KCNE1-Dependent Sumoylation of KV7.1 Subunits Determines the Voltage-Dependence of Cardiac IKs Channels

KV7.1 and KCNE1 subunits assemble to form IKs channels, which are fundamental to cardiac repolarization. Here, we identify SUMOylation of IKs channels as a fundamental regulatory pathway in the heart. A report (Qi et al. Neuron. 2014) that partial deficiency of SENP2 deSUMOylase in mice produced seizures, bradycardia and sudden death in association with hyperSUMOylation of M-current channels (KV7.2/KV7.3) led us to study IKs currents in neonatal mice. We found intracellular application of SENP2 to increase IKs current due to a −20 mV shift in the voltage-dependence of activation (V1/2); in contrast, SUMO2 application decreased the current due to a +20 mV shift in V1/2. A 40 mV excursion in V1/2 between the deSUMOylating and SUMOylating conditions was seen also when mouse or human KV7.1 and KCNE1 subunits were expressed in Chinese hamster ovary (CHO) cells. Forster resonance energy transfer (FRET) confirmed co-assembly of YFP-SUMO2 and IKs channels formed with KV7.1-CFP. Consistent with SUMOylation, mutation of a single target residue in KV7.1 abolished FRET and the effects of SENP2 or SUMO2 on IKs current density. To count the number of SUMO2 subunits in IKs complexes, total internal reflection fluorescence (TIRF) microscopy with simultaneous two-color photobleaching was used. IKs channels carry a maximum of four SUMO2s, one on each Kv7.1 subunit. Unexpectedly, Kv7.1 channels studied in the absence of KCNE1 carry at most two SUMO2s despite having four available Kv7.1 SUMO-sites. Modification of both Kv7.1 channels and IKs channels by two SUMO2s produced a 20 mV shift in V1/2 while four SUMO2s produced a 40 mV shift in IKs channels. We propose that KCNE1-dependent SUMOylation of KV7.1 is required to yield the native biophysical attributes observed for IKs channels in mammalian cardiac myocytes.