Structural Basis for Gating Pore Current in Periodic Paralysis

Potassium-sensitive Hypokalemic and Normokalemic Periodic Paralysis (HypoPP, NormoPP) are inherited skeletal muscle diseases characterized by episodes of flaccid muscle weakness(1,2). They are caused by mutations in one gating charge in an S4 transmembrane segment in the voltage sensor (VS) of voltage-gated sodium channel Na(v)1.4 or calcium channel Ca(v)1.1(1,2). Mutations of the outermost arginine gating charges (R1 and R2) cause HypoPP(1,2) by creating a pathogenic gating pore in the VS through which cations leak in the resting state(3,4). Mutations of the third arginine gating charge (R3) cause NormoPP(5) owing to cationic leak in activated/inactivated states(6). Here we present high-resolution structures of these pathogenic gating pores in the model bacterial sodium channel Na(V)Ab(7,8). Mutation of R2 in Na(V)Ab gives gating pore current in resting states, whereas mutation of R3 gives gating pore current in activated/inactivated states. Mutations R2G and R3G have no effect on backbone structures of VS, but create aqueous space near the hydrophobic constriction site (HCS) that controls gating charge movement through VS. The R3G mutation extends the extracellular aqueous cleft completely through the activated VS. Although the R2G mutation does not create a continuous aqueous pathway in the activated state, molecular modeling of the resting state reveals a complete water-accessible pathway. Crystal structures of Na(V)Ab/R2G in complex with guanidinium define a potential drug target site. Molecular dynamics simulations illustrate the mechanism of Na(+) permeation through the mutant gating pore in concert with conformational fluctuations of gating charge R4. Our results reveal pathogenic mechanisms of periodic paralysis at the atomic level and suggest designs of drugs that may prevent ionic leak and provide symptomatic relief from these episodic diseases.