Recently the switch-motor complex of bacterial flagella was found to be associated with energy-linked proteins, in particular F0F1 ATP synthase which interacts with the flagellar switch protein FliG1. High rates of ATP synthesis and hydrolysis were measured in membrane areas adjacent to the motor. If the function of these associations is to provide a local energy supply to the motor when the PMF is low, we expect localized chemiosmosis in the immediate vicinity of the motor, leading to a localized increase in PMF at the expense of ATP hydrolysis.
Erhardt and co-workers demonstrated that all aerobic OXPHOS complexes within the E. coli cytoplasmic membrane are distributed in patches 2 of radius 150 to 250 nm. Hence it is reasonable to suppose such a patch is centered around the flagellar motor, giving rise to a lateral pH gradient within the periplasmic space. Rieger, Junge, and Busch estimated theoretically a lateral pH profile along the p-side of mitochondrial cristae, where complex IV (in the flat membranes) pumps protons into the intra-cristae space3. These are consumed by F0F1 ATP-synthases lining the rims.
Adapting the calculation of Rieger et al. to a patch buffered at the outer edges surrounding the motor, and assuming the patch has one ring of 17 proton pumps in dynamic equilibrium with the FliG subunits (from steric considerations the largest number able to bind simultaneously), a pH drop was found in the vicinity of the motor of about 1 unit.
1. Zarbiv G, Li H, Wolf A, Cecchini G, Caplan SR, Sourjik V, & Eisenbach M J. Mol. Biol. (2012) 416, 192-207.
2. Erhardt H, Dempwolff F, Pfreundschuh M, Riehle M, Schӓfer C, Pohl T, Graumann P, & Friedrich T MicrobiologyOpen (2014) DOI: 10.1002/mbo3.163.
3. Rieger B, Junge W, & Busch KB Nature Communications (2014) DOI: 10.1038/ncomms 4103.