EPR Spectroscopic Studies of the Voltage Sensor Domain (Q1-VSD) of Human KCNQ1 Potassium Ion Channel in Lipid Bilayers

Abstract

KCNQ1 is a voltage gated potassium ion channel expressed in several tissues of the body and modulated by another membrane protein KCNE1 (E1). Q1 is involved in the repolarization phase of cardiac action potential. Q1/E1 interaction slows down the activation kinetics required for proper channel and heart function. Mutations in Q1 can cause Long-QT syndrome resulting in atrial fibrillation, sudden infant death syndrome, cardiac arrhythmias and congenital deafness. Q1-VSD is important as the channel is activated by a change in voltage, which is sensed by the VSD. Around 40% of the >200 reported disease-related mutations arise from the amino acid substitutions in the VSD, making structural and functional studies of Q1-VSD important. The structure of a membrane protein varies from micelles to lipid bilayer environment. Site-directed spin labeling (SDSL) EPR is a powerful structural biology technique to study the structural dynamics and topology of membrane proteins in lipid bilayers. In this study, the structural dynamics and topology of several residues of Q1-VSD in the POPC/POPG vesicles were studied using CW-EPR line shape analysis and CW power saturation techniques. Spin labeled mutants F130C and F232C were found to be located inside the lipid bilayer, while Q147C and F222C were found to be solvent accessible. The study provides novel insights to the structural details of Q1-VSD reconstituted in POPC/POPG vesicles for more advanced structural and functional studies.