9.5 Type V Secretion Atlas of Bacterial and Archaeal Cell Structure Home
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Mechanism

The type V secretion system is a minimal machine: a channel formed by several copies of one protein (CdiB) in the outer membrane secretes another protein, CdiA. Or rather, it secretes part of it. The first half of CdiA forms the rigid stick you see extending from the cell. The second half loops back to keep the toxin domain at the end of the protein sequestered in the periplasm, awaiting secretion. The tip of the stick (which is the middle of the protein) contains the domain that recognizes and docks to the receptor pore on the target. When it bumps into a neighboring cell and locks on to the receptor, the rest of CdiA is secreted and delivers the toxin into the other cell like a tetherball [83]. (You can watch the full animation of this process on YouTube). Animation by Janet Iwasa.

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Type V Secretion

Other secretion systems independently evolved similar mechanisms, though using very different structures. Some strains of Escherichia coli have an ability, called “Contact-Dependent Inhibition” or CDI, to inhibit the growth of neighboring diderm bacterial cells. They do this by coopting a pore in the outer membrane of the target cell to introduce a toxin that halts its growth. To safeguard themselves, they make an antitoxin that binds to the toxin and renders it harmless. Such antidote systems are common for microbe-produced toxins.

To deliver the toxin to the target, the cells use type V secretion systems (T5SSs) like the ones on this Escherichia coli. In contrast to the nanomachines you have been seeing so far, which contain dozens of unique components, this system is elegantly spare, using just two proteins and a mechanism reminiscent of a medieval ball-and-chain flail (⇩).

Note: This cell belongs to a strain that is normally incapable of CDI, but has been genetically engineered to produce the T5SS components. Once again, this lets us image the machines more easily; in this case, the CDI-practicing strain has a thicker layer of extracellular proteins, which would obscure the needles in a visual haystack.

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