Figure 4. ColE9 threading through the subunits of OmpF disengages the toxin from its outer membrane receptor, BtuB.

1. Cartoon depicting the basis for the control where excess ColE9 R‐domain (residues 348–418, orange) blocks binding of vitamin B₁₂ (red) to BtuB (grey), impairing growth of E. coli 113/3, a B₁₂‐dependent strain (Penfold et al, 2000).
2. Cartoon schematic of the two possible translocon outcomes for binding of ColE9 W39A to BtuB and their implications on vitamin B₁₂ import in E. coli 113/3. The W39A mutation within the TBE of ColE9 (orange) impairs TolB (blue) binding thereby uncoupling the OM components of the translocon from the energised TolQ‐TolR‐TolA complex in the inner membrane. If threading of the ColE9 IUTD through OmpF (green) has no impact on ColE9 binding to BtuB (grey) then vitamin B₁₂ (red) cannot enter cells to support growth. In contrast, if OmpF threading disengages ColE9 from BtuB then vitamin B₁₂ can enter cells to support growth.
3. Growth curves for E. coli 113/3 in defined media and in the presence or absence of B₁₂ (■ and ○, respectively). When challenged with 40 nM isolated R‐domain (▾), cell growth was largely impaired whereas growth in the presence of 40 nM ColE9 W39A (□) approached that of the no‐colicin control, consistent with OmpF threading causing the disengagement of ColE9 from BtuB. Error bars represent the standard deviation across three biological replicates.

Source data are available online for this figure.