The following voltage clamp protocol is used: from V_h −100 mV, 2-s depolarizing test pulses to V_t of −50 to +100 mV in 10-mV increments are applied at an interpulse interval of 30 s. The test pulses are followed by a 2-s step to −60 mV, during which tail currents are measured. The relationship between V_t and peak amplitudes of tail currents (I_tail) is fit with a simple Boltzmann function (except DTT-treated Q1(-Cys)-I145C/E1-G40C): I_tail = I_max/(1 + exp[(z_g(V_0.5 − V_t)F/RT]), where I_max = estimated maximal I_tail, z_g = apparent gating charge, V_0.5 = half-maximum activation voltage, F = Faraday constant, R = gas constant, and T = absolute temperature. Currents from DTT-treated Q1(-Cys)-I145C/E1-G40C manifest a prominent constitutive component. In this case, the data are fit with a modified Boltzmann function: I_tail = I_tail,_step + I_tail,_t-d/(1 + exp[(z_g(V_0.5 − V_t)F/RT]), where I_tail,_step and I_tail,t-d represent tail currents of the constitutive and time-dependent components, respectively. Data are pooled from two to four batches of oocytes (n = total number of oocytes studied). After DTT: oocytes are treated with DTT (10 mM for 10 min or 25 mM for 5 min at room temperature, followed by thorough rinsing) before voltage clamping.