Dual roles of the central domain of colicin D tRNase in TonB-mediated import and in immunity.

Colicin D import into Escherichia coli requires an interaction via its TonB box with the energy transducer TonB. Colicin D cytotoxicity is inhibited by specific tonB mutations, but it is restored by suppressor mutations in the TonB box. Here we report that there is a second site of interaction between TonB and colicin D, which is dependent upon a 45-amino acid region, within the uncharacterized central domain of colicin D. In addition, the 8th amino acids of colicin D (a glycine) and colicin B (a valine), adjacent to their TonB boxes, are also required for TonB recognition, suggesting that high affinity complex formation involves multiple interactions between these colicins and TonB. The central domain also contributes to the formation of the immunity complex, as well as being essential for uptake and thus killing. Colicin D is normally secreted in association with the immunity protein, and this complex involves the following two interactions: a major interaction with the C-terminal tRNase domain and a second interaction involving the central domain of colicin D and, most probably, the alpha4 helix of ImmD, which is on the opposite side of ImmD compared with the major interface. In contrast, formation of the immunity complex with the processed cytotoxic domain, the form expected to be found in the cytoplasm after colicin D uptake, requires only the major interaction. Klebicin D has, like colicin D, a ribonuclease activity toward tRNAArg and a central domain, which can form a complex with ImmD but which does not function in TonB-mediated transport.