The influence of the trichorzianin C-terminal residues on the ion channel conductance in lipid bilayers

New 19-Residue Peptaibols from Trichoderma Clade Viride

Trichoderma koningiopsis and T. gamsii belong to clade Viride of Trichoderma, the largest and most diverse group of this genus. They produce a wide range of bioactive secondary metabolites, including peptaibols with antibacterial, antifungal, and antiviral properties. The unusual amino acid residues of peptaibols, i.e., α-aminoisobutyric acid (Aib), isovaline (Iva), and the C-terminal 1,2-amino alcohol make them unique among peptides. In this study, the peptaibiomes of T. koningiopsis and T. gamsii were investigated by HPLC-ESI-MS. The examined strains appeared to produce 19-residue peptaibols, most of which are unknown from literature, but their amino acid sequences are similar to those of trikoningins, tricholongins, trichostrigocins, trichorzianins, and trichorzins. A new group of peptaibols detected in T. koningiopsis are described here under the name “Koningiopsin”. Trikoningin KA V, the closest peptaibol compound to the peptaibols produced by these two strains, was selected for structural investigation by short MD simulation, which revealed that many residues show high preference for left handed helix formation. The bioactivity of the peptaibol mixtures produced by T. koningiopsis and T. gamsii was tested on agar plates against bacteria, yeasts, and filamentous fungi. The results revealed characteristic differences in bioactivities towards the different groups of target microorganisms, which can be explained with the differences in their cell wall structures.

Template-free self-assembling fullerene and lipopeptide conjugates of alamethicin form voltage-dependent ion channels of remarkable stability and activity

N- and C-terminally modified with fullerene or lipopeptide alamethicin molecules were designed for the formation of template-free, self-assembling, voltage-dependent ion conducting channels. The automated solid phase synthesis of the alamethicin-F30 sequence was performed by in situ fluoride activation on 2-chlorotritylchloride-polystyrene resin and the conjugation with fullerenes-C60 and -C70 was carried out in solution. Voltage-dependent bilayer experiments revealed preferred channel sizes for C-terminal alamethicin F30-fullerene-C60 and -C70 conjugates and higher activity compared with native alamethicin, whereas N-terminally linked fullerene balls destabilize pore formation. C-terminal alamethicin F30-fullerene-C70 conjugates show pore states with remarkably long lifetimes of seconds. C-terminal lipopeptide conjugates of alamethicin were prepared by coupling via short peptide spacers with synthetic tripalmitoyl-S-glyceryl-cysteine. which represents the strong membrane anchoring N-terminus of bacterial lipoprotein. Alamethicin-lipopeptide conjugates exhibit high channel forming activities, whereby they self-assemble and adopt preferred pore states with extremely long lifetimes. The novel membrane modifying peptaibol constructs are valuable lead compounds for developments in sensorics related to transmembrane ion conductance.

Ion channels of alamethicin dimer N-terminally linked by disulfide bond

A covalent dimer of alamethicin Rf30 was synthesized by linking the N-termini by a disulfide bond. When the dimer peptides were added to the cis-side of a diphytanoyl PC membrane, macroscopic channel current was induced only at cis positive voltages. The single-channel recordings showed several conductance levels that were alternately stabilized. These results indicate that the dimer peptides form stable channels by N-terminal insertion like alamethicin and that most of the pores are assembled from even numbers of helices. Taking advantages of the long open duration of the dimer peptide channels, the current-voltage (I-V) relations of the single-channels were obtained by applying fast voltage ramps during the open states. The I-V relations showed rectification, such that current from the cis-side toward the trans-side is larger than that in the opposite direction. The intrinsic rectification is mainly attributed to the macro dipoles of parallel peptide helices surrounding a central pore.

