytic transglycosylase from the Pseudomonas aeruginosa phage varphiKZ. Interactions with zwitterionic lipids characteristic of eukaryotic cells and with anionic lipids characteristic of bacterial cells were studied using fluorescence, solid-state nuclear magnetic resonance, Fourier transform infrared, circular dichroism, Langmuir monolayers, and Brewster angle microscopy (BAM). Gp144 interacted preferentially with anionic lipids, and the presence of gp144 in anionic model systems induced membrane disruption and lysis. Lipid domain formation in anionic membranes was observed by BAM. Gp144 did not induce disruption of zwitterionic membranes but caused an increase in rigidity of the lipid polar head group. However, gp144 interacted with zwitterionic and anionic lipids in a model membrane system containing both lipids. Finally, the gp144 secondary structure was not significantly modified upon lipid binding.

Interaction of three-finger toxins with phospholipid membranes: comparison of S- and P-type cytotoxins

The CTs (cytotoxins) I and II are positively charged three-finger folded proteins from venom of Naja oxiana (the Central Asian cobra). They belong to S- and P-type respectively based on Ser-28 and Pro-30 residues within a putative phospholipid bilayer binding site. Previously, we investigated the interaction of CTII with multilamellar liposomes of dipalmitoylphosphatidylglycerol by wide-line (31)P-NMR spectroscopy. To compare interactions of these proteins with phospholipids, we investigated the interaction of CTI with the multilamellar liposomes of dipalmitoylphosphatidylglycerol analogously. The effect of CTI on the chemical shielding anisotropy and deformation of the liposomes in the magnetic field was determined at different temperatures and lipid/protein ratios. It was found that both the proteins do not affect lipid organization in the gel state. In the liquid crystalline state of the bilayer they disturb lipid packing. To get insight into the interactions of the toxins with membranes, Monte Carlo simulations of CTI and CTII in the presence of the bilayer membrane were performed. It was found that both the toxins penetrate into the bilayer with the tips of all the three loops. However, the free-energy gain on membrane insertion of CTI is smaller (by approximately 7 kcal/mol; 1 kcal identical with 4.184 kJ) when compared with CTII, because of the lower hydrophobicity of the membrane-binding site of CTI. These results clearly demonstrate that the P-type cytotoxins interact with membranes stronger than those of the S-type, although the mode of the membrane insertion is similar for both the types.

Interaction of the P-type cardiotoxin with phospholipid membranes

The cardiotoxin (cytotoxin II, or CTII) isolated from cobra snake (Naja oxiana) venom is a 60-residue basic membrane-active protein featuring three-finger beta sheet fold. To assess possible modes of CTII/membrane interaction 31P- and 1H-NMR spectroscopy was used to study binding of the toxin and its effect onto multilamellar vesicles (MLV) composed of either zwitterionic or anionic phospholipid, dipalmitoylglycerophosphocholine (Pam2Gro-PCho) or dipalmitoylglycerophosphoglycerol (Pam2Gro-PGro), respectively. The analysis of 1H-NMR linewidths of the toxin and 31P-NMR spectral lineshapes of the phospholipid as a function of temperature, lipid-to-protein ratios, and pH values showed that at least three distinct modes of CTII interaction with membranes exist: (a) nonpenetrating mode; in the gel state of the negatively charged MLV the toxin is bound to the surface electrostatically; the binding to Pam2Gro-PCho membranes was not observed; (b) penetrating mode; hydrophobic interactions develop due to penetration of the toxin into Pam2Gro-PGro membranes in the liquid-crystalline state; it is presumed that in this mode CTII is located at the membrane/water interface deepening the side-chains of hydrophobic residues at the tips of the loops 1-3 down to the boundary between the glycerol and acyl regions of the bilayer; (c) the penetrating mode gives way to isotropic phase, stoichiometrically well-defined CTII/phospholipid complexes at CTII/lipid ratio exceeding a threshold value which was found to depend at physiological pH values upon ionization of the imidazole ring of His31. Biological implications of the observed modes of the toxin-membrane interactions are discussed.

Interaction between beta-Purothionin and dimyristoylphosphatidylglycerol: a (31)P-NMR and infrared spectroscopic study

