Structural basis for the RNA-guided ribonuclease activity of CRISPR-Cas13d
CRISPR-Cas endonucleases directed against foreign nucleic acids mediate prokaryotic adaptive immunity and have been tailored for broad genetic engineering applications. Type VI-D CRISPR systems contain the smallest known family of single effector Cas enzymes, and their signature Cas13d ribonuclease employs guide RNAs to cleave matching target RNAs. To understand the molecular basis for Cas13d function, we resolved cryo-electron microscopy structures of Cas13d-guide RNA binary complex and Cas13d-guide-target RNA ternary complex to 3.4 and 3.3 [A] resolution, respectively. Furthermore, a 6.5 [A] reconstruction of apo Cas13d combined with hydrogen-deuterium exchange revealed conformational dynamics that have implications for RNA scanning. These structures, together with biochemical and cellular characterization, explain the compact molecular architecture of Cas13d and provide insights into the structural transitions required for enzyme activation. Our comprehensive analysis of Cas13d in diverse enzymatic states facilitated site-specific truncations for minimal size and delineates a blueprint for improving biomolecular applications of RNA targeting.