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bioRxiv · 10.1101/2025.10.08.681156

Stepwise firing mechanism of an extracellular contractile injection system

Xu, J.·Ericson, C. F.·Toenshoff, E. R.·Pilhofer, M.
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Abstract

Contractile injection systems (CISs) mediate cell-cell interactions and are widespread among bacteria and archaea. These phage tail-like macromolecular machines puncture their target by a tube that is propelled by a contractile sheath. The mechanism underlying CIS firing, which starts with target binding and ends with sheath contraction, remains unclear. Here, we investigate the extracellular CIS from Algoriphagus machipongonensis (AlgoCIS) by a multimodal cryo-electron microscopy approach and structure-guided engineering, which allowed us to arrest AlgoCIS in multiple intermediate states of firing. Together with the post-firing structure, our data suggest a stepwise firing mechanism involving all modules: signal propagation starts with the binding of the tail-fibers, followed by opening of the cage, an expansion of the baseplate iris, and resulting in sheath contraction and the release of cap adaptor. Our study will serve as a framework for understanding the firing mechanism of diverse CISs and will facilitate the engineering of CISs for biomedical applications.

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Xu, J., Ericson, C. F., Toenshoff, E. R., Pilhofer, M.. 2025-10-08. Stepwise firing mechanism of an extracellular contractile injection system. https://doi.org/10.1101/2025.10.08.681156

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