Activation and inactivation of the voltage-gated sodium channel: Role of segment S5 revealed by a novel hyperkalaemic periodic paralysis mutation

Hyperkalaemic periodic paralysis, paramyotonia congenita, and potassium-agg ravated myotonia are three autosomal dominant skeletal muscle disorders lin ked to the SCN4A gene encoding the cu-subunit of the human voltage-sensitiv e sodium channel. To date, similar to 20 point mutations causing these diso rders have been described. We have identified a new point mutation, in the SCN4A gene, in a family with a hyperkalaemic periodic paralysis phenotype. This mutation predicts an isoleucine-to-phenylalanine substitution at posit ion 1495 located in the transmembrane segment S5 in the fourth homologous d omain of the human cr-subunit sodium channel. Introduction of the I1495F mu tation into the wild-type channels disrupted the macroscopic current inacti vation decay and shifted both steady-state activation and inactivation to t he hyperpolarizing direction. The recovery from fast inactivation was slowe d, and there was no effect on channel deactivation. Additionally, a signifi cant enhancement of slow inactivation was observed in the I1495F mutation. In contrast, the T704M mutation, a hyperkalaemic periodic paralysis mutatio n located in the cytoplasmic interface of the S5 segment of the second doma in, also shifted activation in the hyperpolarizing direction but had little effect on fast inactivation and dramatically impaired slow inactivation. T hese results, showing that the I1495F and T704M hyperkalaemic periodic para lysis mutations both have profound effects on channel activation and fast-s low inactivation, suggest that the S5 segment maybe in a location where fas t and slow inactivation converge.