The CRISPR-associated adenosine deaminase Cad1 converts ATP to ITP to provide antiviral immunity

Type III CRISPR-Cas systems provide immunity against genetic invaders through production of cyclic oligo-adenylate (cAn) molecules that bind to and activate effector proteins that contain CARF domains. Previously, CARF effectors harboring adenosine deaminase (ADA) domains have been reported to be genetically linked to type III CRISPR-cas loci; however, whether and how they provide immunity has not been explored. Here we characterized the function and structure of CRISPR-associated adenosine deaminase 1 (Cad1), A CARF-ADA effector. We show that upon binding of cA4 or cA6 to its CARF domain, Cad1 converts ATP to ITP, both in vivo and in vitro. Cryo-EM structural studies on full-length Cad1 reveal an hexameric assembly composed by a trimer of dimers, with bound ATP at inter-domain sites required for activity, and ATP/ITP within deaminase active sites. Upon synthesis of cAn during phage infection, Cad1 activation leads to a growth arrest of the host that prevents viral propagation. Our findings reveal that CRISPR-Cas systems employ a wide range of molecular mechanisms beyond nucleic acid degradation to provide adaptive immunity in prokaryotes.