SODIUM-CHANNEL INACTIVATION IS ALTERED BY SUBSTITUTION OF VOLTAGE SENSOR POSITIVE CHARGES

The role of the voltage sensor positive charges in fast and slow inact ivation of the rat brain IIA sodium channel was investigated by mutati ng the second and fourth conserved positive charges in the S4 segments of all four homologous domains. Both charge-neutralizing mutations (b y glutamine substitution) and charge-conserving mutations were constru cted in a cDNA, encoding the sodium channel oc subunit. To determine i f fast inactivation altered the effects of the mutations on slow inact ivation, the mutations were also constructed in a channel that had fas t inactivation removed by the incorporation of the IFMQ3 mutation in t he III-IV linker (West, J.M.,, D.E. Patron, T. Scheuer, Y. Wang, A.L. Goldin, and W.A. Catterall. 1992. A oc. Natl. Acad. Sci. USA. 89:10910 -10914). Most of the mutations shifted the v(1/2) of fast inactivation in the negative direction, with the largest effects resulting from mu tations in domains I and II. These shifts were in the opposite directi on compared with those observed for activation. The effects of the mut ations on slow inactivation depended on whether fast inactivation was intact or not. When fast inactivation was eliminated, most of the muta tions resulted in positive shifts in the v(1/2) of slow inactivation. Tile largest effects again resulted from mutations in domains I and II . When fast inactivation was intact, the mutations in domains II and I II resulted in negative shifts in the v(1/2) of slow inactivation. Neu tralization of the fourth charge in domain I or II resulted in the app earance of a second component in the voltage dependence of slow inacti vation that was only observable when fast inactivation was intact. The se results suggest the S4 regions of all four domains of the sodium ch annel are involved in the voltage dependence of inactivation, but to v arying extents. Fast inactivation is not strictly coupled to activatio n, but it derives some independent voltage sensitivity from the charge s in the S4 domains. Finally, there is an interaction between the fast and slow inactivation processes.