In vivo properties of colicin A: channel activity is voltage dependent but translocation may be voltage independent.

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RESUMO

The kinetics of K+ efflux caused by colicin A in Escherichia coli-sensitive cells have been investigated by using a K(+)-selective electrode. The order of magnitude of the rate of K+ efflux per colicin molecule was comparable to that of ion channels. The dependence of K+ efflux upon multiplicity, pH, temperature, and membrane potential (delta psi) was determined. The translocation of colicin A from the outer membrane receptor to the inner membrane and insertion into the inner membrane required a fluid membrane, but once inserted, the channel properties showed little dependence upon the state of the lipids. At a given multiplicity, the lag time before the onset of K+ efflux was found to reflect the time required for translocation and/or insertion of colicin into the cytoplasmic membrane. Opening of the channel only occurred above a threshold value of delta psi of 85 +/- 10 and 110 +/- 5 mV at pH 6.8 and 7.8, respectively. Conditions were designed for closing and reopening of the channel in vivo. These conditions allowed us to test separately the delta psi requirements for translocation and channel opening: translocation and/or insertion did not appear to require delta psi. The channel formed in vivo featured properties similar to that of the channel in lipid planar bilayers.

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