The multi-ion nature of the pore in Shaker K+ channels

We have investigated some of the permeation properties of the pore in Shaker K channels. We determined the apparent permeability ratio of K+, Rb+, and NH4+ ions and block of the pore by external Cs+ ions. Shaker channels were expressed with the baculovirus/Sf9 expression system and the channel currents measured with the whole-cell variant of the patch clamp technique. The apparent permeability ratio, PRb/PK, determined in biionic conditions with internal K+, was a function of external Rb+ concentration. A large change in PRb/PK occurred with reversed ionic conditions (internal Rb+ and external K+). These changes in apparent permeability were not due to differences in membrane potential. With internal K+, PNH4/PK was not a function of external NH4+ concentration (at least over the range 50-120 mM). We also investigated block of the pore by external Cs+ ions. At a concentration of 20 mM, Cs+ block had a voltage dependence equivalent to that of an ion with a valence of 0.91; this increased to 1.3 at 40 mM Cs+. We show that a 4-barrier, 3-site permeation model can simulate these and many of the other known properties of ion permeation in Shaker channels.