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J. Bacteriol., May 1996, 2836-2845, Vol 178, No. 10
V Braun, S Gaisser, C Herrmann, K Kampfenkel, H Killmann and I Traub
Ferric siderophores, vitamin B12, and group B colicins are taken up through
the outer membranes of Escherichia coli cells by an energy- coupled
process. Energy from the cytoplasmic membrane is transferred to the outer
membrane with the aid of the Ton system, consisting of the proteins TonB,
ExbB, and ExbD. In this paper we describe two point mutations which
inactivate ExbD. One mutation close to the N-terminal end of ExbD is
located in the cytoplasmic membrane, and the other mutation close to the
C-terminal end is located in the periplasm. E. coli CHO3, carrying a
chromosomal exbD mutation in which leucine at position 132 was replaced by
glutamine, was devoid of all Ton-related activities. A plasmid-encoded ExbD
derivative, in which aspartate at position 25, the only changed amino acid
in the predicted membrane- spanning region of ExbD, was replaced by
asparagine, failed to restore the Ton activities of strain CHO3 and
negatively complemented ExbD+ strains, indicating an interaction of this
mutated ExbD with wild-type ExbD or with another component. This component
was shown to be ExbB. ExbB that was labeled with 6 histidine residues at
its C-terminal end and that bound to a nickel-nitrilotriacetic acid agarose
column retained ExbD and TonB specifically; both were eluted with the ExbB
labeled with 6 histidine residues, demonstrating interaction of ExbB with
ExbD and TonB. These data further support the concept that TonB, ExbB, and
ExbD form a complex in which the energized conformation of TonB opens the
channels in the outer membrane receptor proteins.
Copyright © 1996, American Society for Microbiology
Energy-coupled transport across the outer membrane of Escherichia coli: ExbB binds ExbD and TonB in vitro, and leucine 132 in the periplasmic region and aspartate 25 in the transmembrane region are important for ExbD activity
Mikrobiologie II, Universitat Tubingen, Germany. Vbraun@uni-tuebingen.de
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