Properties of Deactivation Gating Currents in Shaker Channels

The charge versus voltage relation of voltage-sensor domains shifts in the voltage axis depending on the initial voltage. Here we show that in nonconducting W434F Shaker K+ channels, a large portion of this charge-voltage shift is apparent due to a dramatic slowing of the deactivation gating currents, Ig(D) (with tau up to 80 ms), which develops with a time course of similar to 1.8 s. This slowing in Ig(D) adds up to the slowing due to pore opening and is absent in the presence of 4-aminopyridine, a compound that prevents the last gating step that leads to pore opening. A remaining 10-15 mV negative shift in the voltage dependence of both the kinetics and the charge movement persists independently of the depolarizing prepulse duration and remains in the presence of 4-aminopyridine, suggesting the existence of an intrinsic offset in the local electric field seen by activated channels. We propose a new (to our knowledge) kinetic model that accounts for these observations.