Structural Basis for KCNQ1 Long-QT Syndrome Disease causing Mutations

Abstract

The human potassium channel KCNQ1 is a polytopic α-helical membrane protein. KCNQ1 is expressed in both epithelial and heart tissue. In the heart, KCNQ1, in association with KCNE1, mediates the Iks current responsible for the repolarization of the cardiac action potential. Mutations that cause a loss-of-function of KCNQ1 result in a congenital condition known as long-QT syndrome (LQTS). Congenital LQTS predisposes an individual to cardiac arrhythmia and can result in sudden death. While current homology models are sufficient to generate testable structure-function hypotheses, low sequence conservation between KCNQ1 and other potassium channel structures, especially in the voltage-sensing domain, highlight the need for an experimentally determined KCNQ1 structure. Our focus is on the KCNQ1 voltage-sensing domain (Q1-VSD) that mediates the voltage response of the ion channel. Isolated Q1-VSDs exist in equilibrium between an open and closed state, likely favoring the open conformation. To facilitate nuclear magnetic resonance (NMR) structural investigation of the Q1-VSD, we employ state-locking mutations that provide a homogeneous reference spectrum. Our goal is to develop a biochemical model for the disease mechanism of specific LQTS mutations. The structural effects of specific LQTS disease-causing mutations are being evaluated in two ways (1) by comparison to the Q1-VSD ‘locked’ open- and closed-state NMR reference spectra and (2) in the context of current KCNQ1 homology models. Preliminary comparison of wild type Q1-VSD to a ‘locked’ closed-state construct by NMR in model membranes indicates a global conformational rearrangement. Ultimately we seek to leverage the closed-state stabilizing mutations to facilitate NMR structure determination of the Q1-VSD in its inactive “down” state.