Structural highlights
FunctionPublication Abstract from PubMedCas10 is the signature gene for type-III CRISPR-Cas surveillance complexes. Unlike type-I and type-II systems, type-III systems do not require a PAM, and target nascent RNA associated with transcriptionally active DNA. Further, target RNA recognition activates the cyclase domain of Cas10, resulting in the synthesis of cyclic oligoadenylate (cOA) second messengers. These second messengers are recognized by ancillary Cas proteins harboring CARF domains, and regulate the activities of these proteins in response to invading nucleic acid. Csx3 is a distant member of the CARF domain superfamily previously characterized as a Mn2+ dependent deadenylation exoribonuclease. However, its specific role in CRISPR-Cas defense remains to be determined. Here we show Csx3 is strongly associated with type-III systems and that Csx3 binds cyclic tetra-adenylate (cA4) second messenger with high affinity. Further, Csx3 harbors cyclic oligonucleotide phosphodiesterase activity that quickly degrades this cA4 signal. In addition, structural analysis identifies core elements that define the CARF domain fold, and the mechanistic basis for ring nuclease activity are discussed. Overall, the work suggests Csx3 functions within CRISPR-Cas as a counter balance to Cas10 to regulate the duration and amplitude of the cA4 signal, providing an off-ramp from the programmed cell death pathway in cells that successfully cure viral infection. Csx3 is a cyclic oligonucleotide phosphodiesterase associated with type-III CRISPR-Cas that degrades the second messenger cA4.,Brown S, Gauvin CC, Charbonneau AA, Burman N, Lawrence CM J Biol Chem. 2020 Aug 21. pii: RA120.014099. doi: 10.1074/jbc.RA120.014099. PMID:32826317[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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