K+-dependent stability and ion conduction of Shab K+ channels: a comparison with Shaker channels.

K+ depletion exerts dramatically variable effects on different potassium channels. Here we report that Shab channels are rather stable in the absence of either internal or external K+ alone; however, its stability is greater with K+ outside the cell. In contrast, with 0 K+ (non-added) solutions on both sides of the membrane, the conductance (G(K)) is rapidly and irreversibly lost. G(K) is lost with the channels closed and regardless of the composition of the 0 K+ solutions. In comparison, it is known that the Shaker B G(K) collapses only if the channels are gated in 0 K+, Na+-containing solutions. In order to compare the behavior of Shab to that of Shaker, we show that after extensively gating the channels in 0 K+ N-methyl-D-glucamine solutions, most Shaker channels remain stable, and in a conformation where G(K) collapses as soon as there is Na+ in the solutions. Regarding ion conduction, in contrast to Kv2.1 and Shaker A463C that have a sizable G(Na) in 0 K+, Shab, which shares a 463-cysteine and an identical signature sequence with these channels, does not appreciably conduct Na+, although it presents a significant Cs+ conductance. The observations suggest that there are at least two sites where K+ binds and thus maintains Shab G(K) stable, one internal and the other(s) most likely located outside the selectivity filter.