Channel-closing activity of porins from Escherichia coli in bilayer lipid membranes.

The opening and closing of the ompF porin from Escherichia coli JF 701 was investigated by reconstituting the purified protein into planar bilayer membranes. The electrical conductance changes across the membranes at constant potential were used to analyze the size and aggregate nature of the porin channel complexes and the relative number of opening and closing events. We found that, when measured at pH 5.5, the channel conductance diminished and the number of closing events increased when the voltage was greater than 100 mV. The results suggest that the number of smaller sized conductance channels increases above this potential. There was also an increase in the smaller subunits and in the closing events when the pH was lowered to 3.5, and these changes were further enhanced by increasing the voltage. We propose that both lowering the pH and elevating the potential across the membrane stabilize the porin in a conformation in which the subunits are less tightly associated and the subunits open in a non-cooperative manner. These same conditions also appear to stabilize the closed state of the pore.