Pore-forming (alpha) subunit of voltage-gated inwardly rectifying potassium channel. Characterized by unusual gating kinetics by producing relatively small outward currents during membrane depolarization and large inward currents during subsequent repolarization which reflect a rapid inactivation during depolarization and quick recovery from inactivation but slow deactivation (closing) during repolarization. Channel properties are modulated by cAMP and subunit assembly. Forms a stable complex with KCNE1 or KCNE2, and that this heteromultimerization regulates inward rectifier potassium channel activity 