Two‐electrode voltage clamp recordings

For experiments in Xenopus laevis oocytes, complementary RNA was transcribed from the cDNA of several constructs using the mMessage mMachine Kit (Ambion, Austin, TX, USA). Stage V–VI X. laevis oocytes were prepared as described previously and were injected with cRNA (Kim et al. 2015). We used female X. laevis frogs that were 100 g or greater in size and the oocytes were prepared using protocols approved by the University of British Columbia Animal Care Committee, or the University of Alberta Animal Care Committee, in accordance with the Canadian Council for Animal Care guidelines. Frogs were anaesthetized by bathing in 1.5% tricaine methanesulphonate sulphonate solution buffered with bicarbonate, prior to the surgery for harvesting of oocytes. Oocytes were incubated post injection for 12–96 h at 18°C before recording. We recorded voltage‐clamped potassium currents in standard Ringer solution (in mm): 116 NaCl, 2 KCl, 1 MgCl₂, 0.5 CaCl₂ and 5 Hepes (pH 7.4) using an OC‐725C voltage clamp (Warner, Hamden, CT, USA). Glass microelectrodes were backfilled with 3 m KCl and had resistances of 0.1–1 MΩ. Data were filtered at 5 kHz and digitized at 10 kHz using a Digidata 1440A (Molecular Devices) controlled by pClamp, version 10 (Molecular Devices).