Antiamoebin can function as a carrier or as a pore-forming peptaibol

Antiamoebin is a 16-residue polypeptide whose crystal structure and lytic activity in membrane vesicles have recently been reported. It is a bent helical molecule and a member of the peptaibol family of antibiotics. Under conditions which produce voltage-dependent conductance activity by other members of the family, no single-channel conductance was detected for antiamoebin, and a carrier-like mechanism was put forward to account for its mode of action. We now present evidence for pore formation that is largely voltage-insensitive, with large amplitude single-channel events on top of a background conductance that may account for the previously proposed carrier-like activity. Thus, antiamoebin may be the first instance of a peptide which can function both as an ion carrier and a pore former.

Three-dimensional structure of the ion-channel forming peptide trichorzianin TA VII bound to sodium dodecyl sulfate micelles

Trichorzianin TA VII, Ac0 U1 A2 A3 U4 J5 Q6 U7 U8 U9 S10 L11 U12 P13 V14 U15 I16 Q17 Q18 Fol19, is a nonadecapeptide member of the peptaibol antibiotics biosynthesized by Trichoderma soil fungi, which is characterized by a high proportion of the alpha, alpha-dialkylated amino acids, alpha-aminoisobutyric acid (Aib, U) and isovaline (Iva, J), an acetylated N-terminus and a C-terminal phenylalaninol (Pheol, Fol). The main interest in such peptides stems from their ability to interact with phospholipid bilayers and form voltage-dependent transmembrane channels in planar lipid bilayers. In order to provide insights into the lipid-peptide interaction promoting the voltage gating, the conformational study of TA VII in the presence of perdeuterated sodium dodecyl sulfate (SDS-d25) micelles has been carried out. 1H sequential assignment have been performed with the use of two-dimensional homo- and -heteronuclear nmr techniques including double quantum filtered correlated spectroscopy, homonuclear Hartmann-Hahn, nuclear Overhauser effect spectroscopy, 1H-13C heteronuclear single quantum correlation, and heteronuclear multiple bond correlation. Conformational parameters, such as 3JNHC alpha H coupling constants, temperature coefficients of amide protons (delta gamma/delta TNH) and quantitative nuclear Overhauser enhancement data, lead to detailed structural information. Ninety-eight three-dimensional structures consistent with the nmr data were generated from 231 interproton distances six phi dihedral angle restraints, using restrained molecular dynamics and energy minimization calculations. The average rms deviation between the 98 refined structures and the energy-minimized average structure is 0.59 A for the backbone atoms. The structure of trichorzianin TA VII associated with SDS micelles, as determined by these methods, is characterized by two right-handed helical segments involving residues 1-8 and 11-19, linked by a beta-turn that leads to an angle about 90 degrees-100 degrees between the two helix axes; residues 18 and 19 at the end of the C-terminal helix exhibit multiple conformations.

A solution NMR study of the selectively 13C, 15N-labeled peptaibol chrysospermin C in methanol

The conformation of the 19-residue peptaibol chrysospermin C in methanol has been investigated by NMR spectroscopy using selective 15N and 13C labeling of the alpha-aminoisobutyric acid (Aib) residues. Complete 1H and 13C sequential assignments, including stereospecific assignments for the heavily overlapped resonances from the two Cbeta methyl groups of the eight Aib residues, are reported for a peptaibol for the first time. An Aib residue followed by a Pro is an exception to previous suggestions regarding stereospecific assignment of the two Cbeta methyl groups of Aib residues. Local nuclear Overhauser effects and 3J(HNC') and 3J(HNCbeta) scalar couplings indicate that the phi angles of the Aib residues are restricted sterically to local conformations consistent with right-handed helices. Despite these constraints on the eight Aib residues, the NMR data for chrysospermin C in methanol are generally most consistent with an ensemble of transient conformations, including backbone conformations inconsistent with helical structures. Initial NMR measurements for chrysospermin C bound to micelles suggest structural and dynamic differences relative to alamethicin bound to micelles which may be related to differences in gating voltages for formation of ion channels.

