Electrophysiological characterization of the modified hERG(T) 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(T)) was used to obtain this structure. In hERG(T) 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(T) channels. Therefore, this study was undertaken to determine how hERG(T) influences channel function at physiological temperature. Whole‐cell measurements of hERG current (I(hERG)) were made at 37°C from HEK 293 cells expressing wild‐type (WT) or hERG(T) channels. With a standard +20 mV activating command protocol, neither end‐pulse nor tail I(hERG) density significantly differed between WT and hERG(T). However, the I(hERG) deactivation rate was significantly slower for hERG(T). Half‐maximal activation voltage (V(0.5)) was positively shifted for hERG(T) 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(T), but recovery of I(hERG) from inactivation was accelerated for hERG(T) (p < .05 versus WT). Steady‐state “window” current was positively shifted for hERG(T) with a modest increase in the window current peak. Under action potential (AP) voltage clamp, hERG(T) I(hERG) showed modestly increased current throughout the AP plateau phase with a significant increase in current integral during the AP. The observed consequences for hERG(T) I(hERG) of deletion of the two cytoplasmic regions may reflect changes to electrostatic interactions influencing the voltage sensor domain.