Single-molecule investigation of directional transport through a bacterial transmembrane pore

Colicin E9 (ColE9) crosses the outer membrane of E. coli by passing through the trimeric OmpF porin. Entry is initiated by an intrinsically unstructured N-terminal domain (IUTD), which has two OmpF-binding sites (OBS). The IUTD threads through a first subunit of OmpF, reverses direction and enters a second subunit, leaving the IUTD sequence OBS2 within the first subunit, OBS1 within the second subunit and the intervening TolB-binding epitope in the periplasmic space.