Proline at position 14 of alamethicin is essential for hemolytic activity, catecholamine secretion from chromaffin cells and enhanced metabolic activity in endothelial cells

Alamethicin is known to lyse different biological cells and to induce voltage dependent ion channels in lipid bilayers. A set of analogs with proline shifted from position 14 in the native peptide towards the N- and C-terminus was used to investigate the role of proline in: (i) alamethicin induced hemolysis of human red blood cells, (ii) stimulation of catecholamine secretion from bovine adrenal chromaffin cells and (iii) induction of metabolic activity in bovine aortic endothelial cells. Half maximal hemolytic activity was found at 30 microM alamethicin concentration, complete lysis occurred at 100 microM. The stimulation of catecholamine secretion in the presence of extracellular Ca2+ was concentration dependent up to 50 microM alamethicin. At this high concentration mild secretion was also found in the absence of Ca2+ indicating cell membrane damage. Alamethicin transiently stimulated the metabolic rate of endothelial cells in a concentration dependent mode up to 20 microM while the inhibition of metabolism at higher concentrations pointed to a toxic effect. The alamethicin analogs were completely inactive in all the biological assays. The effects correlated with a loss of dye release inducing activities on phosphatidylcholine vesicles and reduction of channel forming properties in lipid bilayers and were associated with modifications of membrane affinity rather than conformational changes of the peptides. The results indicate that proline at position 14 of the native peptide is essential for the interaction with different membrane systems.

Alamethicin-like behaviour of new 18-residue peptaibols, trichorzins PA. Role of the C-terminal amino-alcohol in the ion channel forming activity

The influences of peptide length, absence of a Glx (Gln/Glu) residue and the C-terminal amino alcohol on liposome permeabilization and ion-channel characteristics in planar lipid bilayers were examined with two 18-residue peptaibols, PA V and PA IX. As compared to the 20-residue alamethicin, both peptides belonging to the newly isolated trichorzin family, lack a proline in the N-terminal part and one of the two Gln/Glu residues in the C-terminal part of the sequence. The two analogues studied here differ among themselves in their C-terminal amino alcohol (tryptophanol for PA V and phenylalaninol for PA IX). These alpha-helical peptaibols modify to a similar extent the permeability of liposomes, as measured by leakage of a previously entrapped fluorescent probe. Monitoring tryptophanol fluorescence, a greater embedment of the peptide PA V is observed in cholesterol-free bilayers. Macroscopic conductance studies for PA V and PA IX display alamethicin-like current-voltage curves, with a similar voltage dependence, but a smaller mean number of monomers per conducting aggregate is estimated for the tryptophanol analogue, PA V. Single-channel recordings indicate faster current fluctuations for PA IX, while amplitude histograms show lower conductance levels for PA V. Apart from underlining the role of the mismatch between helix length and bilayer hydrophobic thickness, these results stress that the C-terminal tryptophanol favours a stabilization of the conducting aggregates.

Lateral diffusion and conductance properties of a fluorescein-labelled alamethicin in planar lipid bilayers

In order to follow alamethicin diffusion within membranes under conditions of pore-formation, a fluorescein isothiocyanate (FITC) analogue was synthesized. To test the influence of the fluorescent probe addition on the pore-forming activity of the new analogue, macroscopic and single-channel experiments into planar lipid bilayers were performed. Although the apparent mean number of monomers per conducting aggregate was equivalent, the voltage-dependence of the new analogue was slightly reduced and hysteresses were broader, in agreement with the much longer duration of the open single-channels. Thus, the conducting aggregates seem to be stabilized by the introduction of the probe, presumably through the interaction of the conjugated cycles with the lipid headgroups, while the added steric hindrance may account for the slightly higher conductances of the open substates. Lateral diffusion of the labelled peptide associated with the bilayer was then investigated by the fluorescence recovery after photobleaching technique. Under applied voltage, associated with high conductance, D, the lateral diffusion coefficient, was reduced by 50% when compared to peptide at rest. These results provide new independent experimental evidence for a voltage-driven insertion of the highly mobile surface-associated peptide into the bilayer as a prominent step in pore formation.

