And… cut! - how conformational regulation of CRISPR-Cas effectors directs nuclease activity

Controlling the conformation of dynamic protein, RNA and DNA molecules underpins many biological processes, from the activation of enzymes and induction of signalling cascades to cellular replication. Clustered regularly interspaced short palindromic repeats (CRISPR)-associated (Cas) effectors are enzymes tightly controlled by conformational steps that gate activation of nuclease domains core to their function in bacterial adaptive immunity. These precise conformational checkpoints combined with programmable activation specified by RNA guides have driven the success of CRISPR-Cas tools in biotechnology, medicine and beyond. To illustrate the importance of conformation in controlling CRISPR-Cas activity, we review the discrete conformational checkpoints at play in class 2 CRISPR-Cas systems. Using Cas9, Cas12a and Cas13a as examples, we describe how protein and nucleic acid conformations precisely control the loading of guide RNA, the selection of target nucleic acids and the activation of nuclease domains. Much like a director controls the timing of transitions between scenes in a movie, CRISPR effectors use conformational checkpoints to precisely direct their enzymatic activity.