Constitutively conducting S6 mutants. (A) Conductance–voltage relations for constitutively conducting S6 mutants Q664C, Y667C, and S668C illustrate steady-state current at negative potentials where the channels are normally closed (n = 3–10). Tail currents were normalized as described in Materials and methods to produce G/Gmax. Points are means ± SEM. (B) Scaled tail currents evoked at −100 mV after a step to +60 mV show constitutively conducting mutant channels deactivate more slowly than control channels. Dotted line indicates zero current level. (C) Average deactivation time constant (see Materials and methods) as a function of voltage for constitutive conductors. (D–G) Homology modeling of constitutively conducting mutants reveals a domain of clustered amino acid side chains. Backbone α-helix is represented as gray ribbon; native side chains of affected residues are represented as space-filled spheres, with Q664 in green, Y667 in orange, and S668 in purple. S1–S4 domains have been omitted from the model views for clarity. (D) Model of hERG based on rKv1.2 (open state) in membrane cross-sectional view. (E) Model of hERG based on MlotiK1 (closed state) in membrane cross-sectional view. (F) Model of hERG based on Kv1.2 (open state) viewed from the cytosol. (G) Model of hERG based on MlotiK1 (closed state) viewed from the cytosol. (H) Amplitude of slow and fast time constants of deactivation for constitutively conducting mutants. (I) Application of CnErg-1 reduces the level of hERG constitutive conductance in Q664C, Y667C, and S668C.
