This family groups together integral membrane proteins that appear to be involved translocation of proteins across a membrane. These proteins are probably proton channels. MotA is an essential component of the flageller motor that uses a proton gradient to generate rotational motion in the flageller [1]. ExbB is part of the TonB-dependent transduction complex. The TonB complex uses the proton gradient across the inner bacterial membrane to transport large molecules across the outer bacterial membrane.

1: Function of proline residues of MotA in torque generation by the flagellar motor of Escherichia coli. Braun TF, Poulson S, Gully JB, Empey JC, Van Way S, Putnam A, Blair DF; J Bacteriol 1999;181:3542-3551. PMID:10348868

2: Interactions in the TonB-dependent energy transduction complex: ExbB and ExbD form homomultimers. Higgs PI, Myers PS, Postle K; J Bacteriol 1998;180:6031-6038. PMID:9811664