Probing the Dynamics and Structural Topology of the Reconstituted Human KCNQ1 Voltage Sensor Domain (Q1-VSD) in Lipid Bilayers Using Electron Paramagnetic Resonance Spectroscopy

KCNQ1 (Kv7.1 or KvLQT1) is a potassium ion channel protein found in the heart, ear, and other tissues. In complex with the KCNE1 accessory protein, it plays a role during the repolarization phase of the cardiac action potential. Mutations in the channel have been associated with several diseases, including congenital deafness and long QT syndrome. Nuclear magnetic resonance (NMR) structural studies in detergent micelles and a cryo-electron microscopy structure of KCNQ1 from Xenopus laevis have shown that the voltage sensor domain (Q1-VSD) of the channel has four transmembrane helices, S1-S4, being overall structurally similar with other VSDs. In this study, we describe a reliable method for the reconstitution of Q1-VSD into (POPC/POPG) lipid bilayer vesicles. Site-directed spin labeling electron paramagnetic resonance spectroscopy was used to probe the structural dynamics and topology of several residues of Q1-VSD in POPC/POPG lipid bilayer vesicles. Several mutants were probed to determine their location and corresponding immersion depth (in angstroms) with respect to the membrane. The dynamics of the bilayer vesicles upon incorporation of Q1-VSD were studied using P-31 solid-state NMR spectroscopy by varying the protein:lipid molar ratios confirming the interaction of the protein with the bilayer vesicles. Circular dichroism spectroscopic data showed that the a-helical content of Q1-VSD is higher for the protein reconstituted in vesicles than in previous studies using DPC detergent micelles. This study provides insight into the structural topology and dynamics of Q1-VSD reconstituted in a lipid bilayer environment, forming the basis for more advanced structural and functional studies.