A MUTATION IN THE PORE OF THE SODIUM-CHANNEL ALTERS GATING

Ion permeation and channel gating are classically considered independe nt processes, but site-specific mutagenesis studies in K channels sugg est that residues in or near the ion-selective pore of the channel can influence activation and inactivation. We describe a mutation in the pore of the skeletal muscle Na channel that alters gating. This mutati on, I-W53C (residue 402 in the mu 1 sequence), decreases the sensitivi ty to block by tetrodotoxin and increases the sensitivity to block by externally applied Cd2+ relative to the wild-type channel, placing thi s residue within the pore near the external mouth. Based on contempora ry models of the structure of the channel, this residue is remote from the regions of the channel known to be involved in gating, yet this m utation abbreviates the time to peak and accelerates the decay of the macroscopic Na current. At the single-channel level we observe a short ening of the latency to first opening and a reduction in the mean open time compared with the wild-type channel. The acceleration of macrosc opic current kinetics in the mutant channels can be simulated by chang ing only the activation and deactivation rate constants while constrai ning the microscopic inactivation rate constants to the values used to fit the wild-type currents. We conclude that the tryptophan at positi on 53 in the domain I P-loop may act as a linchpin in the pore that li mits the opening transition rate. This effect could reflect an interac tion of I-W53 with the activation voltage sensors or a more global gat ing-induced change in pore structure.