POINT MUTATIONS IN IIS4 ALTER ACTIVATION AND INACTIVATION OF RAT-BRAIN-IIA NA CHANNELS IN XENOPUS-OOCYTE MACROPATCHES

Macroscopic currents of wild-type rat brain IIA (RBIIA) and mutant Na channels were recorded in excised patches from Xenopus oocytes. A char ge deletion (K859Q) and an adjacent conservative mutation (L860F) in t he second domain S4 membrane-spanning region differentially altered vo ltage sensitivity and kinetics. Analysis of voltage dependence was con fined to Na currents with fast inactivation kinetics, although RBIIA a nd K859Q (but not L860F) also showed proportional shifts between at le ast two gating modes, rendering currents with fast or slow inactivatio n kinetics, respectively. Compared to RBIIA, the midpoint of the activ ation curve was shifted in both K859Q and L860F by 22 mV to more posit ive potentials, yet this shift was not associated with a corresponding change in the voltage dependence of time constants for activation (ta u(a)) or inactivation (tau(h1), tau(h2)). L860F showed faster activati on time constants tau(a) than RBIIA, while K859Q was slower for both t he activation (tau(a)) and the inactivation components (tau(h1)). Simi larly, the steady-state inactivation curve of L860F but not K859Q shif ted by 9 mV in the hyperpolarizing direction. Thus, the fourth charge in the IIS4 transmembrane segment exerts control over voltage sensitiv ity and kinetics of activation and may interact with structure that in fluence other aspects of channel gating.