The gRAMP CRISPR-Cas effector is an RNA endonuclease complexed with a caspase-like peptidase

Type III CRISPR-Cas immunity is widespread in prokaryotes and is generally mediated by multisubunit effector complexes. These complexes recognize complementary viral transcripts and can activate ancillary immune proteins. Here, we describe a type III-E effector from Candidatus “Scalindua brodae” (Sb-gRAMP), which is natively encoded by a single gene with several type III domains fused together. This effector uses CRISPR RNA to guide target RNA recognition and cleaves single-stranded RNA at two defined positions six nucleotides apart. Sb-gRAMP physically combines with the caspase-like TPR-CHAT peptidase to form the CRISPR-guided caspase (Craspase) complex, suggesting a potential mechanism of target RNA-induced protease activity to gain viral immunity.