Potassium channel ancillary subunit that is essential for generation of some native K(+) currents by virtue of formation of heteromeric ion channel complex with voltage-gated potassium (Kv) channel pore-forming alpha subunits. Functions as an inhibitory beta-subunit of the repolarizing cardiac potassium ion channel KCNQ1. Interacts with KCNQ1; impairs KCNQ1 localization in lipid rafts and only conducts current upon strong and continued depolarization. Detected in embryonal dorsal root and nerve ganglia, in the somites and in myoepicardial layer of the developing heart wall. Detected at lower levels in the central nervous system (CNS) and in developing limb. Belongs to the potassium channel KCNE family.