Ion channel permeable for divalent and monovalent cations in native spinach thylakoid membranes

A cation-selective channel was characterized in isolated patches from osmotically swollen thylakoids of spinach (Spinacea oleracea), This channel was permeable for K+ as well as for Mg2(+) and Ca2(+) but not for Cl-. When K+ was the main permeant ion (symmetrical 105 mM KCl) the conductance of the channel was about 60 pS. The single channel conductance for different cations followed a sequence K+ > Mg2+ greater than or equal to Ca2+. The permeabilities determined by reversal potential measurements were comparable for K+, Ca2+, and Mg2+. The cation channel displayed bursting behavior. The total open probability of the channel increased at more positive membrane potentials. Kinetic analysis demonstrated that voltage dependence of the total open probability was determined by the probability of bursts formation while the probability to find the channel in open state within a burst of activity was hardly voltage-dependent. The cation permeability of intact spinach thylakoids can be explained on the single channel level by the data presented here.