Molecular determinants of voltage sensor movement and its transduction to pore opening by the S4-S5 linker in the hERG potassium channel

The hERG potassium channel, found in cardiac tissues, is an important contributor to cardiac repolarization. Loss of function of hERG channels is associated with long QT syndrome, a condition linked to sudden cardiac death. Unlike other Shaker-like voltage-gated potassium (Kv) channels, hERG channel activation (opening) and deactivation (closing) kinetics are unusually slow and this is critical to their unique role in repolarization. Despite this, the mechanistic basis for slow gating kinetics in hERG remains unclear. In Shaker-like Kv channels movement of the voltage sensor upon depolarization is mechanically transduced by the alpha-helical S4-S5 linker to S6 pore opening. Given the unique gating properties of hERG channels, the details of voltage sensor movement and its coupling with the S6 activation gate are of significant interest. My thesis examines the mechanistic basis to S4 voltage sensor movement and its transduction to pore opening. In my first study, I use a flexibility removed hERG mutant (G546L) to examine the role of S4-S5 linker flexibility in the activation gating of hERG channels. The flexibility removed channels are stabilized in the open conformation and when flexibility is reintroduced elsewhere in the S4-S5 linker there is a restoration of WT-like activation gating. This suggest flexibility of the S4-S5 linker regulates hERG channel closed-state stabilization. I then examine the role of the S4 voltage sensor and the S4-S5 linker in slow deactivation. To do this I examined charge conserving mutations within the S4 voltage sensor, single glycine substitutions in the S4-S5 linker and an N-terminally deleted channel. These data suggest that the intrinsic relaxation transition of the voltage sensor causes the slow deactivation kinetics characteristic of hERG. They also suggest that both the S4 voltage sensor and S4-S5 linker are attractive targets for hERG modifying compounds. In my final study, I investigate the utility of the adult zebrafish whole heart translational model in the screening of hERG modifying compounds. Using the oocyte expression system, I conducted a pharmacological characterization of the zebrafish paralog of hERG, zkcnh6a, and using optical mapping of the zebrafish whole heart, I show that this model may serve as a valuable tool in the screening of hERG modifying compounds.