CRISPR RNA binding drives structural ordering that primes Cas7-11 for target cleavage.

Type III-E CRISPR-Cas effectors, referred to as Cas7-11 or giant Repeat-Associated Mysterious Protein, are single proteins that cleave target RNAs (tgRNAs) without nonspecific collateral cleavage, opening new possibilities for RNA editing. Here, biochemical assays combined with amide hydrogen-deuterium exchange mass spectrometry (HDX-MS) experiments reveal the dynamics of apo Cas7-11. The HDX-MS results suggest a mechanism by which CRISPR RNA (crRNA) stabilizes the folded state of the protein and subsequent tgRNA binding remodels it to the active form. HDX-MS shows that the four Cas7 RNA recognition motif (RRM) folds are well-folded, but insertion sequences, including disordered catalytic loops and β-hairpins of the Cas7.2/Cas7.3 active sites, fold upon binding crRNA leading to stronger interactions at domain-domain interfaces, and folding of the Cas7.1 processing site. TgRNA binding causes conformational changes around the catalytic loops of Cas7.2 and Cas7.3. We show that Cas7-11 cannot independently process the CRISPR array and that binding of partially processed crRNA induces multiple states in Cas7-11 and reduces tgRNA cleavage. The insertion domain interacts most stably with mature crRNA. Finally, we show a crRNA-induced conformational change in one of the tetratricopeptide repeat fused with Cas/HEF1-associated signal transducer (TPR-CHAT) binding sites providing an explanation for why crRNA binding facilitates TPR-CHAT binding.