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 multi-subunit 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”, called 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. Intriguingly, the Sb-gRAMP physically combines with the caspase-like TPR-CHAT peptidase to form the Craspase (CRISPR-guided Caspase) complex, pointing at a potential mechanism of target RNA-induced protease activity to gain viral immunity.