A viral SAVED protein with ring nuclease activity degrades the CRISPR second messenger cA ₄

Type III CRISPR systems typically generate cyclic oligoadenylate (cOA) second messengers such as cyclic tetra-adenylate (cA ₄ ) on detection of foreign RNA, activating ancillary effector proteins which elicit a diverse range of immune responses. The CalpLTS system elicits a transcriptional response to infection when CalpL binds cA ₄ in its SAVED (SMODS associated and fused to various effectors domain) sensor domain, resulting in filament formation and activation of the Lon protease domain, which cleaves the anti-Sigma factor CalpT, releasing the CalpS Sigma factor for transcriptional remodelling. Here, we show that thermophilic viruses have appropriated the SAVED domain of CalpL as an anti-CRISPR, AcrIII-2, which they use to degrade cA ₄ . AcrIII-2 dimers sandwich cA ₄ , degrading it in a shared active site to short linear products, using a mechanism highly reminiscent of CalpL. This results in inhibition of a range of cA ₄ activated effectors in vitro . This is the first example of a virally-encoded SAVED domain with ring nuclease activity, highlighting the complex interplay between viruses and cellular defences.