Csx15 is a broad specificity ring nuclease bridging CRISPR-Cas and CBASS cyclic nucleotide signaling systems
Prokaryotic antiviral type III CRISPR-Cas and CBASS systems use cyclic nucleotides as second messengers to activate diverse effector proteins. Whereas CBASS signaling typically triggers abortive infection, type III CRISPR-Cas signaling is tightly regulated through controlled signal production and degradation. Here, we define the molecular mechanism of the CRISPR-associated Csx15 protein family and demonstrate that Csx15 functions as a broad specificity ring nuclease capable of degrading cyclic nucleotide signals from both CRISPR-Cas and CBASS pathways. Structural and biochemical analyses reveal that Csx15 adopts diverse oligomeric states and undergoes ligand-induced conformational transitions that promote filament assembly required for catalytic activity, analogous to filament-forming SAVED-domain effectors. Finally, we provide evidence that Csx15 is a close relative of SAVED domains suggesting that Csx15-like proteins may have also contributed to the evolution of CBASS cyclic nucleotide signaling.