Solvent-dependent structural features of the membrane active peptide trichotoxin A40 as reflected in its dielectric dispersion

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.

Solution conformation of the Pseudomonas syringae pv. syringae phytotoxic lipodepsipeptide syringopeptin 25-A. Two-dimensional NMR, distance geometry and molecular dynamics

Syringopeptin 25-A is a phytotoxic amphiphilic lipodepsipeptide containing 25 amino acid residues, produced by some isolates of the plant pathogenic bacterium Pseudomonas syringae pv. syringae. Previous papers have reported its covalent structure and some of its biological properties. Attention has now been directed to define its conformation in solution, a structural feature regarded as important for understanding its possible role in the bacterial colonization of host plants, and its toxic action on the plant cell. Here we report the stereochemistry of its amino acid components, the complete interpretation of the two-dimensional NMR spectra and NOE data, and finally the structure obtained by computer simulations applying distance geometry and molecular dynamics procedures. The conformation of syringopeptin 25-A in aqueous solution includes three different structural regions interrupted by rigid 2,3-dehydro-2-aminobutyric acid residues: a loop from residue 2 to 6, a helicoidal zone from 8 to 15, and the lactone ring from 18 to 25. The three-dimensional structure of the lactone moiety is very similar to that of two previously studied bioactive lipodepsinonapeptides. Preliminary circular dichroism evidence of conformational variations in solution of trifluoroethanol, which stimulates a membrane-like environment, are also reported.

Conformational mapping of the cytosolic linker between domains III and IV of the cardiac Na+ channel protein and binding studies with a site-directed channel modifying antibody

By combining antibody binding studies with conformational mapping using synthetic peptides, the structure of the cytosolic linker between domains III and IV of the cardiac Na+ channel alpha-subunit was analyzed. Inside-out patch clamp experiments with isolated cardiac Na+ channels from neonatal rat cardiocytes confirmed that a polyclonal antibody against amino acids 1490-1507 of the cardiac Na+ channel recognizes the linker in situ since Na+ inactivation became significantly retarded. Epitope fine mapping with a series of overlapping peptides identified the sequence YYNAMKKLG (corresponding to amino acids 1496-1504 of the cardiac sodium channel alpha-subunit) as the binding locus of the site directed antibody, an interesting result with respect to structure-function relationships because the functionally important hydrophobic amino-acid cluster in position 1487-1489 is not included. Circular dichroism measurements of synthetic 20-mer peptides in hydrophilic and lipophilic environments provided indications for a notable alpha-helical content only for segment GGQDIFMTEEQKKYYNAMKK. This sequence corresponds to amino acids 1483-1502 in the linker and adopts a highly ordered pattern of charge distribution due to this helical conformation. Ordered structure and helix dipole moment represent physical properties which may be important in a refined model for explaining the function of the linker in terminating the open channel configuration.

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.

The solution structure of the lantibiotic gallidermin

The 21-peptide amide antibiotic gallidermin is a potential therapeutic against acne disease. It belongs to the class of polycyclic lanthionine and alpha,beta-didehydroamino acids containing polypeptides, which were named "lantibiotics." The structural gene of the recently elucidated lantibiotic gallidermin encodes a precursor peptide containing Ser, Thr, and Cys residues in the C-terminal prolantibiotic part, and an unusually hydrophilic leader peptide. The ribosomally synthesized pregallidermin is posttranslationally modified and processed to a complex peptide antibiotic with four sulfide rings and two unsaturated residues. The complete solution structure of gallidermin was determined in trifluoroethanol: water (95:5) and dimethylsulfoxide by two-dimensional 1H-nmr at 500 MHz, using a combination of double quantum filtered correlated spectroscopy, homonuclear Hartman-Hahn, and nuclear Overhauser enhancement spectroscopy experiments. Using a total number of 152 distance constraints from NOEs and 14 torsional constraints, derived from coupling constants, we obtained a screwlike solution structure of gallidermin. Restrained molecular dynamics simulations yielded a set of five converging structures with an atomic rms difference of 1.7 A for the backbone atoms, not dependent on the starting structure. The spatial structure model is in excellent agreement with the amphiphilic and channel-forming properties of gallidermin on membranes and its tryptic cleavage at the exposed site between residues 13 and 14.

