Gate Closure Strictly Follows Voltage-Sensor Movements in K V Channels

Abstract

Kv channels are voltage-dependent potassium pores that shape the action potential duration and are critical for cell excitability. Detection of membrane potential (V) is done by a charged (Q) voltage sensor domain (VSD) whose reorientations generate a transient gating current (IQ). Prolonged depolarization of Shaker Kv channels pushes the VSD into the relaxed state, characterized by a slowing in IQOff. Kv channels also have two gates (in series) that seal off K+ permeation: the S6 bundle crossing (BC), directly tied to the VSD, and the selectivity filter (SF). Direct comparison of K+-conduction in Shaker, reflecting the status of the BC gate, with IQ shows a strong correlation between both. As IQOff slowed down with prolonged depolarizations, BC gate closure displayed a similar 2-fold slowing when the duration of a +20mV pre-pulse was increased from 0.2 to 10 seconds. Simultaneous monitoring of the VSD movement (fluorescence recordings) and channel gate closure (ionic recordings) in the TMRM-labeled Shaker mutant M356C showed that the slowing in IQOff and gate closure occurs simultaneously. This indicates that the gate is strictly controlled by the movements of the VSD and most importantly that the BC gate remains open even when the VSD relaxes. Consequently, K+ conduction continues as long as the SF gate does not close (inactivation). Interestingly, in Kv3.1 - a channel that regulates high frequency firing in-vivo - the opposite behavior was observed: prolonged depolarization speeded up both IQOff and gate closure. Thus, the effect of VSD relaxation differs between different subtypes of Kv channels suggesting that relaxation affects the excitability of cells differently depending on their depolarization history, either reducing excitability in cells expressing Shaker or increasing it in case of Kv3.1. (Support: NIH-GM030376, FWO-G025608)