Activity domains of the TonB protein

Summary Escherichia coli and related Gram‐negative bacteria contain an energy‐coupied transport system through the outer membrane which consists of the proteins TonB, ExbB, ExbD anchored in the cytoplasmic membrane and receptors in the outer membrane. Differences in the activities of the Escherichia coli and the Serratia marcescens TonB proteins were used to identify TonB functional domains. In E. coli TonB segments were replaced by equivalent fragments of S. marcescens TonB and the activities of the resulting chimaeric proteins were determined. In addition, E. coli TonB was truncated at the C‐terminal end, and point mutants were generated using bisulphite. From the results obtained we draw the following conclusions: an important site of interaction between TonB and ExbB is located in the M‐terminal region of TonB within or close to the cytoplasmic membrane since an N‐terminal 44‐residue fragment of TonB was stabilized by ExbB and interfered with wild‐type TonB activity. In addition, the activity of a TonB derivative in which histidine residue 20 was replaced by arginine was strongly reduced, and a double mutant containing arginine‐7 to histidine and alanine‐22 to threonine substit