Aims: 

KCNQ1 (Kv7.1) assembles with the KCNE1 auxiliary subunit to form the voltage-dependent cardiac I_KS K⁺ channel. Mutations in either KCNQ1 or KCNE1 genes produce the long-QT syndrome, a life-threatening ventricular arrhythmia. Here we studied the static proximity and the voltage-dependent molecular rearrangements of I_KS channel subunits.

Methods: 

KCNQ1 and KCNE1 were fused to ECFP and/or EYFP, expressed in Xenopus oocytes and simultaneous spectral analysis of the fluorescence resonance energy transfer (FRET) combined with two electrode voltage-clamp recording of K⁺ currents were performed.

Results: 

In the channel closed state, a strong constitutive FRET signal was observed. The static FRET signal between KCNQ1 and KCNE1 was stronger with a C-terminal truncation mutant of KCNQ1 (D622-676). In addition, significant static FRET signals were observed when 1:1 molar ratio of C-terminally tagged KCNQ1-CFP and KCNQ1-YFP were co-expressed. Double labeling of KCNQ1 (N- and C-termini, YFP-KCNQ1-CFP) resulted in a marked FRET signal. A clear voltage-dependent change in the FRET signal was recorded at +30 mV concomitantly with I_KS K⁺ currents, suggesting spatial rearrangement of KCNQ1 and KCNE1 subunits during the gating process. However, no voltage-dependent FRET changes were detected between the C-termini of KCNQ1. Notably, both K⁺ current and dynamic FRET changes were abolished when the D76N KCNE1 LQT5 mutant was co-expressed with KCNQ1.

Conclusion: 

Channel gating is accompanied by a spatial rearrangement of the channel complex that propagates to the C-termini of both subunits. The D76N KCNE1 mutant locks the channel in the closed state and abolishes the voltage-dependent dynamic FRET signal.