Alamethicin. A rich model for channel behavior

Abstract

Alamethicin, a 20-amino acid peptide, has been studied for a number of years as a model for voltage-gated channels. Recently both the x-ray structure of alamethicin in crystal and an NMR solution structure have been published (Fox and Richards, 1982. Bannerjee et al., 1983). Both structures show that the amino end of the molecule forms a stable alpha-helix nine or 10 residues in length and that the COOH-terminal ends exhibits a variable hydrogen bonding pattern. We have used synthetic analogues of alamethicin to test various hypotheses of its mode of action. As a result of these studies we propose a channel structure in which the COOH-terminal residues bond together as a beta-barrel, leaving the alpha- helices free to rotate under the influence of the electric field and gate the channel. Though the number of monomers per channel varies with experimental conditions, the gating charge per monomer stays close to that expected from an alpha-helical gate. We can also alter the sign of the voltage which turns on a channel by varying the charge on the alamethicin analogue. Channels are always slightly cation-selective even though formed by monomers with negative, positive, or zero formal charge. Channels are less stable in low ionic strength solutions than high. Finally, alamethicin conductance parameters vary systematically with changes in membrane thickness. We show how these results and others in the literature can be explained by a fairly detailed structural model. The model can be easily generalized to a form more suited to high molecular weight single-peptide-chain proteins.

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).…”

“…Furthermore, the replacement of Gly 11 or Pro 14 by other, polar side chains modifies but does not abolish channel activities (18). These observations show that Gln 7 (and also Gly 11 and Pro 14 ) are not absolutely required for ion channel formation. Finally, it is worthwhile to mention that at high concentrations of ALM and independently of transmembrane potentials, bulky ions (e.g., N a -benzoyl-L-argininepara-nitroanilide and carboxyfluorescein) are able to pass the membrane (19)(20)(21).…”

“…Notably, it was reported that substitution of the polar Gln 7 by the neutral Ala reduces both the lifetime of channel opening and ion conductance, but does not completely abolish channel activity (17). Furthermore, the replacement of Gly 11 or Pro 14 by other, polar side chains modifies but does not abolish channel activities (18). These observations show that Gln 7 (and also Gly 11 and Pro 14 ) are not absolutely required for ion channel formation.…”

A folding-dependent mechanism of antimicrobial peptide resistance to degradation unveiled by solution structure of distinctin

“…Although the exact mechanism by which some antibiotics (Table 1) kill bacteria is not fully understood, it has been shown that peptide-lipid interactions leading to membrane permeation play a role in their activity. Macroscopic and singlechannel experiments have been used to screen the functional properties of potential pore-former compounds (5,12). Thus, synthetic D1 was incorporated into asolectin or phosphatidylcholine͞phosphatidylethanolamine planar bilayers, which were submitted to repetitive voltage ramps.…”

Voltage‐dependent and multi‐state ionic channels induced by trichorzianines, anti‐fungal peptides related to alamethicin

“…voltages at which the curves cross a reference conductance chosen as 6 nS. Applying an analysis previously described [9], the apparent number of monomers per channel can be estimated as N = Va/Ve, where Va is the Vc shift for an efold change in aqueous peptide concentration and fie, the voltage increment producing an e-fold change in conductance. The results are summarized in table 1 and compared with those of alamethicin.…”