Voltage-Dependent Pore Formation and Antimicrobial Activity by Alamethicin and Analogues

Abstract

Despite the long-standing interest for membrane-active peptides, alamethicin remains the best studied simple model of pores formed by the intramembrane aggregation of amphipathic a-helices. The demonstration of artificial membrane excitability developed by planar lipid bilayers doped with alamethicin [70] indeed struck the minds of many a biophysicist, including electrophysiologists, at a time when the protein nature of ion channels was still a matter of conjecture. Since 1967, the number of articles devoted to alamethicin doubles every decade to presently attain 500 and thus this peptaibol, i.e., a peptide rich in a-aminoisobutyric acid (Aib) or a-methylalanine (a-MeA), opened a classical chapter in biophysics with offshoots such as lipid-peptide interactions and the host of studies presently devoted to antimicrobial peptides (for reviews, see e.g., [32, 43]). Discovered in cultures of the fungus Trichoderma viride [64], alamethicin was assumed to be a cyclic peptide when the ‘barrel-stave’ model was put forward [6, 12]. An NMR spectroscopy study establishing the correct and linear sequence (see the Table) in 1976 [63] was quickly followed by the chemical synthesis of a very similar peptide, with which the essential functional properties were retrieved [36]. After the pioneering studies mentioned above [70], alamethicin single-channel events were recorded with multiple open states [38] and their detailed kinetic analysis led to the proposition of the ‘barrel-stave’ model [12]. In the early eighties, the highresolution (down to 1.5 A) crystallographic structure of alamethicin [33] was a landmark, from which new structural studies sprang also allowing the formulation of hypotheses on the voltage-dependent membrane insertion and the architecture of the oligomeric channel. The purpose of this review is to provide a comprehensive account of the presently available functional and structural data on alamethicin and peptaibols, emphasizing the most recent advances and trends. We shall also concentrate upon comparative macroscopic and singlechannel conductances in planar lipid bilayers and antimicrobial activities of alamethicin, related peptaibols and synthetic analogues designed to test specific residues. Within this class of membrane-active peptides, it is shown that antimicrobial activity obeys the same rules as defined by conductance assays. Several reviews published in the last decade and dealing with this field under different aspects ought to be mentioned [98, 76, 18, 7, 74].