Mutations that alter the pore function of the OmpF porin of Escherichia coli K12

Abstract

We describe the isolation and characterization of mutations in ompF that alter the pore properties of the OmpF porin. The selection makes use of the fact that maltodextrins larger than maltotriose are too large to diffuse through the normal OmpF pore. By demanding growth on maltodextrins (Dex+) in the absence of the LamB protein, which is normally required for the uptake of these large sugars, we are able to obtain ompF mutations. These include transversions, transitions and small deletions. We obtained almost exclusively ompF mutations in spite of the fact that analogous alterations in ompC can result in similar phenotypes. Fifteen independent point mutations identify residues R42, R82, D113 and R132 of the mature peptide as important in pore function. The alterations result in uncharged amino acids being substituted for charged amino acids. Growth tests, antibiotic sensitivities and rates of [14C]maltose uptake suggest that the alterations result in an increased pore size. Small deletions of six to 15 amino acid residues in the region between A108 and V133 of mature OmpF dramatically alter outer membrane permeability to hydrophobic antibiotics and detergents as well as conferring a Dex+ phenotype. We suggest that these mutations affect both the pore function and interactions with other outer membrane components. A model of OmpF protein structure based on general rules for folding membrane proteins and these mutations is presented.