Human ether-a-go-go related gene (hERG) potassium channel gating and drug block

hERG encodes the a-subunit of the rapid delayed rectifier potassium current, a crucial current for normal repolarisation of the cardiac action potential. Pharmacological block of hERG is associated with arrhythmias and sudden death. Given its physiological importance, aspects of both the gating and pharmacology of this channel were investigated.;hERG has unusual gating properties characterised by slow activation and deactivation gating. The roles of conserved S6 glycines (Gly648 and Gly657) in hERG as hinges for activation gating were studied. Glycine residues impart flexibility that is thought to be conducive for channel opening. However, mutations at positions 648 and 657 altered gating in a manner consistent with a role in protein packing rather than flexibility. Deactivation gating in hERG is slow due to interactions between the amino-terminus, the voltage sensor, and the pore that stabilise the open state. The pore mutation V659A dramatically slowed channel deactivation and reduced drug block. Replacing Val659 with larger hydrophobic residues resulted in faster deactivation kinetics, but in contrast, V659G hERG was constitutively open. It was concluded that Val659 mutations influence deactivation through hydrophobic interactions with the S4-S5 linker that couples S6 to the voltage sensor. Effects on drug binding correlated with deactivation rates, indicating that Val659 mutations have allosteric rather than direct effects on drug binding.;Tyr652 is thought to be a critical residue for high affinity drug binding. However, this study showed that the contribution of Tyr652 to drug binding varied considerably among 24 compounds tested, with the majority of low affinity blockers being relatively insensitive to the Y652A mutation. Pharmacophore models generated from the results suggest that higher affinity compounds are longer than lower affinity compounds and simultaneously interact with multiple inner cavity residues. The compact structure of low affinity, Y652A-insensitive drugs permits multiple binding modes, making the compounds less reliant on interactions with Tyr652.