The channel formed by colicin A in planar lipid bilayers has an outsized selectivity for protons compared to any other ion, even though it allows large ions, such as tetraethylammonium, to permeate readily. A mechanism to account for this discrepancy remains obscure. We considered that protons may traverse a separate pathway but were unable to find any evidence for one. Manipulations that interfere with ionic conduction, such as replacing some of the water in the pore with a nonelectrolyte, reduce the proton current along with the ionic current. Lipids have been proposed to play a structural role in the channel, but we found that the proton selectivity was unaffected by various gross changes in the lipid composition of the bilayer, effectively ruling out any specific effect of lipids in the selectivity and offering no support for their role in structure. The 10-helix channel-forming domains of colicins Ia and E1 are structurally homologous to that of colicin A but do not select so remarkably for protons; thus we were able to use them to probe for the regions responsible for the high selectivity. Using hybrids made by helix swapping among these proteins, we found that the anomalous selectivity could be localized to the five C-terminal helices of colicin A.
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