How does the rotary type 9 secretion system of gliding bacteria anchor to the peptidoglycan?

The bacterial Type 9 Secretion System (T9SS) is essential for the secretion of >300,000 different proteins and enzymes encoded by bacteria of the phylum Bacteroidetes. PG1058 (PorE) is an essential component of the T9SS of the human oral pathogen Porphyromonas gingivalis (P.gingivalis), which has been suggested to anchor T9SS to the cell wall and connect the inner leaflet of the outer membrane and the peptidoglycan layer by the OmpA_C-like domain present in this protein subunit. Hydrogen bonds are used to form a bridge between the OmpA_C-like domain and peptidoglycan layer found between the inner membrane and outer membrane. Loss of PG1058 in P. gingivalis results in a defect in proteolytic activity, colonial pigmentation, and absence of visible electron defense surface layer that contains gingipains, all essential for the functioning of T9SS (3). Bacteria such as Flavobacterium johnsoniae (F. johnssoniae) utilize the T9SS for gliding motility and thus far nothing is known about the T9SS to peptidoglycan connection of gliding bacteria. To further understand the role of PG1058-like proteins in F. johnssoniae, Δfjoh_1647, the most similar homolog protein when compared to PG1058 in P.gingivalis, was deleted. Overall, it was found that the Δfjoh_1647 mutant did not exhibit any significant difference in gliding motility from the wild type fjoh_1827. Microscopic videos of gliding bacteria were collected and analyzed via custom Python scripts. The mean speeds were 1.689 ± 0.366 µm/s and 1.70 ± 0.683 µm/s for wild type and mutant respectively. Additionally, the mutant colonies exhibited similar long-range trajectories and swarming patterns on Peptone Yeast Extract (PYE) plates.