Gating currents from a Kv3 subfamily potassium channel: charge movement and modification by BDS-II toxin

Abstract

Kv3 channels have a major role in determining neuronal excitability, and are characterized by ultra-rapid kinetics of gating and a high activation threshold. However, the gating currents, which occur as a result of positional changes of the charged elements in the channel structure during activation, are not well understood. Here we report a study of gating currents from wild-type Kv3.2b channels, expressed in human embryonic kidney (HEK) cells to facilitate high time-resolution recording. On-gating currents (I_g,on) had extremely rapid kinetics such that at +80 mV, the time constant for the decay of I_g,on was only ∼0.3 ms. Decay of I_g,on appeared mono-exponential at all potentials studied, and in support of this, the charge–voltage (Q–V) relationship was fitted with a single Boltzmann function, supporting the idea that only one charge system is required to account for the time course of I_g,on and the voltage dependence of Q_on. The voltage (V_½) for half movement of gating charge was −8.4 ± 4.0 mV (n = 6), which closely matches the voltage dependence of activation of Kv3.2b ionic currents reported before. Depolarizations to more positive potentials than 0 mV decreased the amplitude and slowed the decay of the off-gating currents (I_g,off), suggesting that a rate-limiting step in opening was present in Kv3 channels as in Shaker and other Kv channels. Return of charge was negatively shifted along the potential axis with a V_½ of Q_off of −80.9 ± 0.8 mV (n = 3), which allowed ∼90% charge return upon repolarization to −100 mV. BDS-II toxin apparently reduced I_g,on, and greatly slowed the kinetics of I_g,on, while shifting the Q–V relationship in the depolarizing direction. However, the Q–V relationship remained well fitted by a single Boltzmann function. These data provide the first description of Kv3 gating currents and give further insight into the interaction of BDS toxins and Kv3 channels.