On the nature of the structural change of the colicin E1 channel peptide necessary for its translocation-competent state

Acidic pH conditions required in vitro for membrane binding and activity of the channel-forming colicin E1 resulted in an increased susceptibility to proteases of the 178-residue thermolytic channel peptide, an increased accessibility to acrylamide of a fluorescence probe linked to cysteine-505 of the peptide, and an increased partition into nonionic detergent. The structural change in the peptide sensed by the fluorescence probe caused by a transition from pH 6.0 to 3.5 occurred in less than 1 s. The presence of low concentrations of detergents (0.001% SDS or 0.44% octyl beta-D-glucoside) or urea (0.2 M) at pH 6 or 4 also increased the susceptibility of the channel peptide to proteases. The increase in protease susceptibility and acrylamide accessibility at low pH, as well as partition of the peptide into nonionic detergent, suggested that acidic pH or the detergents might cause peptide unfolding. However, the hydrodynamic radius of the channel peptide at pH 6, 21-23 A, was not changed at pH 3.5 or by detergents or urea under conditions that increased the susceptibility of the peptide to protease. The activity of the channel peptide at pH 6 measured with liposomes and planar bilayers, which was a factor of 10(3)-10(4) smaller than that at pH 4, was increased by 2-4 orders of magnitude by 0.001% SDS or 0.44% octyl beta-D-glucoside, with an additional small increment of activity on planar bilayers caused by 0.01% SDS. A small increase in Stokes radius of the peptide in the presence of SDS could be detected that was approximately correlated with increased activity.