Ion-channels formed by hypelcins, antibiotic peptides, in planar bilayer lipid membranes

Ion-channel properties of native hypelcins (HP) A-I, A-V and B-V isolated from Hypocrea peltata and a synthetic analog, HP-A-Pheol, were studied in planar bilayer lipid membranes by a single-channel recording technique. The native and synthetic hypelcins formed ion-channels with three conductance levels for 3 mole dm(-3) KCl: < or = 0.09 nS at 225 mV (level 0, only detectable at voltages above 200 mV), approximately 0.6 nS at 150 mV (level 1, most common level) and approximately 3 nS at 150 mV (level 2). The effects of the C-terminal aminoalcohol on the channel properties were examined with HP-A-I, HP-A-V and HP-A-Pheol, whose C-termini are leucinol (Leuol), isoleucinol (Ileol) and phenylalaninol (Pheol), respectively. The substitution of Pheol for Leuol and Ileol prolonged the open channel lifetime. A comparison of HP-A-V (Gln18) and HP-B-V (Glu18) indicated that the carboxyl group at position 18 increased both the open channel lifetime and the magnitude of unitary channel conductance at each conductance level. The pores of level 1 showed poor ion-selectivity for K+ over Cl-. The selectivity order of alkali metal cations was Rb > or = Cs > or = K > Na > Li for level 1 and Cs > Rb > K > Na > Li for level 0. The unitary current-voltage characteristics showed non-linear relationships, which were simulated by a Nernst-Planck approach with a simple barrier model.

Role of the Gln/Glu residues of trichocellins A-II/B-II in ion-channel formation in lipid membranes and catecholamine secretion from chromaffin cells

Trichocellins (TC) A-II and B-II, 20-residue peptaibols isolated from conidia of the fungus Trichoderma viride, have the same sequence except for the residue at position 18. Both TCs were found to form voltage-dependent ion-channels in bilayer lipid membranes (BLM) and to induce catecholamine secretion from bovine adrenal chromaffin cells through Ca2+ influx. TC-A-II (Gln18, neutral) was more effective than TC-B-II (Glu18, charged) for macroscopic current induction in BLMs and for catecholamine secretion from chromaffin cells, suggesting that Glu18 is unfavorable for the ion-channel formation in BLMs and chromaffin cell membranes. Nevertheless, single-channel recordings indicated that TC-B-II forms larger pores with longer open lifetimes than those of TC-A-II. This indicates that the negatively charged carboxyl group of Glu at position 18 stabilizes larger pores. The effects of the negative charge of Glu18 on the activities were confirmed by the use of a TC-B-II analog containing the methyl ester of Glu18.

Interaction of the 14-residue peptaibols, harzianins HC, with lipid bilayers: permeability modifications and conductance properties

Harzianins HC are a series of 14-residue peptaibols containing three Aib-Pro motives separated by sequences of two usual amino acids (Aib-Pro-Xaa-Xaa)n. They are organized in a subtype of the 3(10)-helix, which results in an approximate length of about 27-30 A for the helical rods, allowing them to span a bilayer. Permeabilization of small unilamellar vesicles composed of zwitterionic lipids (egg phosphatidylcholine/cholesterol 7/3 and 8/2) by harzianins HC was observed as well as voltage-gated macroscopic conductance and single-channel formation in planar lipid bilayers (DOPE/POPC 7/3) The permeabilization process was shown to increase with increasing the helix global hydrophobicity. The ion channel-for ming properties appeared rather favoured by an increase in the peptide amphipathicity. The set of conductance levels increasing in geometrical progression, reflecting the sequential uptake and release of monomers which is characteristic of the barrel-stave model for ion-channels described for alamethicin was not observed. The passage of ions through the bilayer would rather be the result of a set of aggregates with fixed numbers of monomers formed in the bilayer. The permeability process and the voltage-gated properties could thus result from different mechanisms showing that harzianins HC can permeabilize membranes via bilayer destabilization or channels, depending on the membrane system, composition and application of voltage.

