Rotation of the bacterial flagellum is powered by a proton influx through the peptidoglycan (PG)-tethered stator ring MotA/B. MotA and MotB form an inner-membrane complex that does not conduct protons and does not bind to PG until it is inserted into the flagellar motor. The opening of the proton channel involves association of the plug helices in the periplasmic region of the MotB dimer into a parallel coiled coil. Here, we have characterised the structure of a soluble variant of full-length Helicobacter pylori MotB in which the plug helix was engineered to be locked in a parallel coiled coil state, mimicking the open state of the stator. Fluorescence resonance energy transfer measurements, combined with PG-binding assays and fitting of the crystal structures of MotB fragments to the small angle X-ray scattering (SAXS) data revealed that the protein's C-terminal domain has a PG-binding-competent conformation. Molecular modelling against the SAXS data suggested that the linker in H. pylori MotB forms a subdomain between the plug and the C-terminal domain, that 'clamps' the coiled coil of the plug, thus stabilising the activated form of the protein. Based on these results, we present a pseudo-atomic model structure of full-length MotB in its activated form