Presence of the nucleic acid channel in renal brush-border membranes: allosteric modulation by extracellular calcium

Abstract

We have previously described a cell surface channel complex that is highly selective for nucleic acid (6, 7). The channel complex was purified to homogeneity by solubilizing renal brush-border membranes (BBM) with CHAPS and separation by liquid chromatography. It was characterized by reconstitution in planar lipid bilayers. The channel consists of a pore-forming subunit that is blocked by heparan sulfate and a regulatory subunit that is blocked by L-malate (7). The current studies were performed to compare the characteristics of the nucleic acid-conducting channel in native BBM with the characteristics that have been determined for the complex reconstituted from purified proteins. BBM were purified by differential centrifugation and reconstituted in lipid bilayers. Current was not observed until oligodeoxynucleotide (ODN) was added. Conductance was 9.1 +/- 0.9 pS; rectification and voltage dependence were not observed. Reversal potential (E(rev)) shifted to +14 +/- 0.1 mV by a 10-fold gradient for ODN but was not altered when gradients were created for any other ion. Open probability increased significantly with an increase in Ca(2+) on the trans chamber of the bilayer apparatus. Changes in cis Ca(2+) were without effect. Addition of L-malate to the cis chamber or heparan sulfate to the trans chamber significantly reduced the open probability of the channel. These data demonstrate that the nucleic acid channel in BBM is electrophysiologically and pharmacologically identical to that previously reported for purified protein and demonstrate that a nucleic acid-conducting channel is a component of renal BBM.