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.

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We have examined the causes of the asymmetry of the current-voltage curve induced by addition of alamethicin to one side of a black lipid membrane. We find that the alamethicin-induced current-voltage (I-V) curve has an inherent asymmetry. If it were possible to confine all alamethicin molecules to one side of the membrane, the I-V curve would exhibit a positive branch (voltage being measured with respect to the side of the membrane trans to the alamethicin addition) of steeper logarithmic slope than the negative branch and at a lower absolute value of potential. This condition is not usually realized, however, because alamethicin can leak through the membrane, so that, except at very high alamethicin concentrations and in certain kinds of membranes, the positive branch of the current-voltage curve has the same logarithmic slope as the negative branch and appears to arise from alamethicin which diffuses from the cis to the trans side of the membrane. We develop simple quantitative models for these two cases.

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