Passive potassium transport by symbiosomes from broad bean root nodules

Passive potassium release from symbiosomes isolated from broad bean root nodules and placed into K+-free assay medium was investigated by following acidification of the symbiosome interior and tetraphenylphosphonium (TPP+)-induced change in turbidity of symbiosome suspension observed under the chosen experimental conditions. The kinetics of acid pH shift inside the symbiosomes was substantially accelerated by the K+-specific ionophore valinomycin, the protonophore carbonyl cyanide 4-trifluoromethoxyphenylhydrazone (FCCP), but rapidly reversed by centimolar concentrations of K+ as well as by TPP+ added to the assay medium. In addition, the acidification process was shown to be markedly inhibited by tetraethylammonium (TEA), the known blocker of K+-channels in cell membranes, and to be strongly suppressed by removal of free calcium ions from the assay medium with the Ca2+ chelator EGTA. TPP+ was found to rapidly trigger the increase in turbidity of symbiosome suspension in the same assay medium. The kinetics of the TPP+-triggered process appeared to be markedly inhibited by TEA as well. Taken together, these results indicate relatively high potassium permeability of the peribacteroid membrane (PBM) of broad bean root nodules and provide evidence that the responses tested are due to K+ release from the symbiosomes through TEA-sensitive K+-permeable channel in the PBM down transmembrane electrochemical gradient of K+ ions. The data obtained suggest that such a channel is voltage-gated, and Ca2+ ions on the cytosolic side of the PBM are required for functioning of this transporter.