Abstract
Voltage-activated potassium (Kv) channels open in response to membrane depolarization and subsequently inactivate through distinct mechanisms. For the model Shaker Kv channel from Drosophila, fast N-type inactivation is thought to occur by a mechanism involving blockade of the internal pore by the N-terminus, whereas slow C-type inactivation results from conformational changes in the ion selectivity filter in the external pore. Kv channel inactivation plays critical roles in shaping the action potential and regulating firing frequency, and has been implicated in a range of diseases including episodic ataxia and arrhythmias. Although structures of the closely related Shaker and Kv1.2 channels containing mutations that promote slow inactivation both support a mechanism involving dilation of the outer selectivity filter, mutations in the outer pores of these two Kv channels have been reported to have markedly distinct effects on slow inactivation, raising questions about the extent to which slow inactivation is related in both channels. In this study, we characterized the influence of a series of mutations within the external pore of Shaker and Kv1.2 channels and observed many distinct mutant phenotypes. We find that mutations at four positions near the selectivity filter promote inactivation less dramatically in Kv1.2 when compared to Shaker, and they identify one key variable position (T449 in Shaker and V381 in Kv1.2) underlying the different phenotypes in the two channels. Collectively, our results suggest that Kv1.2 is less prone to inactivate compared to Shaker, yet support a common mechanism of inactivation in the two channels.