Hyperpolarization-activated ion channel exhibiting weak selectivity for potassium over sodium ions. Contributes to the native pacemaker currents in heart (If) and in neurons (Ih). May mediate responses to sour stimuli. Activated by cAMP, and at 10-100 times higher concentrations, also by cGMP. cAMP binding promotes tetramerization and formation of an active channel. Compared to other family members, cAMP has less stimulatory effect on HCN1 because part of the molecules already contain bound cAMP and form homotetramers when cAMP levels are low. Homotetramer. Heterotetramer with HCN2. The potassium channel is composed of a homo- or heterotetrameric complex of pore-forming subunits. Interacts with KCNE2. Interacts with the SH3 domain of CSK. Detected in myocytes in heart sinoatrial node (SAN) and in brain, in particular in the granule cell layer and in Purkinje neuron bodies in the cerebellum. 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. It is uncertain whether Met-1 or Met-30 is the initiator.