Human ether á go-go related gene (hERG) potassium channels play a central role in cardiac repolarization where channel closing (deactivation) regulates current density during action potentials. Consequently, mutations in hERG that perturb deactivation are linked to long QT syndrome (LQTS), a catastrophic cardiac arrhythmia. Interactions between an N-terminal domain and the pore-forming "core" of the channel were proposed to regulate deactivation, however, despite its central importance the mechanistic basis for deactivation is unclear. Here, to more directly examine the mechanism for regulation of deactivation, we genetically fused N-terminal domains to fluorescent proteins and tested channel function with electrophysiology and protein interactions with Förster resonance energy transfer (FRET) spectroscopy. Truncation of hERG N-terminal regions markedly sped deactivation, and here we report that reapplication of gene fragments encoding N-terminal residues 1-135 (the "eag domain") was sufficient to restore regulation of deactivation. We show that fluorophore-tagged eag domains and N-truncated channels were in close proximity at the plasma membrane as determined with FRET. The eag domains with Y43A or R56Q (a LQTS locus) mutations showed less regulation of deactivation and less FRET, whereas eag domains restored regulation of deactivation gating to full-length Y43A or R56Q channels and showed FRET. This study demonstrates that direct, noncovalent interactions between the eag domain and the channel core were sufficient to regulate deactivation gating, that an LQTS mutation perturbed physical interactions between the eag domain and the channel, and that small molecules such as the eag domain represent a novel method for restoring function to channels with disease-causing mutations.
