2 DIFFERENT CONDUCTANCES CONTRIBUTE TO THE ANION CURRENTS IN COFFEA-ARABICA PROTOPLASTS

The anion conductance of the plasma membrane of Coffea arabica protopl asts was isolated and characterized using the whole-cell patch clamp t echnique. Voltage pulse protocols revealed two components: a voltage-g ated conductance (G(s)) and a voltage-independent one (G(l)). G(s) is activated upon depolarization (e-fold activation every +36 mV) with ti me constants of 1 sec and 5 sec at all potentials. G(l) and G(s) also differ by their kinetic and biophysical properties. In bi-ionic condit ions the current associated with G(s) shows strong outward rectificati on and its permeability sequence is F- > NO3- > Cl-. In the same condi tions the current associated with G(l) does not rectify and its permea bility sequence is F(-)much greater than NO3- = Cl-. Furthermore, at p otentials over +50 mV G(s), but not G(l), increases with a time consta nt of several minutes. Finally the gating of G(s) is affected by stret ch of the membrane, which leads to an increased activation and a reduc ed voltage sensitivity. Anion conductances similar to the ones describ ed here have been found in many plant preparations but G(l)-type compo nents have been generally interpreted as the background activation of the slow voltage-gated channels (corresponding to G(s)). We show that in coffee protoplasts G(l) and G(s) are kinetically and biophysically distinct, suggesting that they correspond to two different molecular e ntities.