Permeability and voltage-gating properties of channels formed by colicin E1 and its mutants in planar phospolipid bilayer membranes
Abrams, Charles K.
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Colicin E1 is a soluble, bacteriocidal protein that forms voltage-gated channels in planar lipid bilayers. I examined the ion selectivity and voltage gating of channels formed by wild type colicin E1, chemically derivatized wild type colicin E1, and mutants of colicin E1.;All of the replacements, with the exception of those at position 440, resulted in a channel that, at low pH, had an ion selectivity identical to that of wild type. At higher pH, however, one of these mutations, which replaced the aspartate at position 473, resulted in a channel that was less cation-selective. By comparing the ion selectivity of the cys473 mutant to that of the wild type as a function of the pH on the cis and trans sides of the membrane, I concluded that residue 473 lies close to the cis side.;Previous work on planar phospholipid bilayers and lipid vesicles indicates that voltage gating of colicin E1 channels involves translocation of peptide segments of the molecules into and across the membrane. I have identified histidine residue 440 as a gating charge associated with this translocation. Using site-directed mutagenesis to convert his440 to a neutral cysteine, I found that the channels formed by this mutant are less voltage dependent than are wild-type channels; the magnitude of the change in voltage dependence is consistent with residue 440 moving from the trans to the cis side of the membrane in association with channel closure. The effect of trans pH changes on the ion selectivity of channels formed by the carboxymethylated derivative of the cysteine 440 mutant independently established that in the open channel state, residue 440 lies on the trans side of the membrane. Accordingly, I propose that the voltage-gated opening of colicin E1 channels is accompanied by the insertion into the bilayer of a helical hairpin loop extending with the extrusion of this loop from the interior of the bilayer back to the cis side.;The results of my studies have generated an experimentally testable mechanism of voltage gating of the colicin E1 channel and have placed important constrains on models of the channel structure.
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