Molecular determinants of external barium block in Shaker potassium channels

Mutations in the outer pore region of Shaker K+ channels (T-449 and D-447) can influence external Ba2+ block. Substitution of T-449 by A, V or Y differentially reduced Ba2+ block primarily by decreasing the blocking rate, Substitution of D-447 by N resulted in a non-conducting channel with apparently normal gating currents. External Ba2+ can speed the OFF gating current of a different non-conducting mutant, W434F; this effect was markedly attenuated by the D447N substitution, These results suggest that D-447 contributes to an external Ba2+ binding site while T-449 imposes a barrier to the access of that site.