Structural insights into RNA-guided RNA editing by the Cas13b-ADAR2 complex

Cas13 is an RNA-guided RNA endonuclease derived from the type VI CRISPR-Cas system, which has been used in numerous RNA-targeting technologies, such as RNA knockdown, detection and editing. The catalytically inactive Prevotella sp. Cas13b (dPspCas13b) fused to the human adenosine deaminase acting on RNA 2 (ADAR2) deaminase domain can edit adenosine in target transcripts to inosine, in an RNA-editing technology called REPAIR (RNA editing for programmable A-to-I replacement), which has potential for gene therapy. Here we report the cryo-electron microscopy structures of the PspCas13b-guide RNA binary complex, the PspCas13b-guide RNA-target RNA ternary complex and the dPspCas13b-ADAR2-guide RNA-target RNA complex. These structures provide mechanistic insights into RNA cleavage and editing. We applied our structural insights to engineer a compact and efficient dPspCas13b-ADAR2 complex (REPAIR-mini). Overall, our findings advance the understanding of CRISPR-Cas13 effector nucleases and could enable the development of improved RNA-targeting technologies.