In situ imaging of the bacterial flagellar motor reveals stable inner- and outer-membrane associated sub-complexes during motor assembly and disassembly

Using electron cryo-tomography (ECT), we show that, in three bacterial species, the flagellar motor disassembly process results in stable outer-membrane-embedded sub-complexes. These sub-complexes consist of the periplasmic embellished P- and L-rings, and bend the membrane inward while it remains apparently sealed. Our experiments show that these sub-complexes are more enriched in the cells at high optical-density (OD 600 ). In order to identify candidates for the protein(s) that may play a role in sealing the outer membrane we performed mass spectrometry experiments on membrane vesicles isolated from Shewanella oneidensis MR-1 cells grown to two different optical densities (OD 600 sample 1 ~ 5, OD 600 sample 2 ~ 0.6). Based on certain criteria, we identified few candidate proteins that are overexpressed in cells grown to high OD 600 and that might be involved in sealing the membrane after flagellar motor disassembly.