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 KCNH2/HERG is proposed to form the rapidly activating component of the delayed rectifying potassium current in heart (IKr). May associate with KCNQ2 and/or KCNQ3 and modulate the native M-type current. May associate with KCNQ1/KVLTQ1 and elicit a voltage-independent current. May associate with HCN1 and HCN2 and increase potassium current. Defects in KCNE2 are the cause of long QT syndrome type 6 (LQT6). Long QT syndromes are heart disorders characterized by a prolonged QT interval on the ECG and polymorphic ventricular arrhythmias. They cause syncope and sudden death in response to exercise or emotional stress. KCNE2 mutants form channels that open slowly and close rapidly, thereby diminishing potassium currents. Defects in KCNE2 are the cause of familial atrial fibrillation type 4 (ATFB4). Atrial fibrillation is a common disorder of cardiac rhythm that is hereditary in a small subgroup of patients. It is characterized by disorganized atrial electrical activity and ineffective atrial contraction promoting blood stasis in the atria and reduces ventricular filling. It can result in palpitations, syncope, thromboembolic stroke, and congestive heart failure. Belongs to the potassium channel KCNE family. Note: This description may include information from UniProtKB.