A central, shared role for disulfide bonds in Bacteroidota protein transport and gliding motility

The gliding motility and Type 9 Secretion Systems (T9SSs) of Bacteroidota share a Power Chain that in current models uses a Hub complex to distribute energy to both the T9SS translocon and a moving gliding track. In Flavobacterium johnsoniae the cell surface gliding adhesins are attached to the internal gliding track by the outer membrane-spanning protein SprF. We show that both SprF and the T9SS component PorG contain twin-cysteine motifs that form a disulfide link to a shared cysteine residue in the Hub component GldK. These linkages are crucial for gliding motility and T9SS export, respectively. Since SprF moves with the gliding track, the SprF-GldK bond indicates that the Hub components are part of the gliding track in gliding Bacteroidota. We determine the structure of the 3 MDa Hub complex from the non-gliding bacterium Porphyromonas gingivalis and used this to model the F. johnsoniae gliding track.