The Gram-negative bacterium Vibrio cholerae (causative agent of cholera) is armed with a nano-speargun, which it uses to combat unwelcome competitors. University of Basel Professor Marek Basler and his colleagues from Switzerland and Germany have now gained insights into the construction, mode of action and recycling of this bacterial weapon.
Vibrio cholerae’s speargun (also known as the type VI secretion system, or T6SS) is composed of various components, including a sheath and a spear with a sharp tip.
The sheath consists of over 200 connected cogwheel-like protein rings that are assembled around the inner rigid spear.
When T6SS fires, the sheath rapidly contracts and pushes the toxic spear out of the cell, which can then penetrate into neighboring cells where it releases deadly toxins.
“So far, there have only been assumptions as to how the structure of the T6SS sheath changes during contraction,” Professor Basler said.
“Using cryo-electron microscopy, we have now obtained an image of the spear and the extended sheath in atomic resolution.”
By comparing the structures of the extended and contracted states, Professor Basler and co-authors were able to model how T6SS works in detail.
“During the sheath contraction, ring after ring turns and gets closer to the previous ring, while the ring diameter expands and thus releases the spear,” Professor Basler said.
“This combination of sheath shrinking and turning results in drilling a hole into the target cells.”
“Within less than two milliseconds, the T6SS sheath contracts to half of its length and at the same time the toxic spear spirals out like a screw. Therefore, the bacteria have an extremely powerful drill.”
According to the team, after firing T6SS, Vibrio cholerae bacteria re-use the individual components of the sheath to assemble a new speargun.
“For a long time, it was not clear why only the contracted, but not the extended sheath is disassembled,” Professor Basler said.
“Now, we could see that a certain protein domain is exposed on the surface of the sheath during contraction and can be recognized by a specific protein responsible for dismantling the sheath. In the extended sheath state, this domain is hidden and the T6SS sheath is therefore protected from disassembly.”
Details of the research are published in the journal Nature Microbiology.
Jing Wang et al. Cryo-EM structure of the extended type VI secretion system sheath-tube complex. Nature Microbiology, published online September 25, 2017; doi: 10.1038/s41564-017-0020-7