Hyperpolarization-activated potassium channel. May also facilitate the permeation of sodium ions. Inhibited by Cs(1+) and ZD7288. Is apparently not activated by cAMP. Homotetramer. The potassium channel is probably composed of a homo- or heterotetrameric complex of pore-forming subunits. Interacts with KCTD3 and PEX5L. 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 HCN family.