Charge Reversal of the Uppermost Arginine in Sliding Helix S4-I Affects Gating of Cardiac Sodium Channel.

Several mutations of the uppermost arginine, R219, in the voltage-sensing sliding helix S4 of cardiac sodium channel Nav1.5 are reported in the ClinVar databases, but the clinical significance of the respective variants is unknown (VUSs). AlphaFold 3 models predicted a significant downshift of S4 in the R219C VUS. Analogous downshift S4, upon its in silico deactivation, resulted in a salt bridge between R219 and the uppermost glutamate, E161, in helix S2. To understand how salt bridge elimination affects biophysical characteristics, we generated mutant channel R219E, expressed it in the HEK293-T cells, and employed the patch-clamp method in a whole-cell configuration. Mutation R219E did not change the peak current density but shortened time to the peak current at several potentials, significantly enhanced activation, enhanced steady-state inactivation and steady-state fast inactivation, and slowed recovery from inactivation. Taken together, these data suggest that mutation R219E destabilized the resting state of Nav1.5. Cardiac syndromes associated with mutations R219P/H/C/P or E161Q/K are consistent with the observed changes of biophysical characteristics of mutant channel R219E suggesting pathogenicity of the respective VUSs, as well as ClinVar-reported VUSs involving arginine or glutamate in homologous positions of several Nav1.5 paralogs.