Collapse of conductance is prevented by a glutamate residue conserved in voltage-dependent K+ channels

Voltage-dependent K+ channel gating is influenced by the permeating ions. E xtracellular K+ determines the occupation of sites in the channels where th e cation interferes with the motion of the gates. When external [K+] decrea ses, some K+ channels open too briefly to allow the conduction of measurabl e current. Given that extracellular K+ is normally low, we have studied if negatively charged amino acids in the extracellular loops of Shaker K+ chan nels contribute to increase the local [K+]. Surprisingly, neutralization of the charge of most acidic residues has minor effects on gating. However, a glutamate residue (E418) located at the external end of the membrane spann ing segment S5 is absolutely required for keeping channels active at the no rmal external [K+]. E418 is conserved in all families of voltage-dependent K+ channels. Although the channel mutant E418Q has kinetic properties resem bling those produced by removal of K+ from the pore, it seems that E418 is not simply concentrating cations near the channel mouth, but has a direct a nd critical role in gating. Our data suggest that E418 contributes to stabi lize the S4 voltage sensor in the depolarized position, thus permitting mai ntenance of the channel open conformation.