Members of this family are Gram-negative bacterial inner membrane proteins, generally designated ExbD, related to the TolR family modeled by TIGRFAMs TIGR02801. Members always are encoded next to a protein designated ExbB (TIGR02797), related to the TolQ family modeled by TIGRFAMs TIGR02796. ExbD and ExbB together form a proton channel through which they can harness the proton-motive force to energize TonB, which in turn energizes TonB-dependent receptors in the outer membrane. TonB-dependent receptors with known specificity tend to import siderophores or vitamin B12. A TonB system and Tol-Pal system often will co-exist in a single bacterial genome.