Weird Ion…Weird Ion Channels Mediating Resistance to Fluoride Toxicity

Abstract

F‐ specific ion channels of the newly discovered “Fluc” family act as F‐ exporters protect many microorganisms from the toxic effects of this environmentally pervasive halide. The channel is constructed as an four‐TM helix dual‐topology homodimer, in which the two subunits adopt antiparallel orientation within the membrane. We previously used fibronectin domain‐based “monobodies” selected by phage display methods to establish this unusual membrane architecture, showing that these small proteins block single Fluc channels from either side of the membrane. We now ask whether the two blocking sites on opposite sides of the membrane can both be occupied simultaneously by monobodies. Our analysis examines the dependence of single‐channel block‐times as a function of monobody concentration varied symmetrically on the two sides of the channel. If both sides can be blocked symmetrically, a linear blocking scheme should be followed:
Open ‐‐‐‐ Block(1) ‐‐‐‐Block(2),Where both blocked states are nonconducting. This scheme makes quantitative predictions for the increase in block‐times with monobody concentration that provide information about any cooperativity, negative or positive, that may operate between the two sites.