Calcium regulation of a cell surface nucleic acid channel

We have recently purified a 45 kDa protein from rat renal brush border membrane that functions as a macromolecular nucleic acid channel when reconstituted in artificial lipid bilayers. To explore the role of calcium in the regulation of this channel, purified protein was reconstituted in bilayers and calcium concentration was altered while voltage clamp experiments were performed. Open probability of the channel was less than 1% in 0 calcium and increased an average of 19-fold when calcium concentration was increased from 0 mM to 0.5 mM and 100-fold when increased to 1.0 mM. Equimolar calcium and EDTA resulted in a fall in open probability to 1%. Increasing calcium concentration had no effect on current/voltage relationship or mean open time of the channel, but decreased the mean closed time significantly. Cooperative channel gating was observed at 1.0 mM. These data demonstrate that open probability of the renal nucleic acid channel is calcium-dependent. The increase in open probability is due to an increase in channel gating activity rather due to an increase in the duration of the open state.