Entropic Modulation of Ion Transport through OmpF Channel. Molecular Basis of pH Sensing Derived from Cooperative Interactions

Abstract

Porins are channel-forming proteins that are located in the outer membranes of Gram-negative bacteria and allow the influx of hydrophilic nutrients and the extrusion of waste products. The inactivation of these wide channels is thought to play an important role in the survival of the bacteria in acidic media. We investigate here the mechanism responsible for pH sensing in the trimeric porin OmpF, of E. coli. Planar lipid bilayer electrophysiology was used to study the effect of pH on the transport properties of the OmpF channel in its fully open, ‘‘nongated’’ conformation. At low pH we observe a drastic drop in the OmpF open channel conductance that is accompanied by a huge increase of the current noise and inversion of the channel selectivity. All these channel features are strongly dependent on the salt concentration and, according to the Hill formalism, might be related to negative cooperative phenomena. Based on our studies with OmpF mutants, we propose that the underlying mechanism is a competitive binding of cations and protons occurring in the narrow central constriction of the channel. Temperature-controlled experiments suggest that the entropic benefit arising from cooperative interactions is the driving force behind the pH sensitivity of the channel.