Bacteria use secretion systems to transport proteins across their membranes, allowing them to interact with their environment and cause disease. The Type 7 Secretion System (T7SS) is a multi-protein complex that moves specific effector proteins out of the bacterial cell. First discovered in Mycobacterium, the T7SS is important for bacterial survival within a host organism. Understanding this system reveals vulnerabilities that can be exploited for medical purposes.
Core Components and Structure
The Type 7 Secretion System is built from multiple protein components that form a large complex spanning the bacterial cell envelope. In bacteria like Mycobacterium with complex, multi-layered cell walls, this structure creates a continuous channel for proteins to pass through. The T7SS components can be separated into the membrane complex that forms the channel and the substrate proteins that are transported through it.
The core of the T7SS is a large protein assembly embedded within the bacterium’s inner membrane, primarily composed of “ESX conserved components,” or Ecc proteins. These proteins come together to form a six-sided pore that acts as the secretion channel and foundation of the apparatus. The Ecc proteins provide both the physical channel and the energy-generating components needed to drive transport. These proteins include:
- EccB
- EccC
- EccD
- EccE
The proteins transported by the T7SS are known as substrates, with the most well-known belonging to the Esx family. These are small proteins, with classic examples being EsxA (also called ESAT-6) and EsxB (also called CFP-10), which are often secreted as a pair. The T7SS is highly specific, ensuring that only the correct cargo is moved out of the cell.
Mechanism of Secretion
Secretion through the T7SS is an energy-dependent process powered by ATP. It involves recognizing specific substrate proteins, delivering them to the secretion channel, and transporting them across the cell envelope.
The process begins when the large ATPase protein, EccC, recognizes and binds to substrates like the EsxA/EsxB heterodimer inside the bacterium. This protein has domains that extend into the cytoplasm, allowing it to bind to the substrates. This recognition step is highly specific, ensuring that only designated T7SS substrates are engaged by the system.
Once a substrate is bound, EccC uses energy from ATP hydrolysis to power its transport. This energy drives conformational changes in the secretion complex, pushing or pulling the substrate through the channel. The exact mechanism of passage is still under investigation, but it is believed that the substrate proteins may need to be at least partially unfolded to fit through the narrow pore.
A protease enzyme, MycP, is also part of the T7SS machinery. This enzyme can cleave certain components of the system or its substrates, which may be a regulatory step or a requirement for proper function.
Role in Bacterial Pathogenesis
The T7SS contributes to the ability of several bacteria to cause disease. In Mycobacterium tuberculosis, the cause of tuberculosis, the T7SS is necessary for pathogenesis. Its secreted proteins interact with the host’s cells and immune system, creating a favorable environment for the bacteria to survive. The most studied of these systems in M. tuberculosis is known as ESX-1.
One of the most documented functions of the ESX-1 system is its ability to disrupt membranes within host cells. When M. tuberculosis is engulfed by an immune cell, it is enclosed in a compartment called a phagosome. The secreted T7SS effector proteins, particularly ESAT-6, can form pores in the phagosomal membrane. This damage allows the bacterium to escape into the host cell’s cytoplasm or to allow nutrients from the cytoplasm to leak into the phagosome.
The T7SS also manipulates the host’s immune response. T7SS activity allows bacterial components to enter the host cytoplasm, triggering specific cellular signaling pathways. M. tuberculosis uses this process to its advantage, often promoting a type of cell death called necrosis. This ruptures the host cell, releasing the bacteria and allowing them to infect neighboring cells.
By controlling the location and timing of its protein secretion, the bacterium can subvert immune functions that would normally clear the infection. This allows M. tuberculosis to establish the chronic, long-term infection characteristic of tuberculosis.
Distribution Among Bacteria
While best known for its role in Mycobacterium tuberculosis, the T7SS is not exclusive to this species. Related systems are found across a range of bacteria, primarily within the Actinobacteria and Firmicutes phyla.
The phylum Actinobacteria is the primary group where T7SS is found. This includes the Mycobacterium genus, with species like M. leprae (the cause of leprosy) and numerous non-pathogenic, environmental mycobacteria. In these bacteria, different versions of the T7SS, named ESX-1 through ESX-5, exist, each responsible for secreting different sets of proteins and contributing to distinct functions like nutrient uptake.
The T7SS is also present in the phylum Firmicutes, which includes several medically important Gram-positive bacteria. For example, the pathogen Staphylococcus aureus possesses a T7SS that it uses to secrete toxins that attack competing bacteria. Other Firmicutes, like Listeria monocytogenes, also have T7SS-like systems that contribute to their ability to cause disease.
Therapeutic and Diagnostic Applications
The T7SS’s role in the virulence of pathogens makes it a focus for medical research. Because the system is present in bacteria like M. tuberculosis but absent in humans, it is a target for developing new drugs for both treatment and diagnosis.
From a therapeutic standpoint, the T7SS is a target for new antibiotics. A drug designed to inhibit T7SS function could disarm the bacteria without directly killing them. For example, a compound that blocks the ATP-binding site of the EccC component could shut down the entire secretion process. This would prevent the bacteria from secreting the effector proteins needed to damage host cells, rendering them harmless.
In diagnostics, proteins secreted by the T7SS have already led to major advances. The effector proteins ESAT-6 and CFP-10 are highly immunogenic, provoking a strong immune response in individuals infected with M. tuberculosis. This is exploited in modern blood tests for tuberculosis called Interferon-Gamma Release Assays (IGRAs). These tests measure the release of interferon-gamma by a patient’s T-cells when they are exposed to ESAT-6 and CFP-10. Because these proteins are specific to M. tuberculosis, IGRAs are more accurate than the traditional tuberculin skin test.