Toxin Insertion Into Lipid Monolayers at the Air/Water Interface is Promoted by the Cognate Receptor Protein

Abstract

Bacterial surfaces mediate many interactions important for health. We describe the production and analysis of a membrane protein containing lipid monolayer, at the air-water interface. Monolayers were formed by spreading a solution of outer membrane protein F (OmpF) / DPPG vesicles at the air-liquid interface. Monolayer structure was analysed parallel and perpendicular to the interface using surface pressure isotherms (SP), Brewster angle microscopy (BAM), see figure, and neutron reflectometry (NR). The domain structure of an OmpF containing monolayer was distinct from that of the protein free monolayer over all surface pressures. OmpF is the receptor for the antibacterial toxin colicin N (ColN) and, by an unknown mechanism, enables the toxin to translocate across the bacterial outer membrane barrier in order to kill E. coli cells. SP and NR data indicated that OmpF enabled ColN to penetrate deeply into the anionic lipid monolayer, and BAM showed a more selective interaction than that seen for lipid monolayers. This suite of methods including selective deuteration can thus provide membrane penetration data for a range of receptor-mediated translocation events in biology where resolution in multiple axes is required.