Abstract
The synthetic ciguatoxin, CTX3C has four distinct effects on Na channels:1) to speed up time-to-peak Na current (INa), 2) to suppress peak INa amplitude, 3) to shift the activation curve in the hyperpolarizing direction, and 4) to delay recovery from "slow inactivation." In this study, we explored possible sites modulating CTX effects. Mutant channels N434A, L437A or A438K lacked effect 3 of CTX3C. These sites in D1S6 selectively modulate activation mechanisms via CTX binding. In separate experiments, we systematically replaced positively charged amino acids (Arg or Lys) with a neutral residue (Gln) in each S4 segment of the four domains. In all domains, neutralization of positive charges did not suppress the hyperpolarizing shift of the activation curve induced by the toxin (effect 3). However, CTX3C(3μM) did not suppress INa in the four mutants with neutralized charges in D2 (lack of effect 2). Interestingly, some of the D2 mutants even exhibited an increase in INa in the presence of CTX3C: INa grew progressively larger as the location of the mutations approached the extracellular face of the membrane. CTX3C is a membrane-spanning molecule having a length of 30 Å and possessing a relatively constrained structure. Thus, CTX3C may affect the voltage sensor by accessing the sodium channel from its exterior surface through the membrane lipid phase. [J Physiol Sci. 2006;56 Suppl:S80]
