Translocator hunt comes full Cir-Col

The ability of Escherichia coli to kill other E. coli using protein antibiotics known as colicins has been known for many years, but the mechanisms involved poorly understood. Recent progress has been rapid, however, particularly concerning events on either side of the outer membrane (OM). Structures of colicins bound to OM receptors have been determined and we have detailed mechanistic information on how colicins subvert the periplasmic complexes of TolQRAB/Pal or TonB/ExbB/ExbD to trigger cell entry. In this issue of Molecular Microbiology, Jakes and Finkelstein answer a long-standing problem concerning the uptake mechanism of the pore-forming colicin ColIa: How does the TonB box of the colicin cross the OM following high-affinity binding of ColIa to its primary receptor, the siderophore transporter Cir? Through a series of chimeric protein constructions tested for their activity against a range of mutants and in cell death protection assays, the authors come up with the surprising observation that following binding of ColIa to Cir it recruits another Cir protein as its OM translocator. Not only does this settle various conundrums in the literature, but the translocation mechanism that stems from their study will likely be applicable to many TonB-dependent colicins.