Mutating R217 affects both voltage dependence and kinetics of charge movement. (A) Structural model of the VSD of Ci-VSP built using the package MODELLER and taking the structure of the chimeric potassium channel Kv1.2/Kv2.1 (2R9R; Long et al., 2007) as template. Segments S1 to S3 are displayed in gray. The S4 segment (green) contains five arginines (blue). Arginine 217 faces the extracellular space, whereas the remaining four arginines point toward the core of the VSD. (B) Sensing currents from mutants C363S, R217Q-C363S, and R217E-C363S. The HP for these recordings was −60 mV for C363S, −90 mV for R217Q-C363S (hereafter R217Q), and −120 mV for R217E-C363S (hereafter R217E). Respectively, voltage ranged from −80 to 160 mV, −120 to 40 mV, and −140 to 20 mV. (C) Net charge versus potential (Q-V) curves for the mutants C363S, R217Q, and R217E. Fitted parameters are reported in Table 1. (D) Time constant for the sensing currents of the mutants in B and C. Both R217Q and R217E displayed faster ON-sensing current than the background construct C363S at all potentials tested (see R217 modulates the voltage dependence of Ci-VSP for detail). Error bars are SD.