Type IX secretion system

Type IX Secretion System

Discovered	2010
Discovered by	M.J. McBride and K. Nakayama
Organisms	Bacterial species from the phylum Bacteroidetes
Function	Protein secretions, virulence factor exportation, motility (gliding)
Pathogenicity	Periodontal disease, immune evasion, tissue disruption
Key proteins	PorL, PorM, PorN, PorK, PorT, SprA/Sov,

The Type IX secretion system is a specialized protein secretion system found in the Fibrobacteres-Chlorobi-Bacteroidetes superphylum. It plays a crucial role in various cellular processes, including gliding motility and the secretion of virulence factors in Porphyromonas gingivalis. To date, at least nineteen components of the T9SS have been identified, though their precise architecture and mechanistic functions remain incompletely understood.

An illustration of the various types of secretion systems that are used in bacteria.

Secretion systems come in several different varieties. These are intricate complexes of proteins that are incorporated within the membranes of many different species of bacteria. These proteins are used by the bacteria to expel and transport intracellular enzymes, proteins, and molecules across the cytoplasmic membrane into a host cell or into the surrounding extracellular space. The type of secretion system used is dependent upon the function required and the type of cell that is utilizing it. A gram-negative, pathogenic diderm might employ a secretion system's membrane bound proteins to inject toxins into the host cell, while that of a Type IX Secretion System (T9SS) may only be used to secrete proteins into the extracellular space.

Discovery and epidemiology

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.

Formerly known as Porphyromonas secretion systems (PorSS), due to its discovery on Porphyromonas gingivalis, Type IX Secretion Systems were officially recognized and renamed in 2010 as the ninth secretion system by research groups that were headed by M.J. McBride and K. Nakayama. 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. Further research that was carried out by S.S. Abby found that about 62% of members from the phylum Bacteroidetes contain the T9SS.

The only phylum of bacteria to house a T9SS, Bacteroidetes are largely found throughout the gastrointestinal tracts of mammals. While there presence is stronger within fecal material, as much as 20% of all bacteria present within the oral cavities of mammals can belong to the phylum Bacteroidetes. 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.

Composition and function

This illustrates the basic structure of Por proteins, a β-barrel.

The Type IX bacterial secretion system contains 18 genes that are needed for proper function. 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. The subunits that make up this system are found in sub-complexes that contribute to the overall structure.

There are three sub-complexes found within the secretion system:

1. Rotary Motor
2. Outer Membrane Complex
3. Translocon

Other subunits include GldO, GldJ, β-barrel, and plug proteins.

1) PorLM/GldLM rotary motor

PorLM and GldLM proteins form a complex on the inner membrane in a ratio of 5:2. The overall protein structure crosses the membrane twice (transmembrane) (PorL/GldL), while also containing a portion that can be found in they cytoplasm (PorM/GldM). This motor system works similarly to the proton motive force, where it uses energy to transport proteins through the secretion system, specifically by deriving its energy from the inner membrane gradient.

2) PorKN/GldKN outer membrane complex

PorK/GldK and PorN/GldN are proteins that associate with the outer membrane. GlddJ and GldO may also be present in certain species of bacteria, like Flavobacterium johnsoniae. PorK/GldK are lipoproteins found at the outmembrane, while PorN/GldN are proteins found within the periplasmic space. In the Porphyromonas class, a ring structure is formed by the interactions of PorK and PorN, which forms a structure in the periplasm that resembles a cage.

3) Sov/SprA Translocon

Sov/SprA form a translocon, which makes up a β-barrel. This a large protein, consisting of many strands. At the extracellular side of the protein, the barrel is closed. In the Flavobacterium class, it has been shown that SprA associate with either the β-barrel on PorV, or a plug protein which closes the translocon opening in the periplasmic side. On the other hand, the PorV β-barrel can insert itself into an opening found on the translocon, probably interacting when the translocon is active. Regulation of the translocon (movement of proteins through it) comes through the plug protein and the PorV β-barrel, which can associate/deassociate at different times.

Other components of the T9SS

Besides the important components already mentioned, this system can have other components that also play a role in the overall function. These can include PorQ β-barrels PorZ proteins.

The GldBDHI and GldAFG proteins haven't been shown to have a specific purpose, but scientists believe that they play a part in motility purposes. SprCD/REM proteins also play a part in motility, specifically for the Flavobacteria class.

Function

Protein secretion from this system goes through two steps:

1. Proteins are transported from the cytoplasm into the periplasm through the use of a Sec translocon. When this hapens, the protein is not folded, but will get folded in the periplasm
2. A C-terminal domain will help the protein interact with the secretion system. The exact method that this occurs is unknown, but it is proposed that the proteins bind to the ring structure at the outer membrane and are transferred to the Soc/SprA translocons through the PorM/GldM movement of the ring. After the proteins arrive at the translocon, they can be pumped through the PorV β-barrel to PorQUZ, which then get attached to the cell or can be released from the system.

Through these steps, proteins are exported through the secretion system.

Utility

A closer illustration of the T9SS of a diderm bacterial cell.

Rotation of the T9SS can be used to enable motility for the cell in the form of gliding motility. It can also be used to secrete a variety of proteins into the extracellular environment. These secreted proteins include: virulence factors, 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.

Secreted virulence factors are used as a coating for the cell and the cargo vesicles that it releases. This coating allows these packaged vesicles to enter into a host cell and impair immune response in the host. Virulence factors on vesicles contribute to immune evasion but may also trigger inflammatory responses in host tissues.

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.

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. The T9SS also can be a major component to motility for various bacterial systems. The proteins that make up this externally assembled rotary system can be recognized, much like other pathogen-associated molecular patterns (PAMPs), by a host's innate immune system, resulting in complement cascades. These plasma proteins meet the enzymatic T9SS cargo proteins and become susceptible to degradation.

An instance of periodontitis that illustrates inflamed and receding gingiva and the appearance of gaps forming between the teeth.

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.

Therapeutic treatments for bacteria with T9SS release include the administration of proper antibiotics, which can also target the proteolytic enzymes that T9SSs secrete. Cranberry and rice extracts were also seen to have a degree of success with inhibiting the activity of gingipains and in preventing pathogenic biofilm formations and growth.