SAVED by a toxin: Structure and function of the CRISPR Lon protease.

CRISPR antiviral defense systems such as the well-known DNA-targeting Cas9- and the more complex RNA-targeting type III systems are widespread in bacteria and archea. The type III systems can orchestrate a complex antiviral response that is initiated by the synthesis of cyclic oligoadenylates (cOAs) upon foreign RNA recognition. These second messenger molecules bind to the CARF (CRISPR associated Rossmann-fold) domains of dedicated effector proteins that are often DNAses, RNAses, or putative transcription factors. The activated effectors interfere with cellular pathways of the host, inducing cell death or a dormant state of the cell that is better suited to avoid propagation of the viral attack. Among a large set of proteins that were predicted to be linked to the type III systems, the CRISPR-Lon protein caught our attention. The protein was predicted to be an integral membrane protein containing a SAVED- instead of a CARF-domain as well as a Lon protease effector domain. Here, we report the crystal structure of CRISPR-Lon. The protein is a soluble monomer and indeed contains a SAVED domain that accommodates cA4. Further, we show that CRISPR-Lon forms a stable complex with the 34 kDa CRISPR-T protein. Upon activation by cA4, CRISPR-Lon specifically cleaves CRISRP-T, releasing CRISPR-T23, a 23 kDa fragment that is structurally very similar to MazF toxins and is likely a sequence specific nuclease. Our results describe the first cOA activated proteolytic enzyme and provide the first example of a SAVED domain connected to a type III CRISPR defense system. The use of a protease as a means to unleash a fast response against a threat has intriguing parallels to eukaryotic innate immunity. ### Competing Interest Statement The authors have declared no competing interest.