Alamethicin and related peptaibols ? model ion channels

Abstract

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.

Alamethicin Supramolecular Organization in Lipid Membranes from 19F Solid-State NMR

“…As many as 20 distinguishable conductance states have been described (12). The fluctuations between the stepwise K þ and Cl À conductivities are consistent with the various sizes of the transmembrane channels due to the presence of different oligomeric states of the helix bundles (13,14) (reviewed in (1,2)). It has been suggested that these pore structures may consist of at least three (15), four (1), or even five subunits (16).…”

“…4 d highly dynamic within its own structure but the size of the supramolecular arrangement can also change over time, for example, in electrophysiological single-channel measurements where different levels in conductivity are obtained depending on the detailed experimental conditions (1,16). Indeed, when the peptide/lipid ratio is diminished, smaller oligomers predominate (Fig.…”

“…Alamethicins (ALMs) belong to a family of antibiotic peptides (peptaibols) of fungal origin that are known to selfassociate upon binding to lipid membranes (reviewed in (1)(2)(3)(4)(5)(6)(7)(8)). The most extensively studied isoform, ALM F50/5, is characterized by a linear sequence of 19 amino acids, eight of which are a-aminoisobutyric acid residues.…”

“…The peptide is acetylated at the N-terminus and the main chain is extended with a 1,2-aminoalcohol (1,2-phenylalaninol). This membrane-active peptide antibiotic is of interest as a model for more complex antimicrobial sequences, but also in analogy to larger voltage-gated channels (1,9). In electrophysiological recordings, ALM exhibits a welldefined pattern of successive increases in conductance levels, each with a duration of a few milliseconds (10,11).…”

“…Structural analyses of ALM in membrane mimetic and micellar environments indicate a high propensity for helical conformations. Based on oriented circular dichroism and solid-state NMR spectroscopies (22,23), a two-state model was proposed (1,24) in which the peptide adopts the transmembrane state above a threshold concentration. Continuous-wave electron paramagnetic resonance (EPR) studies at room temperature suggest that the majority of the membrane-bound ALM molecules are probably in the monomeric transmembrane oriented state, and upon a reduction in temperature they begin to self-assemble below the lipid-phase transition temperature (25).…”