Effects of MTS reagents on the ionic selectivity of colicin Ia cysteine-mutant channels. V_rs were measured before and after MTS addition in the presence of a salt gradient of 100 mM KCl (cis): 1 M KCl (trans), pH 7.2 buffer. The V_r was determined either by zeroing the current (taking into account any non-gating “leak” current) or by finding the voltage at which the decay of the tail currents reversed direction. V_r is given as the mean ± standard deviation, with the number of experiments, n, in parentheses. In many cases (marked “trans/cis”), the MTS reagent was added to both sides of the membrane. For N578C channels after MTSET treatment, the tail currents had two components with different selectivities; the table gives V_r for the faster-closing component, which we estimated to represent about 70% of the conductance at +50 mV. The slower-closing component had V_r = −13.5 ± 0.5 mV (n = 2).