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.