What if your cell wanted to go a step further and deliver toxins directly into its target? Perhaps it needs a syringe. One of the most common, and best-studied, pathogenic secretion systems is the type III secretion system (T3SS) that assembles a needle-like toxin delivery system called the injectisome. T3SS injectisomes and flagellar motors are closely related, and the basal machines have similar structures (⇩). Rather than assembling a long helical filament for motility, though, injectisomes form a shorter needle, which you can see on this Pseudomonas aeruginosa. The needle can penetrate a eukaryotic cell, delivering effectors directly into its cytoplasm that block the immune response of the host, and lead to cell lysis. (You can watch an animation of this process on YouTube.)
Note: It would be wasteful, or detrimental, for your cell to secrete toxins all the time. Pathogens typically have multi-stage lifecycles, often passing through an intermediate host and/or the environment. Pathogenic machinery like secretion systems are typically only produced when conditions signal that the cell has reached its target (e.g. increased temperature or salt concentration). Until then, the genes encoding the protein components are turned off. This cell is missing a protein (ExsD) that turns off expression of these injectisome components. As a result, the strain makes injectisomes even outside the host, enabling us to image their structure.
T3SSs are closely related to type II secretion systems (T2SSs) and share a common element: the channel embedded in the outer membrane, formed by many copies of a protein called a Secretin (⇩). The rest of the machine looks very different, though, reflecting their evolution into separate mechanisms.
This average of T3SS injectisomes from many Shigella flexneri cells shows the overall structure, including the large “sorting platform” in the cytoplasm which selects proteins for secretion through the channel of the needle, whose atomic structure you see here [82].
Many copies of a Secretin protein come together to form a channel, as in this one from a Vibrio cholerae type II secretion system [83]. In addition to the main channel embedded in the outer membrane, additional domains of the protein form a series of rings in the periplasm that interact with other components of the system. The Secretin channel forms the core of both type II and type III secretion systems, showing that they evolved from the same ancestral machine, adding and repurposing other components over time to serve their different functions.
