Influence of Permeant Ions on Voltage Sensor Function in the Kv2.1 Potassium Channel

We previously demonstrated that the outer vestibule of activated Kv2.1 potassium channels can be in one of two conformations, and that K⁺ occupancy of a specific selectivity filter site determines which conformation the outer vestibule is in. These different outer vestibule conformations result in different sensitivities to internal and external TEA, different inactivation rates, and different macroscopic conductances. The [K⁺]-dependent switch in outer vestibule conformation is also associated with a change in rate of channel activation. In this paper, we examined the mechanism by which changes in [K⁺] modulate the rate of channel activation. Elevation of symmetrical [K⁺] or [Rb⁺] from 0 to 3 mM doubled the rate of on-gating charge movement (Q_on), measured at 0 mV. Cs⁺ produced an identical effect, but required 40-fold higher concentrations. All three permeant ions occupied the selectivity filter over the 0.03–3 mM range, so simple occupancy of the selectivity filter was not sufficient to produce the change in Q_on. However, for each of these permeant ions, the speeding of Q_on occurred with the same concentration dependence as the switch between outer vestibule conformations. Neutralization of an amino acid (K356) in the outer vestibule, which abolishes the modulation of channel pharmacology and ionic currents by the K⁺-dependent reorientation of the outer vestibule, also abolished the K⁺-dependence of Q_on. Together, the data indicate that the K⁺-dependent reorientation in the outer vestibule was responsible for the change in Q_on. Moreover, similar [K⁺]-dependence and effects of mutagenesis indicate that the K⁺-dependent change in rate of Q_on can account for the modulation of ionic current activation rate. Simple kinetic analysis suggested that K⁺ reduced an energy barrier for voltage sensor movement. These results provide strong evidence for a direct functional interaction, which is modulated by permeant ions acting at the selectivity filter, between the outer vestibule of the Kv2.1 potassium channel and the voltage sensor.