pH-sensitive gating kinetics of the maxi-K channel in the tonoplast of Chara australis.

The most frequently observed K+ channel in the tonoplast of Characean giant internodal cells with a large conductance (ca. 170 pS; Lühring, 1986; Laver & Walker, 1987) behaves, although inwardly rectifying, like animal maxi-K channels. This channel is accessible for patch-clamp techniques by preparation of cytoplasmic droplets, where the tonoplast forms the membrane delineating the droplet. Lowering the pH of the bathing solution, that virtually mimics the vacuolar environment, from an almost neutral level to values below pH 7, induced a significant but reversible decrease in channel activity, whereas channel conductance remained largely unaffected. Acidification (pH 5) on both sides of the membrane decreased open probability from a maximum of 80% to less than 20%. Decreasing pH at the cytosolic side inhibited channel activity cooperatively with a slope of 2.05 and a pKa 6.56. In addition, low pH at the vacuolar face shifted the activating voltage into a positive direction by almost 100 mV. This is the first report about an effect of extraplasmatic pH on gating of a maxi-K channel. It is suggested that the Chara maxi-K channel possesses an S4-like voltage sensor and negatively charged residues in neighboring transmembrane domains whose S4-stabilizing function may be altered by protonation. It was previously shown that gating kinetics of this channel respond to cytosolic Ca2+ (Laver & Walker, 1991). With regard to natural conditions, pH effects are discussed as contributing mainly to channel regulation at the vacuolar membrane face, whereas at the cytosolic side Ca2+ affects the channel. An attempt was made to ascribe structural mechanisms to different states of a presumptive gating reaction scheme.