FhuA plug occluded FhuA barrel. Current traces begin with either 4 M urea (A) or 3 M glycerol (B) in the cis compartment, and the voltage held at −20 mV throughout the experiment. Conductance was induced by the addition of FhuA barrel (9 nM) to the cis solution, and unlike whole FhuA, this conductance persisted even when urea was removed. (Also unlike whole FhuA, conductance was induced by 3 M glycerol.) Furthermore, the addition of FhuA plug (45 nM) to either side of the membrane resulted in some loss of conductance. (Addition of cis plug resulted in a 25% and 22% loss of conductance in panels A and B, respectively; addition of trans plug resulted in a 50% and 40% loss of conductance in panels A and B, respectively).