Diphtheria toxin forms transmembrane channels in planar lipid bilayers
When exposed to a lipid bilayer, diphtheria toxin binds to it and forms transmembrane, voltage-dependent, anion-selective channels. The mean (± SD) conductance of these channels in asolectin membranes is 6.2± 0.7 pmho (pS) in 0.2 M NaCl and 20± 2 pmho in 1.0 M NaCl. The rate of channel formation depends on the pH in the toxin-containing compartment; it is very low at pH >5.0 and increases abruptly as the pH decreases from 4.9 to 4.0. Binding of toxin to the membrane is also pH dependent, being unmeasurable at pH 7 and increasing monotonically with decreasing pH. The rate of channel formation depends upon membrane potential as well; channels form only at negative potentials. These channels are permanent in the time scale of the experiments (about 1 hr). The membrane conductance caused by the channels is also voltage dependent, being constant at positive potentials and decreasing at negative potentials. Hence, the current-voltage curve is linear at positive potentials and sublinear at negative potentials. The conditions necessary for insertion of toxin into the bilayer and formation of channels are similar to those that prevail inside the lysosome. Thus, these results lend credence to the idea that toxin enters the cytoplasm from the lysosomal compartment.