A novel Beta Subunit Mechanism in a Critical Heart K Channel: KCNE1 Alters the Voltage Sensor Movements Necessary to Open the KCNQ1 Channel Gate

Abstract

The delayed rectifier IKs potassium channel, a major contributor to repolarization of the human cardiac action potential, forms by co-assembly of alpha (KCNQ1) and beta (KCNE1) subunits. Though KCNQ1 encodes a functional voltage-gated potassium channel, it requires the presence of KCNE1 to reproduce the biophysical characteristics of the native IKs channel. Despite much study, the mechanism(s) through which KCNE1 modulates KCNQ1 are not well understood, partly due to the lack of a voltage sensor assay. Voltage clamp fluorometry (VCF) is a method for measuring voltage sensor movements by monitoring changes in the environment of a fluorophore, attached to the voltage sensor, during gating. Here we present the first VCF study of KCNQ1 and KCNQ1/KCNE1 channels. In KCNQ1, while the time course of the fluorescence signal narrowly precedes current, steady state voltage dependence of fluorescence and current are indistinguishable, suggesting a one-to-one relationship between voltage sensor movement and channel opening. Mutational analysis of the KCNQ1 fluorescence signal provides novel mechanistic insight into the gating of this channel and validates the use of VCF in this channel. Co-expression of KCNE1 dramatically separates the voltage dependence of fluorescence and current signals, suggesting multiple independent voltage sensor movements are now required for channel opening. This prediction is further supported by measurement of a Cole-Moore effect in KCNQ1/KCNE1 but not KCNQ1 alone, and by computer-based Markov modeling of channel kinetics. Taken together, our results indicate a KCNE1-dependent change in the coupling between the voltage sensor and the channel gate. This represents the first measurements of KCNQ1 voltage sensor using VCF and elucidates a novel role for a potassium channel beta-subunit: to alter the relationship between voltage sensor movement and channel opening.