Asymmetric block of the plant vacuolar Ca2+-permeable channel by organic cations

In this work we have analysed the voltage-dependent block of the slow activating channel from red beet vacuoles by Tris, quaternary ammonium ions and the natural polyamines putrescine, spermidine and spermine. All these organic cations when applied from the cytosolic side blocked the channel by binding apparently deep (z delta values in the range of 0.65-1.35) within the pore. Tetraethylammonium ion did not pass the selectivity filter, whereas the cations with a smaller cross-section and Tris could pass across the entire pore, as evidenced by a relief of block at high positive voltages. Voltage dependence of the establishment of block from cytosolic side and of its relief was anomalously strong in the sense that the total charge moved across the pore for all blockers tested, with a notable exception of spermine, was in excess of their actual valence. This behaviour is consistent with the existence of multiple binding sites within a long pore, their simultaneous occupancy and interaction between different ions. In contrast, binding of blockers from the vacuolar (lumenal) side appears to Follow a single-ion handling rule, with a common binding site for all amines located at approximately 30% of the electrical distance from the lumenal side.