enCas7-11S3: A compact Cas7-11 variant with enhanced RNA cleavage and minimal collateral activity

In the CRISPR-Cas system, the RNA-targeting single-protein effectors include the Class 2 Type VI Cas13 and Class 1 Type III-E Cas7-11. Cas7-11 from Desulfonema ishimotonii is a large single-protein effector containing four Cas7 domains and one Cas11 domain, and its substantial size hinders delivery via a single adeno-associated virus (AAV) vector. To address this limitation, a compact Cas7-11 variant (Cas7-11S) was engineered by deleting the insertion (INS) domain. However, this truncation reduced target RNA cleavage activity compared to full-length Cas7-11. Here, we engineered Cas7-11S to improve its RNA cleavage efficiency while further reducing its size. We designed a dual-fluorescence reporter system in mammalian cells to evaluate the RNA cleavage efficiency of Cas7-11S. We found that adding a nucleocytoplasmic shuttling signal to the C-terminus of Cas7-11S greatly improved its RNA cleavage efficiency. We then systematically screened Cas7-11S mutants in HEK293T cells and identified variants with stronger RNA cleavage activity, as well as truncated variants with more compact structures. Finally, by combining these beneficial modifications, we generated an enhanced Cas7-11S variant called enCas7-11S3 that is both more compact and exhibits higher RNA cleavage activity. Importantly, enCas7-11S3 retains almost no collateral activity, broadening the potential application of Cas7-11 in RNA editing.