ExbB

The Structure of ExbB^[1]

Structure

ExbB consists of three transmembrane domains (spanning from residues 16-39, 128-155 and 162-194), with two large portions of the protein representing the majority of the protein in the cytoplasm and two smaller portions in the periplasm^[1].

ExbB can exist in the TonB system either in a complex with ExbD (to a ratio of 3.5:1) or on its own (where no ExbD is present), which has been suggested to play a part in the diverse roles of TonB^[2].

ExbB has a similar sequence and physiological structure to TolQ and is therefore thought to be evolutionarily linked^[3].

Function

ExbB is essential for TonB-dependent energy transduction as the absence of ExbB prevents TonB responding to the proton motive force, as well as the change of the high-affinity association of TonB for the outer membrane to the cytoplasmic membrane^[2].

Loss of ExbB function can be partially replaced by TolQ and vice versa - reduced activities of either of these proteins via a mutant form can be reversed by introducing double mutants^[3].