Type IX secretion system

Various components of this system had been previously discovered as early as 2005. Namely, PorT and its ability to transport Gingipains in then-novel organisms Flavobacterium johnsoniae and Porphyromonas gingivalis. These components eventually led to the differentiation of the T9SS, setting it apart from the others. Through the conglomeration of other research done on these two novel organisms, specific proteins were identified in patterns as being used for similar functions across Bacteroidetes. GldK, GldL, GldM, and GldN proteins were observed in F. johnsoniae to be necessary for the cells to have motility and the ability to use chitin. And protease transportation was only enabled in P. gingivalis specimens if PorT, PorL, PorM, PorN, PorK, SprA/Sov proteins were present and functional within the cell. A later discovery that PorT was also necessary for the membrane facilitation of chitinase in F. johnsoniae led to the subsequent observation that the aforementioned list of proteins made up an entirely unique secretion system. These research groups found that Type IX secretion systems are exclusive to the phylum of Bacteroidetes and that they are present within a majority of species within that phylum. found that about 62% of members from the phylum Bacteroidetes contain the T9SS. Though mammals house a strong presence of these T9SS bacteria, Bacteroidetes can also be found within echinoderm, arthropod, and avian species. This illustrates that the presence of T9SS is relatively widespread. == Evolutionary relationship ==

The type IX secretion system likely evolved from ancient protein transport systems adapted to gliding motility and environmental interactions in Bacteroidetes. Genomic studies suggest that components of T9SS may have evolved in parallel with those of the type VI Secretion System (T6SS), sharing structural and energy-transducing similarities. Unlike injectisome-type systems, T9SS developed primarily for secretion into the extracellular environment rather than into host cells, supporting its unique ecological roles. There are many genes in the P. gingivalis genome that code for specific parts of the secretion system that are found in various areas, while the genes PorK-PorL-PorM-PorN-PorP are transcribed together. There are three sub-complexes found within the secretion system Source: • PorLM and GldLM rotary motor • PorKN and GldKN which associate with the outer membrane • Soc and SprA translocon Other subunits include GldO, GldJ, β-barrel, and plug proteins. T9SS subcomplex overview Rotary motor • PorL, PorM, GldL, GldM • Function: Generates energy using the proton motive force (PMF) Outer membrane scaffold • PorK, PorN, GldK, GldN • Function: Structural support and protein recruitment Translocon • SprA, Sov, Soc • Function: Moves proteins across the outer membrane Accessory proteins • GldO, GldJ, plug proteins • Function: Help stabilize and regulate the system Mechanism of energy utilization The PorLM/GldLM motor uses the proton motive force (PMF) across the inner membrane to power movement. GldL and GldM form a proton channel. As protons flow through, this generates torque that moves proteins like SprA through the outer membrane. This process supports secretion of enzymes like chitinases and proteases and helps build biofilms, especially in bacteria like Flavobacterium johnsoniae and Porphyromonas gingivalis. Structural studies Advances in cryo-electron microscopy have resolved the ring-like architecture of PorK and PorN complexes, revealing a periplasmic channel that aligns with the outer membrane translocon. These structures highlight the modularity and coordination of energy use and substrate specificity across the system. A 2025 cyro-EM work shows the PorKN ring in higher resolution. This work combines the use of cyro-EM and AlphaFold-predicted structures. == Utility == s, adhesins, protective surface proteins, cargo proteins, and enzymes such as hydrolytic enzymes, cellulases, chitinases, and proteases, each of which vary in utility for the cell. Adhesins act to fasten the cell to other cells and to ensure that it can dock and lock onto other surfaces that would be more beneficial for the cell's survival. These secreted adhesins help to establish biofilms around the cells which contribute to resisting external distress and an increase in cellular resilience to the environment. The enzymes that can be secreted are used for the breakdown of extracellular molecules for the acquisition of nutrients from the environment, or for protection by cleaving complement plasma proteins or peptides found in the environment. T9SS also helps non-pathogenic bacteria survive in nature. In marine species, it supports the breakdown of seaweed and contributes to nutrient recycling and carbon cycling. T9SS is not just important for infections. It plays a key role in the environment, especially in marine bacteria that use it to break down complex carbohydrates like chitin and cellulose. This helps recycle nutrients in aquatic ecosystems. Researchers are also studying T9SS for industrial uses, including enzyme production, wastewater treatment, and converting plant material into energy. Its unique secretion mechanism may be useful for future biotechnological applications. == Medical significance ==

T9SS is used by a bacteria for the release of a diverse array of proteins including virulence factors that can add to the bacterial pathogenicity. Some of the main pathogens are referred to as gingipains and are Kgp, RgpA, and RgpB. Gingipains are virulence factors that cause around 85% of protein degradation or proteolysis, greatly contributing to inflammatory conditions and the destruction of periodontal tissue. Porphyromonas peptidyl arginine deiminase (PPAD) is an enzyme that was additionally discovered to strictly be used with a T9SS in P. gingivalis that breaks down and alters protein structures by converting any arginine residues within the proteins into a neutrally charged citrulline. Secretion of PPADs can contribute to various deregulatory and inflammatory diseases. Periodontitis and rheumatoid arthritis (RA) are among the more common diseases that PPAD can contribute to, other diseases include psoriasis, multiple sclerosis (MS), Alzheimer's, and even some forms of cancer. == References ==