Ancillary protein that assembles as a beta subunit with a voltage-gated potassium channel complex of pore-forming alpha subunits. Modulates the gating kinetics and enhances stability of the channel complex. Associated with KCNQ1/KVLTQ1 inhibits potassium current (By similarity). Interacts with KCNQ1; impairs KCNQ1 localization in lipid rafts and inhibits voltage-gated potassium channel activity. Belongs to the potassium channel KCNE family.