Caspase recruitment domains (CARDs) and pyrin domains are important facilitators of inflammasome activity and pyroptosis. Upon pathogen recognition by NLR proteins, CARDs recruit and activate caspases, which, in turn, activate gasdermin pore forming proteins to and induce pyroptotic cell death. Here we show that CARD-like domains are present in defense systems that protect bacteria against phage. The bacterial CARD is essential for protease-mediated activation of certain bacterial gasdermins, which promote cell death once phage infection is recognized. We further show that multiple anti-phage defense systems utilize CARD-like domains to activate a variety of cell death effectors. We find that these systems are triggered by a conserved immune evasion protein that phages use to overcome the bacterial defense system RexAB, demonstrating that phage proteins inhibiting one defense system can activate another. We also detect a phage protein with a predicted CARD-like structure that can inhibit the CARD-containing bacterial gasdermin system. Our results suggest that CARD domains represent an ancient component of innate immune systems conserved from bacteria to humans, and that CARD-dependent activation of gasdermins is conserved in organisms across the tree of life.
Caspase recruitment domains (CARDs) and pyrin domains are important facilitators of inflammasome activity and pyroptosis. Upon pathogen recognition by NLR proteins, CARDs recruit and activate caspases, which, in turn, activate gasdermin pore forming proteins to and induce pyroptotic cell death. Here we show that CARD-like domains are present in defense systems that protect bacteria against phage. The bacterial CARD is essential for protease-mediated activation of certain bacterial gasdermins, which promote cell death once phage infection is recognized. We further show that multiple anti-phage defense systems utilize CARD-like domains to activate a variety of cell death effectors. We find that these systems are triggered by a conserved immune evasion protein that phages use to overcome the bacterial defense system RexAB, demonstrating that phage proteins inhibiting one defense system can activate another. We also detect a phage protein with a predicted CARD-like structure that can inhibit the CARD-containing bacterial gasdermin system. Our results suggest that CARD domains represent an ancient component of innate immune systems conserved from bacteria to humans, and that CARD-dependent activation of gasdermins is conserved in organisms across the tree of life.
Pore-forming proteins called gasdermins control cell-death response to infection in animals. Gasdermins are also present in bacteria where they have been shown to act as an abortive infection system that permeabilizes the cell membrane before phage release :ref{doi=10.1126/science.abj8432,10.1101/2023.05.28.542683}. In *Lysobacter*, the gasdermin operon includes two genes encoding trypsin-like protease domains, and a gene encoding an ATPase domain :ref{doi=10.1101/2023.05.28.542683}. Intact active sites for the second protease and the ATPase, but not the first protease, are required for succesful phage defense :ref{doi=10.1126/science.abj8432}. The domain architecture of the ATPase suggests it belongs to a protein family that is considered the ancestor of the eukaryotic nucleotide oligomerization domain (NOD)-like receptor (NLR) protein family :ref{doi=10.1101/2023.05.28.542683}. In animals, NLR initiates the formation of the inflammasome complex :ref{doi=10.1126/science.abe3069}. The second protease contains a region with similar structure to human CARD domain :ref{doi=10.1101/2023.05.28.542683}. The CARD domain takes part on the assembly of immune protein complexes :ref{doi=10.1038/sj.cdd.4401890}. The CARD-like domain in the *Lysobacter* system is required for succesful phage defense :ref{doi=10.1101/2023.05.28.542683}. Homology searches recovered multiple bacterial operons that include two proteases, one of them containing a CARD-like domain, and a NLR-like protein. In most cases, the effector gasdermin gene was replaced by another gene:ref{doi=10.1101/2023.05.28.542683}. The operon found in *Pedobacter rhizosphaerae* exhibits phage defense capabilities and contains a protein with phospholipase and endonuclease domains replacing the gasdermin gene. This system confers protection against the same phages as the *Lysobacter* gasdermin containing system, suggesting that the proteases and ATPase participate in phage specificity and recognition.
## To do
## Molecular mechanisms
For the *Lysobacter* system, the effector has been described as a pore-formin protein that disrupts the cell membrane :ref{doi=10.1101/2023.05.28.542683}. To our knowledge, other parts of the molecular mechanisms have yet to be elucidated.
## Example of genomic structure
## Example of genomic structure
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@@ -49,13 +59,3 @@ The system was detected in 57 different species.
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@@ -49,13 +59,3 @@ The system was detected in 57 different species.
Proportion of genome encoding the CARD_NLR system for the 14 phyla with more than 50 genomes in the RefSeq database.
Proportion of genome encoding the CARD_NLR system for the 14 phyla with more than 50 genomes in the RefSeq database.