The effect of free radicals on the conductance induced by alamethicin in planar lipid membranes: activation and inactivation

Exposure to ionizing radiation of planar lipid membranes doped with alamethicin gives rise to an increase and to a subsequent decrease of the membrane conductance. Both effects are due to the presence of radiation-induced free radicals of water radiolysis as was shown by addition of various radical scavengers. The increase of the conductance was found to be a consequence of free radical-initiated lipid peroxidation favouring the formation of active ion channels. The decrease of the conductance observed at larger radiation doses is due to an inactivation of alamethicin monomers. The characteristic D37 dose of inactivation was found to be about two orders of magnitude larger than in the case of gramicidin A. The comparatively high sensitivity of the latter is due to the presence of its four tryptophan residues. Inactivation of trichorzianine AIIIc, an analogue of alamethicin with a C-terminal tryptophanol residue, occurs at radiation doses two orders of magnitude lower than observed with alamethicin.

Formation of membrane channels by chrysospermins, new peptaibol antibiotics

The four homologous chrysospermins (Ia-d) are new 19 amino acid peptaibols which form cation selective ion channels in artificial lipid bilayer membranes. Conductance of channels formed by chrysospermins B (Ib) and D (Id) was twice as high (640 pS in 100 mM KCl) as found with chrysospermins A (Ia) and C (Ic). Single channel current traces were recorded for each of the four peptides even at very low (even zero) membrane voltages suggesting that non-gated channels are formed.

Structure and function of channel-forming peptaibols

Transport of ions through channels is fundamental to a number of physiological processes, especially the electrical properties of excitable cells (Hille, 1992). To understand this process at a molecular level requires atomic resolution structures of channel proteins.

Alamethicin and related peptaibols--model ion channels

Peptaibols are considered as models of those ion channels which consist of a bundle of transbilayer helices surrounding a central pore. X-Ray diffraction and NMR studies have yielded high resolution structures for several peptaibols. In conjunction with other spectroscopic investigations and molecular dynamics simulations, these studies suggest that peptaibols form proline-kinked alpha-helices, and that there may be "hinge-bending" movement of the helix in the region of the central proline residue. The amphipathicity of peptaibol helices is analyzed in relation to their channel-forming properties. Studies of the interactions of peptaibols with lipid bilayers suggest that they are helical when in a membrane-like environment, and that the helix orientation relative to the bilayer is sensitive to the peptaibol:lipid ratio, and to the degree of hydration of the bilayer. Electrical studies reveal that many peptaibols form multiple-conductance level channels in a voltage-dependent fashion. Analysis of conductance levels provides support for the "barrel stave" model of channel formation, whereby different conductance levels correspond to different numbers of monomers in a helix bundle. Alternative models for voltage-activation are discussed, and the roles of molecular dipoles and of hinge-bending in this process are considered. Two molecular models for an N = 6 bundle of alamethicin helices are presented and their electrostatic properties analyzed. The relevance of studies of peptaibols to channel and transport proteins in general is considered.

Membrane-modifying properties of the pore-forming peptaibols saturnisporin SA IV and harzianin HA V

Harzianin HA V and saturnisporin SA IV are alpha-amino isobutyric-containing peptides with 18- and 20-residue chain length, respectively. They were isolated from in vitro cultures of Trichoderma species and their sequences were determined by the combined use of positive ion FAB mass spectrometry and NMR. In organic solvent solution, both peptides exhibited the same predominant alpha-helical secondary structure including a hinge at the level of the central Pro residue, as deduced from NMR data. Their interaction with neutral phospholipid bilayers was shown to induce leakage of the material entrapped in small unilamellar vesicles composed of egg phosphatidylcholine/cholesterol (7/3). When incorporated into neutral planar lipid bilayers, they promoted voltage-gated channels. The concentration- and voltage-dependences of the ionic conductances induced by these peptides were studied in macroscopic current-voltage experiments. Single-channel measurements showed that whilst SA IV developed non-integral multi-open states similar to those induced by alamethicins, but with faster kinetics, the shorter analogue, HA V promoted much smaller-sized conducting aggregates in agreement with macroscopic conductance data.

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.