Passive receptor dissociation driven by porin threading establishes active colicin transport through Escherichia coli OmpF

Abstract

Bacteria deploy weapons to kill their neighbours during competition for resources and aid survival within microbiomes. Colicins were the first antibacterial system identified yet how these bacteriocins cross the outer membrane of Escherichia coli is unknown. Here, by solving the structures of translocation intermediates and imaging toxin import, we uncover the mechanism by which the Tol-dependent nuclease colicin E9 (ColE9) crosses the outer membrane. We show that threading of ColE9s disordered domain through two pores of the trimeric porin OmpF causes the colicin to disengage from its primary receptor, BtuB, and reorganise the translocon either side of the membrane. These rearrangements prime the toxin for import through the lumen of a single OmpF subunit, which is driven by the proton motive force-linked TolQ-TolR-TolA-TolB assembly. Our study explains why OmpF is a better translocator than OmpC and reconciles the mechanisms by which Ton- and Tol- dependent bacteriocins cross the bacterial outer membrane.