Genetic identification of the pore domain of the OmpC porin of Escherichia coli K-12

We have isolated and characterized 31 mutations in the ompC gene which allow Escherichia coli to grow on maltotriose (Dex+) in the absence of the LamB and OmpF porins. These ompC(Dex) mutations include single-base-pair substitutions, small deletions, and small insertions. DNA sequence analysis shows that all of the alterations occur within the coding region for the first 110 amino acids of mature OmpC. The 26 independent point mutations repeatedly and exclusively alter residues R37, R74, and D105 of mature OmpC. In each case, a charged amino acid is changed to an uncharged residue. Biochemical and physiological tests suggest that these alterations increase the size of the pore channel. Starting with three different ompC(Dex) strains with alterations affecting R74, we isolated mutants that could grow on maltohexose (Hex+). These mutants each contained a second alteration in the ompC gene involving residues R37, D105, or R124. The combined effects on pore function of the two mutations appear to be additive. These experiments suggest that we have identified the important residues of OmpC peptide involved in pore function. On the basis of these mutations and general rules for membrane protein folding, a model for the topology of the OmpC protein is proposed.