Molecular determinants of external barium block in Shaker potassium channels.

Mutations in the outer pore region of Shaker K+ channels (T449 and D447) can influence external Ba2+ block. Substitution of T449 by A, V or Y differentially reduced Ba2+ block primarily by decreasing the blocking rate. Substitution of D447 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 D447 contributes to an external Ba2+ binding site while T449 imposes a barrier to the access of that site.