Negative Countercharges and S4 Interaction in Domain IV of Nav1.4

The voltage sensor module comprises the positively charged S4 residues and negative countercharges in S1 to S3 segments. In the present study the putative interaction of these charges in domain IV during fast inactivation of the human voltage gated sodium channel of skeletal muscle was investigated. The effects of charge reversing mutations of R1448 and R1457 were compared to those for mutations at N1366, E1373, N1389 and D1420. Mutations slowed the entry of channels into fast inactivation, slowed or accelerated the recovery of channels from fast inactivation, and reduced the gating charge. Charge swapping mutations had a more pronounced effect to rescue the wild type phenotype on recovery from fast inactivation than for entry into fast inactivation. These results suggest specific residue interactions that promote S4 translocation during the deactivating transition leading to recovery from the fast inactivated state.