Physical Interactions Between The Cytoplasmic Domains Of KCNQ1 And KCNE1 Channel Subunits

KCNE1 associates with KCNQ1 to form the slowly activating K+ current (IKs) that is critical for myocardial repolarization. Like other voltage-gated K+ channels, it is tetrameric with N- and C-termini oriented intracellular. The C-terminus (CT) is large and predicted to contain various functional properties influencing channel folding, assembly, trafficking and gating. Interactions between the transmembrane segments of KCNE1 and KCNQ1 govern rates of activation and channel conductance. Less is known regarding possible interactions of the C-termini. Many Long QT (LQT) mutations occur in the C-termini of both KCNE1 and KCNQ1. We investigated possible physical interactions between the C-termini of KCNQ1 and KCNE1. Recombinant proteins (KCNE1-CT and KCNQ1-CT), expressed in HEK293 cells are capable of co-precipitation and co-localization within the cell. The direct physical interaction between KCNQ1-CT and KCNE1-CT expressed and purified from E.Coli indicated a direct interaction. Analysis of purified subdomains of KCNQ1-CT further localized the binding region for KCNE1-CT to a region just after the last transmembrane segment, close to the inner membrane surface (349-398). This KCNQ1 segment (Q1C1) was sufficient for account for KCNQ1-CT biding to KCNE1-CT since subdomains C-terminal to this region did not physically interact with KCNE1. The kinetics studies of the interaction between C-terminal cytoplasmic domains of KCNQ1 and KCNE1 proteins, as exhibited by surface plasmon resonance analysis, indicate that Q1C1 region contributes to the bimolecular interaction with dissociation constant of ∼ 4 μM. LQT mutants of KCNE1-CT, D76N and W87F retained binding to Q1C1 with the similar affinity, indicating that disease-causing mutations do not disrupt the association. Our results indicate that the C-termini of KCNQ1 and KCNE1 comprise and interaction domain contains that may play a role in IKs channel behavior.