ANIONS PERMEATE AND GATE GCAC1, A VOLTAGE-DEPENDENT GUARD-CELL ANION CHANNEL

GCAC1 is a strongly voltage-dependent anion channel in the guard-cell plasma membrane of Vicia faba. In patch-clamp experiments, we have inv estigated the permeation and gating properties of GCAC1 with respect t o its anion dependence in the whole-cell and excised-patch configurati on. The relative permeability followed the order SCN- > NO3- > Br- > C l- while the single-channel conductances in symmetrical anionic soluti ons exhibited a nearly inverse sequence. The Cl- dependence of inward currents (Cl- release) is characterized by a maximum single-channel co nductance of 89 pS half-saturating at 87 mM cytoplasmic chloride. In a ddition to this substrate saturation, anion release was also dependent on the external Cl- activity (K-m = 16 mM). In the presence of SCN- a nd Cl-, the single-channel conductance exhibited an anomalous mole-fra ction dependence, identifying GCAC1 as a multi-ion single-file pore. U sing anions with increasing ionic size, a minimum pore diameter of 0.5 nm was assumed from their relative permeabilities. In line with an an ion-selective channel, a tenfold increase in the extracellular anion a ctivity shifted the reversal potential by -59.8 mV. Simultaneously, th e half-activation potential shifted negatively by about 23 mV. A furth er analysis of the anion dependence revealed that extracellular rather than cytosolic anions affect the gating process of GCAC1. From anion substitution experiments, we conclude that anion concentration and spe cies determines both permeation and gating of the plant anion channel GCAC1.