ArticleNucleic acids research2026
Structural basis of assembly-dependent activation of the MAD3-MAD4 nuclease in the Pseudomonas aeruginosa MADS defense system.
Article in Nucleic acids research, 2026. The graph could read no effect estimate from its abstract, so it casts no vote on the map. Not yet cited in PubMed.
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Abstract
The methylation-associated defense system (MADS) is a bacterial antiphage immune system that cooperates with CRISPR-Cas systems to restrict phage infection. Within MADS, the MAD3-MAD4 module has been proposed to function as an ATPase-nuclease module related to the OLD nuclease family, but the molecular basis of its assembly and function remains unknown. Here, we combined cryo-electron microscopy, X-ray crystallography, biochemical assays, and mutagenesis to elucidate the architecture and assembly mechanism of the MAD3-MAD4 complex. We found that MAD3 and MAD4 assemble into a stable 2:2 heterotetramer. Structural and biochemical analyses revealed that MAD3 alone lacks detectable ATPase activity, whereas MAD4 is insoluble when expressed alone. Assembly of the MAD3-MAD4 complex induces a conformational rearrangement in which MAD3 adopts an ATPase-competent dimeric conformation, while MAD4 is stabilized through intersubunit interactions. This heterotetrameric assembly exhibits both ATPase and DNA cleavage activities. Overall, our findings reveal the molecular basis of MAD3-MAD4 heterotetramer assembly and provide the first structural and biochemical insights into the function of the MAD3-MAD4 module within the MADS defense system.
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