ExbB and ExbD are inner-membrane proteins that use the proton motive force to energize TonB, which contacts TonB-dependent outer-membrane transporters; ExbB forms a pentameric motor surrounding an ExbD dimer (ExbB5:ExbD2).

In pseudomonads, TonB together with ExbB and ExbD transmits proton motive force energy from the inner membrane to outer-membrane receptors for ferric-siderophore and heme uptake.

The exbB-exbD-tonB genes are closely linked and likely cotranscribed; the TonB-ExbB-ExbD inner-membrane complex is required for iron uptake, and an exbD insertion mutant fails to grow under iron limitation.

Genome sequence of P. putida KT2440 in which the exbB locus (PP_5306, UniProt Q88C77) was annotated.