Electrophysiological characterization of the modified hERG potassium channel used to obtain the first cryo-EM hERG structure.

The voltage-gated hERG (human-Ether-à-go-go Related Gene) K channel plays a fundamental role in cardiac action potential repolarization. Loss-of-function mutations or pharmacological inhibition of hERG leads to long QT syndrome, whilst gain-of-function mutations lead to short QT syndrome. A recent open channel cryo-EM structure of hERG represents a significant advance in the ability to interrogate hERG channel structure-function. In order to suppress protein aggregation, a truncated channel construct of hERG (hERG ) was used to obtain this structure. In hERG cytoplasmic domain residues 141 to 350 and 871 to 1,005 were removed from the full-length channel protein. There are limited data on the electrophysiological properties of hERG channels. Therefore, this study was undertaken to determine how hERG influences channel function at physiological temperature. Whole-cell measurements of hERG current (I ) were made at 37°C from HEK 293 cells expressing wild-type (WT) or hERG channels. With a standard +20 mV activating command protocol, neither end-pulse nor tail I density significantly differed between WT and hERG . However, the I deactivation rate was significantly slower for hERG . Half-maximal activation voltage (V ) was positively shifted for hERG by ~+8 mV (p < .05 versus WT), without significant change to the activation relation slope factor. Neither the voltage dependence of inactivation, nor time course of development of inactivation significantly differed between WT and hERG , but recovery of I from inactivation was accelerated for hERG (p < .05 versus WT). Steady-state "window" current was positively shifted for hERG with a modest increase in the window current peak. Under action potential (AP) voltage clamp, hERG I showed modestly increased current throughout the AP plateau phase with a significant increase in current integral during the AP. The observed consequences for hERG I of deletion of the two cytoplasmic regions may reflect changes to electrostatic interactions influencing the voltage sensor domain.