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Tad pili with adaptable tips mediate contact-dependent killing during bacterial predation

By Julien Herrou and Tâm Mignot

What is bacterial predation?

Bacterial predation is the ability of certain bacteria, like Myxococcus xanthus, to attack and kill other microorganisms to feed and grow. Upon contact with prey, Myxococcus halts its motility and assembles two predatory nanomachines at the contact site: the Kil system, which is essential for prey intoxication [1], and the needleless T3SS*, required for prey lysis [2].

While the discovery of the Kil and T3SS* was a major step forward in understanding Myxococcus contact-dependent predation, it remained unclear whether the Kil system, a member of the Tad pilus family, could actually assemble a predatory pilus. Additionally, the specific components of the Kil pilus fiber were still unidentified.

The breakthrough

A major breakthrough came with the emergence of powerful and revolutionary protein structure prediction and comparison tools such as AlphaFold [3] and Foldseek [4]. Using these tools, we discovered that the Kil pilus is composed of fourteen pilin proteins. By tagging the main pilin with a fluorescent marker, we were able to demonstrate, for the first time, that Myxococcus extends a predatory pilus at the prey contact site, leading to prey killing and lysis.

Structural predictions also revealed that the extremity of this pilus is functionalized by four distinct minor pilin complexes, referred to as “Tips”. These Tips work complementarily and in specific combinations to effectively detect and kill a wide range of prey species. This combinatorial strategy may explain how Myxococcus is able to prey on a broad variety of microorganisms. Importantly, the toxic activity of a subset of these Tips relies on the T3SS*, highlighting a functional link between the Kil system and the T3SS*.

Why is this important?

First, the discovery of the Kil system and its associated Tips highlights the remarkable diversification of Tad pili in bacteria. It provides the first evidence that the extremity of a Tad pilus can be functionalized by minor pilins to intoxicate other microorganisms. We also identified structural analogs of the Tip complexes in non-predatory bacteria, including pathogens. This suggests that other bacteria may also customize the end of their Tad pili with "Tip-like" complexes, further expanding the functional repertoire of these systems.

Second, the unique association between the Kil and the T3SS* suggests that these apparatuses work together to deliver toxic effectors during predation. Our current model proposes that, in the absence of a needle, the T3SS* translocates lytic and toxic molecules from the cytoplasm to the periplasm, where they may interact with the Kil pilus and its Tips to intoxicate prey cells.

Third, the Kil and T3SS* assemble exclusively at the prey contact site, coinciding with motility arrest. This implies that a unique “bacterial sense of touch” enables prey detection during predation. Uncovering the mechanism behind this process will shed light on novel sensory capabilities in bacteria.

References

[1] Seef, S. et al. A Tad-like apparatus is required for contact-dependent prey killing in predatory social bacteria. eLife 10, e72409 (2021).

[2] Thiery, S., Turowski, P., Berleman, J. E. & Kaimer, C. The predatory soil bacterium Myxococcus xanthus combines a Tad- and an atypical type 3-like protein secretion system to kill bacterial cells. Cell Rep. 40, (2022).

[3] Abramson, J. et al. Accurate structure prediction of biomolecular interactions with AlphaFold 3. Nature 630, 493–500 (2024).

[4] van Kempen, M. et al. Fast and accurate protein structure search with Foldseek. Nat. Biotechnol. 42, 243–246 (2024).