Cloning, purification and crystallization of MotB, a stator component of the proton-driven bacterial flagellar motor

MotB is an essential component of the proton motive force-driven bacterial flagellar motor. It binds to the stress-bearing layer of peptidoglycan in the periplasm, anchoring the MotA/MotB stator unit to the cell wall. Proton flow through the channel formed by the transmembrane helices of MotA and MotB generates the turning force (torque) applied to the rotor. Crystals of recombinant Helicobacter pylori MotB have been obtained by the sitting-drop vapour-diffusion method using ammonium sulfate as a precipitant. These crystals belong to space group P4(1)2(1)2 or its enantiomorph P4(3)2(1)2, with unit-cell parameters a = 75.2, b = 75.2, c = 124.7 A. The asymmetric unit appears to contain one subunit, corresponding to a packing density of 3.4 A(3) Da(-1). The crystals diffract X-rays to at least 1.8 A resolution on a synchrotron-radiation source.