The CRISPR ring nuclease Csx15 oligomerises on cyclic nucleotide binding to regulate antiviral defence
Chi, H.; McMahon, S.; Graham, S.; White, M. F.
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Prokaryotic type III CRISPR systems signal infection by generating cyclic oligoadenylate (cOA) second messengers, which activate defence proteins allosterically, providing immunity. cOA molecules are typically degraded by extrinsic, stand-alone ring nuclease (RN) enzymes with phosphodiesterase activity, or by the intrinsic RN activity of the effectors themselves. Viruses and plasmids also encode RNs, which can function as anti-CRISPRs (Acr). Eight different families of extrinsic RNs are currently known. Here, we report the structural and biochemical analysis of one of these families: Csx15. We show that Csx15 is a dimeric protein with the ability to bind cyclic tetra-adenylate (cA4) molecules in a shared binding site formed by the head-to-tail stacking of dimers in a filament conformation. Some family members are non-enzymatic, relying on the sequestration (sponging) of cA4 to regulate the host immune response, while others act as canonical RNs, slowly degrading cA4.
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