Helical-wheel diagrams showing the pattern of MTS reaction in the colicin Ia hydrophobic segment. The name of each mutant showing an MTS effect on the single-channel conductance is enclosed by a rectangular box. Ovals enclose mutants that showed a less certain effect: a questionable effect on selectivity (I598C), a very slow effect on the macroscopic conductance (G602C), or a small macroscopic conductance effect in 0.1 M KCl buffer (D613C and E614C). The clustering of reactive residues on one face of each helix clearly indicates that these protein segments are α-helical in the open channel state. (This excludes E614C, which presumably lies beyond the end of H9.) The helices are positioned so as to mimic the relative orientation of H8 and H9 in the aqueous crystal structure.