The ion-channel forming colicins A, B, E1, Ia, Ib and N all kill bacterial cells selectively by co-opting bacterial active-transport pathways and forming voltage-gated ion conducting channels across the plasma membrane of the target bacterium(1,2). The crystal structure of colicin Ia reveals a molecule 210 Angstrom long with three distinct functional domains arranged along a backbone of two extraordinarily long alpha-helices. A central domain at the bend of the hairpin-like structure mediates specific recognition and binding to an outer-membrane receptor(3). A second domain mediates translocation across the outer membrane via the TonB transport pathway(4); the TonB-box(5) recognition element of colicin Ia is on one side of three 80 Angstrom-long helices arranged as a helical sheet. A third domain is made up of 10 alpha-helices which form a voltage-activated and voltage-gated ion conducting channel across the plasma membrane of the target cell. The two 160 Angstrom-long alpha-helices that link the receptor-binding domain to the other domains enable the colicin Ia molecule to span the periplasmic space and contact both the outer and plasma membranes simultaneously during function(6,7).