Structure and function analysis of the KCNQ1 channel in relation to the LQT1 syndrome

• Main Author: Boulet, Inge R
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2007
• Subjects: Biochemistry
• ISBN: 0549384987; 9780549384984

Voltage gated potassium channels are comprised of four subunits, each subunit contains 6 transmembrane helices. These channels open and close upon depolarization or repolarisation of the cellular membrane as they posses a voltage sensor, which enables them to sense alterations in the membrane voltage. Upon a change in membrane voltage, the voltage sensor moves across the membrane, thereby inducing a conformational change in the channel protein, leading to channel opening or closing. KCNQ1 is a voltage-gated potassium channel, eliciting a rapidly activating K+ current. Association with the KCNE1 β-subunit generates the IKs current which is implicated in the repolarising phase of the action potential (AP) in the ventricle. Mutations in the KCNQ1 or KCNE1 gene can lead to a rare disease, the Long QT syndrome (LQT). This disease is characterized by a prolongation of the QT interval on the ECG of the affected patients, which reflects a prolonged repolarisation of the AP in the ventricle. We studied the electrophysiological consequences of a LQT1 mutation "Q357R" in the KCNQ1 gene. This mutation caused significant alterations of the functional properties of the IKs channel, showing a slowing of the activation kinetics and a shift of the voltage dependence towards positive potentials. Furthermore, the mutation decreased the membrane expression of the mutant channel complexes. However, the patient affected with the mutation showed a normal "QTc" interval, further strengthening the concept that IKs forms a "repolarisation reserve in the ventricle. Probably, the IKs current does not significantly contribute to normal repolarisation in the ventricle, but would form a reserve mechanism when repolarisation is prolonged by e.g. drug block of the other repolarising current IKr (rapid) or in conditions of increased adrenergic stress. A substitution scan of the 56 region (6th transmembrane segment) or activation gate region of the KCNQ1 channel revealed several important residues in channel gating. One residue (L353) was crucial for channel closure as the channel was unable to close upon substitution to an alanine or a charged residue. Recently, it was demonstrated that the S6 region interacts with the S4-S5 linker region to couple the activation gate to the voltage sensor (S4). Therefore, we further investigated the S4-S5 linker region of the KCNQ1 channel. The results demonstrated that the S4-S5 linker region forms an α-helix, with all the relevant residues located at one side of the helix. Furthermore, all LQT1 mutations identified in this region coincide at this side of the α-helical pattern indicating the relevance of this region in the IKs channel.