Cellular import of colicin E3 is initiated by the Escherichia coli outer membrane cobalamin transporter, BtuB. The 135-residue 100-A coiled-coil receptor-binding domain (R135) of colicin E3 forms a 1:1 complex with BtuB whose structure at a resolution of 2.75 A is reported. Binding of R135 to the BtuB extracellular surface (DeltaG(o) = -12 kcal mol(-1)) is mediated by 27 residues of R135 near the coiled-coil apex. Formation of the R135-BtuB complex results in unfolding of R135 N- and C-terminal ends, inferred to be important for unfolding of the colicin T-domain. Small conformational changes occur in the BtuB cork and barrel domains but are insufficient to form a translocation channel. The absence of a channel and the peripheral binding of R135 imply that BtuB serves to bind the colicin, and that the coiled-coil delivers the colicin to a neighboring outer membrane protein for translocation, thus forming a colicin translocon. The translocator was concluded to be OmpF from the occlusion of OmpF channels by colicin E3.