Ionic currents and G-V curve of Shaker tetramers wt (4wt), with two (2wt/2mut) or three subunits (1wt/3mut) with neutralized S4 gating charges. (A; Top) Scaled ionic currents from oocytes expressing Shaker zH4 IR (black line) or our tetramer 4wt (red line) for selected membrane potentials, as indicated. The maximal current recorded at +80 mV was around 5 µA for each oocyte. (Bottom) Averaged normalized conductance (G)-membrane potential (V) relation taken from the tail currents, Shaker zH4 IR (n = 4; filled black squares) and tetramer 4wt (n = 4; filled red circles). Holding membrane potential was −90 mV, and each depolarization was preceded by a 20-ms prepulse to −120 mV. The internal and external solutions contained 120 and 12 mM K⁺, respectively. (B) Ionic currents from oocytes expressing 4wt, 2wt/2mut, or 1wt/3mut heterotetramers. The voltage pulse protocol is shown on top. The HP was −90 mV. Pulse from −140 to +40 mV in 10-mV increments was applied, and the tail currents were measured at −50 mV for 4wt and at −80 mV for 2wt/2mut and 1wt/3mut. (C) Normalized G-V curve for 4wt (filled squares), 2wt/2mut (filled circles), and 1wt/3mut (filled triangles). The lines represent the global fit of a simple Boltzmann distribution for 1wt/3mut and Boltzmann distribution taken to the “n^th” power for 2wt/2mut and 4wt, all sharing the parameters V_1/2 and z. The HP was −90 mV for 4wt and −120 mV for 2wt/2mut and 1wt/3mut, and depolarization from −70 to +40 mV was applied for 4wt and from −120 to +40 mV for 2wt/2mut and 1wt/3mut. The conductance was measured using the tail currents. The internal and external solutions both contained 120 mM K⁺. Cut-open voltage clamp measurements.