Single OmpC porin channels have been reconstituted in planar bilayer membranes. Wild-type OmpC forms trimers which are largely insensitive to voltages below 250 mV. A single-point mutation of the ompC gene has been prepared resulting in replacement of Trp56 by Cys in the eyelet region of the channel wall in a highly conserved segment of the polypeptide. The monomeric channels of which the trimer is composed have smaller conductivity in 1 M NaCl (400 ± 20 pS, mean and S.E.M., n = 30) and increased voltage sensitivity by comparison with the wild-type under similar conditions, whereas other (Dex) mutants form larger channels and display different behaviour. Further, by treatment in SDS solutions at different temperatures, the W56C mutant has been shown to be less stable than either the wild-type or the Dex mutants.
