A, ColE9 R-domain binds BtuB with nM affinity, positioning the T-domain and IUTD above a neighbouring OmpF trimer. B, The ColE9 IUTD translocates through subunit 2 of OmpF to deposit the TBE in the periplasm and capture TolB at the expense of the OM lipoprotein Pal. The TBE allosterically promotes displacement of TolB's N-terminus which constitutes the TolA binding site. We propose this complex equates to the full translocon cryo-EM structure. C, ColE9 OBS1 binds subunit 1 of OmpF from the periplasm. Threading through two of OmpFs three subunits triggers dissociation of the ColE9 R-domain-BtuB complex and reorients the T-domain above subunit 2 below which TolB is positioned in the periplasm. The docking of OBS1 also results in TolB moving closer to the opening of subunit 2. This complex, which we propose equates to the partial translocon cryo-EM structure, is now primed for contact with TolA in the inner membrane. TolA extension through the periplasm is coupled to the PMF via its stator proteins, TolQ and TolR. D, Retraction of TolA (the molecular mechanism of which remains to be established) provides the driving force for pulling ColE9 bound to TolB into the periplasm through subunit 2 of OmpF, accompanied by unfolding of its constituent domains. It is at this point the immunity protein Im9 (not shown) would be displaced at the cell surface. E, The TolA-TolB complex is thought to retract through the cell wall, which would bring ColE9 close to the cytoplasmic membrane. The toxin likely refolds prior to transport across the cytoplasmic membrane, which involves the AAA+ATPase/protease FtsH (not shown) Walker et al., 2007(). 