Potassium channel subunit that does not form functional channels by itself. Modulates KCNB1 and KCNB2 channel activity by shifting the threshold for inactivation to more negative values and by slowing the rate of inactivation. Can down-regulate the channel activity of KCNB1, KCNB2, KCNC4 and KCND1, possibly by trapping them in intracellular membranes. Heteromultimer with KCNB1 and KCNB2. Interacts with KCNC4 and KCND1. Has to be associated with another potassium channel subunit to get inserted in the plasma membrane. Remains intracellular in the absence of KCNB2. Detected in brain. The segment S4 is probably the voltage-sensor and is characterized by a series of positively charged amino acids at every third position. Belongs to the potassium channel family. V (TC 1.A.1.2) subfamily. Kv8.1/KCNV1 sub-subfamily.