Pi. Higgs et al., INTERACTIONS IN THE TONB-DEPENDENT ENERGY TRANSDUCTION COMPLEX - EXBBAND EXBD FORM HOMOMULTIMERS, Journal of bacteriology (Print), 180(22), 1998, pp. 6031-6038
The cytoplasmic membrane proteins ExbB and ExbD support TonB-dependent
active transport of iron siderophores and vitamin B-12 across the ess
entially unenergized outer membrane of Escherichia coil. In this study
, in vivo formaldehyde cross-linking analysis was used to investigate
the interactions of T7 epitope-tagged ExbB or ExbD proteins. ExbB and
ExbD each formed two unique cross-linked complexes which were not depe
ndent on the presence of TonB, the outer membrane receptor protein Fep
A, or the other Exb protein. Cross-linking analysis of ExbB- and ExbD-
derived size variants demonstrated instead that these ExbB and ExbD co
mplexes were homodimers and homotrimers and suggested that ExbB also i
nteracted with an unidentified protein(s). Cross-linking analysis of e
pitope-tagged ExbB and ExbD proteins with TonB antisera afforded detec
tion of a previously unrecognized TonB-ExbD cross-linked complex and c
onfirmed the composition of the TonB-ExbB cross-linked complex. The im
plications of these findings for the mechanism of TonB-dependent energ
y transduction are discussed.