The interaction of beta-purothionin, a small basic and antimicrobial protein from the endosperm of wheat seeds, with multilamellar vesicles of dimyristoylphosphatidylglycerol (DMPG) was investigated by (31)P solid-state NMR and infrared spectroscopy. NMR was used to study the organization and dynamics of DMPG in the absence and presence of beta-purothionin. The results indicate that beta-purothionin does not induce the formation of nonlamellar phases in DMPG. Two-dimensional exchange spectroscopy shows that beta-purothionin decreases the lateral diffusion of DMPG in the fluid phase. Infrared spectroscopy was used to investigate the perturbations, induced by beta-purothionin, of the polar and nonpolar regions of the phospholipid bilayers. At low concentration of beta-purothionin, the temperature of the gel-to-fluid phase transition of DMPG increases from 24 degrees C to ~33 degrees C, in agreement with the formation of electrostatic interactions between the cationic protein and the anionic phospholipid. At higher protein concentration, the lipid transition is slightly shifted toward lower temperature and a second transition is observed below 20 degrees C, suggesting an insertion of the protein in the hydrophobic core of the lipid bilayer. The results also suggest that the presence of beta-purothionin significantly modifies the lipid packing at the surface of the bilayer to increase the accessibility of water molecules in the interfacial region. Finally, orientation measurements indicate that the alpha-helices and the beta-sheet of beta-purothionin have tilt angles of ~60 degrees and 30 degrees, respectively, relative to the normal of the ATR crystal.

Membrane binding motif of the P-type cardiotoxin

Carditoxins (CTXs) from cobra snake venoms, the basic 60-62 residue all-beta sheet polypeptides, are known to bind to and impair the function of cell membranes. To assess the membrane induced conformation and orientation of CTXs, the interaction of the P-type cardiotoxin II from Naja oxiana snake venom (CTII) with perdeuterated dodecylphosphocholine (DPC) was studied using ( 1 )H-NMR spectroscopy and diffusion measurements. Under conditions where the toxin formed a well-defined complex with DPC, the spatial structure of CTII with respect to the presence of tightly bound water molecules in loop II, was calculated using the torsion angle dynamics program DYANA. The structure was found to be similar, except for subtle changes in the tips of all three loops, to the previously described "major" form of CTII in aqueous solution illustrated by the "trans" configuration of the Val7-Pro8 peptide bond. No "minor" form with the "cis" configuration of the above bond was found in the micelle-bound state. The broadening of the CTII backbone proton signals by 5, 16-doxylstearate relaxation probes, together with modeling based on the spatial structure of CTII, indicated a periphery mode of binding of the toxin molecule to the micelle and revealed its micelle interacting domain. The latter includes a hydrophobic region of CTII within the extremities of loops I and III (residues 5-11, 46-50), the basement of loop II (residues 24-29,31-37) and the belt of polar residues encircling these loops (lysines 4,5,12,23,50, serines 11,46, histidine 31, arginine 36). It is suggested that this structural motif and the mode of binding can be realized during interaction of CTXs with lipid and biological membranes.

Effect of angiotensin II non-peptide AT(1) antagonist losartan on phosphatidylethanolamine membranes

Losartan was found to affect both the thermotropic behavior and molecular mobility of dimyristoyl- and dipalmitoyl-phosphatidylcholine membranes (Theodoropoulou and Marsh, Biochim. Biophys. Acta 1461 (1999) 135-146). At low concentrations, the antagonist is located close to the interfacial region of the phosphatidylcholine bilayer while at high mole fractions it inserts deeper in the bilayers. In the present study, we investigated the interactions of losartan with phosphatidylethanolamine membranes using differential scanning calorimetry (DSC), electron spin resonance (ESR) and 31P nuclear magnetic resonance (NMR) spectroscopy. DSC showed that the antagonist affected the thermotropic transitions of dimyristoyl-, dipalmitoyl- and dielaidoyl-phosphatidylethanolamine membranes (DMPE, DPPE and DEPE, respectively). ESR spectroscopy showed that the interaction of losartan with phosphatidylethanolamine membranes is more superficial than in the case of phosphatidylcholine bilayers. Additionally, losartan increased the spin-spin broadening of 12-PESL spin labels in the gel phase of DMPE and DPPE membranes, while in the case of DEPE membranes the opposite effect was observed. (31)P-NMR showed that the antagonist stabilizes the fluid lamellar phase of DEPE membranes relative to the hexagonal H(II) phase. Our results show that losartan affects the thermotropic behavior of phosphatidylethanolamine membranes, while the molecular mobility of the membranes is not affected greatly. Furthermore, its interactions with phosphatidylethanolamine membranes are more superficial than with phosphatidylcholine bilayers.