Location and dynamics of alamethicin in unilamellar vesicles and thylakoids as model systems. A spin label study

Location and dynamics of the voltage-dependent pore-forming icosapeptide alamethicin have been studied using spin labels which were linked directly and via spacers to the C-terminus of the amphiphilic alpha-helix. Ion-transport activities of these derivatives were found to be very similar to those of natural alamethicin in green plant thylakoids chosen as a model system. The shape of the electron spin resonance spectra indicates segmental motion of the nitroxide rather than rotation of the whole peptide. A population of spins showing narrow lines in the presence of thylakoids or lipid vesicles is attributed to alamethicin in the aqueous solution. A second population shows rotational correlation times greater than 10(-9) s and is bound to the membranes, the C-termini residing in an environment with a polarity close to that of water. This population is inaccessible to the hydrophilic, charged line broadening agent chromium oxalate. Since spectral shapes and amplitudes of spectra are unchanged by additions of unlabelled peptide, it is concluded that the ESR detectable spins are bound to peptides essentially in the monomeric state. Alamethicin induced pore formation under flash illumination is demonstrated by measurement of kinetics of proton deposition in the thylakoid interior. When pores are opened by illuminating thylakoids and thus applying a membrane potential, mainly the bound population is affected by a process reversibly suppressing the signal, whereas only limited disappearance of label from the external medium is detected. Apparently, the potential causes a change in the conformation of the peptide which leads to a further immobilisation of the label, possibly due to a deeper insertion of the alpha-helices into the lipid membrane. However, evidence has been presented experimentally that there is no detectable change of potential prior to the opening of the pore.

Voltage-dependent channel formation by rods of helical polypeptides

The voltage-dependence of channel formation by alamethicin and it natural analogues can be described by a dipole flip-flop gating model, based on electric field-induced transbilayer orientational movements of single molecules. These field-induced changes in orientation result from the large permanent dipole moment of alamethicin, which adopts alpha-helical conformation in hydrophobic medium. It was, therefore, supposed that the only structural requirement for voltage-dependent formation of alamethicin-type channels might be a rigid lipophilic helical segment of minimum length. In order to test this hypothesis we synthesized a family of lipophilic polypeptides--Boc-(Ala-Aib-Ala-Aib-Ala)n-OMe, n = 1-4--which adopt alpha-helical conformation for n = 2-4 and studied their interaction with planar lipid bilayers. Surprisingly, despite their large difference in chain length, all four polypeptides showed quantitatively similar behavior. At low field strength of the membrane electric field these polypeptides induce a significant, almost voltage-independent increase of the bilayer conductivity. At high field strength, however, a strongly voltage-dependent conductance increase occurs similar to that observed with alamethicin. It results from the opening of a multitude of ion translocating channels within the membrane phase. The steady-state voltage-dependent conductance depends on the 8th-9th power of polypeptide concentration and involves the transfer of 4-5 formal elementary charges. From the power dependences on polypeptide concentration and applied voltage of the time constants in voltage-jump current-relaxation experiments, it is concluded that channels could be formed from preexisting dodecamer aggregates by the simultaneous reorientation of six formal elementary charges. Channels exhibit large conductance values of several nS, which become larger towards shorter polypeptide chain length. A mean channel diameter of 19 A is estimated corresponding roughly to the lumen diameter of a barrel comprised of 10 alpha-helical staves. Similar to experiments with the N-terminal Boc-derivative of alamethicin we did not observe the burst sequence of nonintegral conductance steps typical of natural (N-terminal Ac-Aib)-alamethicin. Saturation in current/voltage curves as well as current inactivation in voltage-jump current-relaxation experiments are found. This may be understood by assuming that channels are generated as dodecamers but, while reaching the steady state, reduce their size to that of an octamer or nonamer. We conclude that the overall behavior of these synthetic polypeptides is very similar to that of alamethicin.(ABSTRACT TRUNCATED AT 400 WORDS)

Conformation of sequential polypeptide poly(Leu-Leu-D-Phe-Pro) and formation of ion channel across bilayer lipid membrane

Sequential polypeptide, poly(Leu-Leu-D-Phe-Pro), containing a part of beta-turn sequence in gramicidin S, was synthesized and investigated as a model for ion channels. Sequential peptides, Boc-(Leu-Leu-D-Phe-Pro)n-OBzl1 (n = 1-4), were also synthesized to acquire conformational information about this polypeptide. From the analyses by NMR, CD, and IR measurements, intramolecular hydrogen bonds were found in the sequential peptides with n larger than two and Boc-(Leu-Leu-D-Phe-Pro)3-OBzl was deduced to adopt a 3(10)-helical conformation. Poly(Leu-Leu-D-Phe-Pro) was also suggested to have this conformation. With the addition of this polymer to oxidized cholesterol membrane, current-voltage response across the membrane was observed. Stepwise fluctuation of current was recorded under a positive electric field to support the channel formation. This polymer might form bundles of 3(10)-helices across the bilayer lipid membrane to pass through the ion.