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. 2012 Sep;140(3):293–306. doi: 10.1085/jgp.201110761

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© 2012 Tan et al.

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Figure 6. — Voltage-clamp fluorometry of residues contributing to coupling. (A) Summary of the voltage-dependent equilibria of activation (i) and deactivation (ii) of ionic current for R4A, R5A, and G6A mutants in the E518C background. Solid lines are fits of the Boltzmann equation to data. (B) Example fluorescent traces recorded from MTSR-labeled R4A E518C hERG using voltage-clamp fluorometry in response to the protocols shown to measure upward motion (i) and return (ii) of the VSD. (C) Summary of the voltage-dependent equilibria of VSD upward motion and return. V_0.5 values for VSD upward motion were −1.4 ± 2 mV, 20.9 ± 2.6 mV, 10 ± 3.5 mV, and 6.2 ± 3.8 mV for E518C, R4A, R5A, and G6A, respectively (SEM; n > 4). V_0.5 values for VSD return were −46.4 ± 1 mV, −34.5 ± 7.2 mV, −29.6 ± 3.8 mV, and −29.0 ± 4.4 mV for E518C, R4A, R5A, and G6A, respectively (SEM; n > 4). (D) Difference between free energy terms (ΔΔG₀) associated with the voltage-dependent equilibrium of current deactivation relative to current activation (open) and VSD return relative to VSD upward motion (gray; SEM; n ≥ 6).