Two forms of cytotoxin II (cardiotoxin) from Naja naja oxiana in aqueous solution: spatial structures with tightly bound water molecules

1H-NMR spectroscopy data, such as NOE intraprotein and (bound water)/protein contacts, 3J coupling constants and deuterium exchange rates were used to determine the in-solution spatial structure of cytotoxin II from Naja naja oxiana snake venom (CTII). Exploiting information from two 1H-NMR spectral components, shown to be due to cis/trans isomerization of the Val7-Pro8 peptide bond, spatial structures of CTII minor and major forms (1 : 6) were calculated using the torsion angle dynamics algorithm of the DYANA program and then energy refined using the FANTOM program. Each form, major and minor, is represented by 20 resulting conformers, demonstrating mean backbone rmsd values of 0.51 and 0.71 A, respectively. Two forms of CTII preserve the structural skeleton as three large loops, including two beta-sheets with bend regions, and demonstrate structural differences at loop I, where cis/trans isomerization occurs. The CTII side-chain distribution constitutes hydrophilic and hydrophobic belts around the protein, alternating in the trend of the three main loops. Because of the Omega-shaped backbone, formed in participation with two bound water molecules, the tip of loop II bridges the tips of loops I and III. This ensures the continuity of the largest hydrophobic belt, formed with the residues of these tips. Comparison revealed pronounced differences in the spatial organization of the tips of the three main loops between CTII and previous structures of homologous cytotoxins (cardiotoxins) in solution.

Membrane structure and dynamics as viewed by solid-state NMR spectroscopy

The purpose of the present study is the investigation of the structure and dynamics of biological membranes using solid-state nuclear magnetic resonance (NMR) spectroscopy. Two approaches are used in our laboratory. The first involves the measurement of high-resolution 13C and 1H spectra obtained by the magic angle spinning (MAS) technique while the second approach involves the measurement of 31P and 2H powder spectra in static samples. This paper will present some recent results obtained by high-resolution solid-state 1H NMR on the conformation of gramicidin A incorporated in a phosphatidylcholine bilayers. More specifically, we were able to observe changes in the gramicidin spectra as a function of the cosolubilization solvent initially used to prepare the samples. The interaction between lipid bilayers and an anticancer drug derived from chloroethylurea was also investigated using proton NMR spectroscopy. Finally, we have studied the interaction between cardiotoxin, a toxic protein extracted from snake venom, and negatively charged lipid bilayers using 31P solid-state NMR spectroscopy.

Action of Taiwan cobra cardiotoxin on membranes: binding modes of a beta-sheet polypeptide with phosphatidylcholine bilayers

The interaction of Taiwan cobra cardiotoxin (CTX A3), a basic polypeptide consisting of three-fingered loops and five-strand beta-sheet structure, with zwitterionic dipalmitoylphosphatidylcholine (DPPC) has been studied by 31P and 2H NMR to understand the binding modes of CTX in membrane bilayers. The results, in conjunction with DPH fluorescence anisotropy and differential scanning calorimetry studies, show that CTX may penetrate and lyse the bilayers into small aggregates at a lipid/protein molar ratio of about 20 in the ripple Pbeta' phase. Elevating the temperature to that of the liquid crystalline Lalpha phase leads to the fusion of the small aggregates into larger ones as evidenced by the change of the isotropic signal into a magnetically aligned 31P signal with a marked reduction in the chemical shift anisotropy. 2H NMR study on deuterium-labeled DPPC in the head group and fatty acyl region as a function of temperature and CTX concentration reveals a molecular model that CTX undergoes a redistribution between penetrating and peripheral binding states depending on the temperature studied. In addition, both the conformational and dynamic states of the phosphocholine head group of DPPC bilayers are significantly perturbed in the presence of CTX. Structural consideration of the CTX molecule indicates that the penetration binding mode of CTX with the DPPC bilayer may involve a novel membrane-binding motif identified recently in the three-fingered loops of P-type CTX. CTX can only bind to DPPC membrane peripherally in the Lalpha phase due to the mismatch of their hydrophobic lengths.

Interactions of thionin from Pyrularia pubera with dipalmitoylphosphatidylglycerol large unilamellar vesicles

The peptide toxin thionin from Pyrularia pubera binds to dipalmitoylphosphatidylglycerol (DPPG) large unilamellar vesicles as shown by an increase in the intensity and blue-shift of the fluorescence emission spectrum of the single tryptophan residue of the protein. The magnitude of these fluorescence changes increased with temperature near the thermotropic phase transition of DPPG (about 40 degrees C). Fluorescent probes sensitive to the structure and dynamics of the membrane were used to assess the effect of thionin binding on bilayer properties. The fluorescence emission spectra of Prodan, Patman, and Laurdan all showed spectral changes consistent with an increase in bilayer polarity at temperatures below the DPPG phase transition but a decrease in polarity at higher temperatures. Fluorescence polarization experiments and the ratio of monomer-to-excimer fluorescence of the probe 1,3-bis(1-pyrene)propane suggested that thionin increases the bilayer order above the transition temperature. Differential scanning calorimetry revealed that thionin broadens the transition and either increases or decreases the melting temperature depending on the concentration of the peptide. Taken together, the data are consistent with at least three distinct interactions of thionin with the bilayer: (1) thionin bound electrostatically to the bilayer surface; (2) tryptophan of the bound thionin inserted into the bilayer; (3) high-order aggregates of thionin-bound vesicles.