Characterization ofEscherichia coliExbD protein modificationin vivo

Abstract

ABSTRACTThe TonB system ofEscherichia colicouples the protonmotive force of the cytoplasmic membrane to active transport of nutrients across the outer membrane. In the cytoplasmic membrane, this system consists of three known proteins, TonB, ExbB, and ExbD. ExbB and ExbD appear to harvest the protonmotive force and transmit it to TonB, which then makes direct physical contact with TonB-dependent active transport proteins in the outer membrane. Using two-dimensional gel electrophoresis, we found that ExbD exists as two different species with the same apparent molecular mass but with different pIs. The more basic ExbD species was consistently present, while the more acidic species arose when cells were starved for iron by the addition of iron chelators. The cause of the modification was, however, more complex than simple iron starvation. Absence of either TonB or ExbB protein also gave rise to modified ExbD under iron-replete conditions where the wild-type strain exhibited no ExbD modification. The effect of thetonBorexbBmutations were not entirely due to iron limitation since an equally iron-limitedaroBmutation did not replicate the ExbD modification. This constitutes the first report ofin vivomodification for any of the TonB system proteins.