Physical and functional interaction sites in cytoplasmic domains of KCNQ1 and KCNE1 channel subunits.

The cardiac potassium IKs current is carried by a channel complex formed from α-subunits encoded by KCNQ1 and β-subunits encoded by KCNE1. Deleterious mutations in either gene are associated with hereditary long QT syndrome. Interactions between the transmembrane domains of the α- and β-subunits determine the activation kinetics of IKs. A physical and functional interaction between COOH termini of the proteins has also been identified that impacts deactivation rate and voltage dependence of activation. We sought to explore the specific physical interactions between the COOH termini of the subunits that confer such control. Hydrogen/deuterium exchange coupled to mass spectrometry narrowed down the region of interaction to KCNQ1 residues 352-374 and KCNE1 residues 70-81, and provided evidence of secondary structure within these segments. Key mutations of residues in these regions tended to shift voltage dependence of activation toward more depolarizing voltages. Double-mutant cycle analysis then revealed energetic coupling between KCNQ1-I368 and KCNE1-D76 during channel activation. Our results suggest that the proximal COOH-terminal regions of KCNQ1 and KCNE1 participate in a physical and functional interaction during channel opening that is sensitive to perturbation and may explain the clustering of long QT mutations in the region.NEW & NOTEWORTHY Interacting ion channel subunits KCNQ1 and KCNE1 have received intense investigation due to their critical importance to human cardiovascular health. This work uses physical (hydrogen/deuterium exchange with mass spectrometry) and functional (double-mutant cycle analyses) studies to elucidate precise and important areas of interaction between the two proteins in an area that has eluded structural definition of the complex. It highlights the importance of pathogenic mutations in these regions.