A SINGLE AMINO-ACID SUBSTITUTION ALTERS CONDUCTANCE AND GATING OF OMPC PORIN OF ESCHERICHIA-COLI

We have reconstituted into liposomes outer-membrane fractions from Escherichia coli strains which express OmpC porins with altered pore properties. Single-channel experiments were performed with the patch-clamp technique on blisters generated from the reconstituted liposomes. Our goal was to identify positively the activity pattern of OmpC in our reconstituted system. The properties of the parent strain were compared to those of a strain whose OmpC porin has a single amino acid substitution in a postulated transmembrane segment. The parent and the mutant strain each exhibit a cation-selective channel of high open probability and gating to closed levels of various amplitudes. However, the mutant channel appeared to be 9 to 30% larger in unit conductance. It tended to close and reopen most often in groups of three units, as opened to two units in the parent channel. The results are discussed in terms of the observed phenotype and of their implication as to the structure-function relationship of the porin channels.