Sulfate is both a substrate and an activator of the voltage-dependent anion channel of Arabidopsis hypocotyl cells

On the basis of the anion content of in vitro-cultured Arabidopsis plantlets, we explored the selectivity of the voltage-dependent anion channel of the plasma membrane of hypocotyl cells. In the whole-cell configuration, substitution of cytosolic Cl^- by different anions led to the following sequence of relative permeabilities: NO₃^- (2.6) ≥ SO₄^2- (2.0) > Cl^- (1.0) > HCO₃^- (0.8) >> malate^2- (0.03). Large whole-cell currents were measured for NO₃^- and SO₄^2-, about five to six times higher than the equivalent Cl^- currents. Since SO₄^2- is usually considered to be a weakly permeant or non-permeant ion, the components of the large whole-cell current were explored in more detail. Aside from its permeation through the channel with a unitary conductance, about two-thirds that of Cl^-, SO₄^2- had a regulatory effect on channel activity by preventing the run-down of the anion current both in the whole-cell and the outside-out configuration, increasing markedly the whole-cell current. The fact that the voltage-dependent plasma membrane anion channel of hypocotyl cells can mediate large NO₃^- and SO₄^2- currents and is regulated by nucleotides favors the idea that this anion channel can contribute to the cellular homeostasis of important metabolized anions.