High Salt Concentrations Increase Permeability through OmpC channels of Escherichia coli [Membrane Biology]

August 1st, 2014 by Kojima, S., Nikaido, H.

OmpF and OmpC porin channels are responsible for the passage of small, hydrophilic solutes across the outer membrane of Escherichia coli. While these channels are two of the most extensively studied porin channels, what had yet remained elusive was the reason why OmpC shows markedly lower permeability than OmpF, despite having little difference in its channel size. OmpC channel, however, is known to contain a larger number of ionizable residues than the OmpF channel. We examined in this study the channel property of OmpF and OmpC using intact cell of E. coli, and found that the permeability of several β-lactams and lactose through OmpC became increased to the level comparable to OmpF with up to 0.3 M salt that may increase the Debye-Hückel shielding, or with 2% ethanol or 0.3 M urea that may perturb the short-range ordering of water molecules. Replacing 10 pore-lining residues that show different ionization behavior between OmpC and OmpF led to substantial conversion of channel property with respect to their permeability and response to external salt concentration. We thus propose that overall configuration of ionizable residues in the channel that may orient water molecules, and the electrostatic profile of the channel, play a decisive role in defining the channel property of OmpC porin rather than its channel size.