Structure of the HERG K⁺ channel S5P extracellular linker: Role of an amphipathic α-helix in C-type inactivation

The HERG K⁺ channel has very unusual kinetic behavior that includes slow activation but rapid inactivation. These features are critical for normal cardiac repolarization as well as in preventing lethal ventricular arrhythmias. Mutagenesis studies have shown that the extracellular peptide linker joining the fifth transmembrane domain to the pore helix is critical for rapid inactivation of the HERG K⁺ channel. This peptide linker is also considerably longer in HERG K⁺ channels, 40 amino acids, than in most other voltage-gated K⁺ channels. In this study we show that a synthetic 42-residue peptide corresponding to this linker region of the HERG K⁺ channel does not have defined structural elements in aqueous solution; however, it displays two well defined helical regions when in the presence of SDS micelles. The helices correspond to Trp⁵⁸⁵-Ile ⁵⁹³ and Gly⁶⁰⁴-Tyr⁶¹¹ of the channel. The Trp⁵⁸⁵-Ile⁵⁹³ helix has distinct hydrophilic and hydrophobic surfaces. The Gly⁶⁰⁴-Tyr⁶¹¹ helix corresponds to an N-terminal extension of the pore helix. Electrophysiological studies of HERG currents following application of exogenous S5P peptides show that the amphipathic helix in the S5P linker interacts with the pore region of the channel in a voltage-dependent manner.