6q1h
Structure of P. aeruginosa ATCC27853 NucC, cAAA-bound form
Structural highlights
FunctionNUCC_PSEAI CBASS (cyclic oligonucleotide-based antiphage signaling system) provides immunity against bacteriophage. The CD-NTase protein synthesizes cyclic nucleotides in response to infection; these serve as specific second messenger signals. The signals activate a diverse range of effectors, leading to bacterial cell death and thus abortive phage infection. A type III-C(AAA) CBASS system (PubMed:32839535).[1] [2] A cyclic nucleotide-activated dsDNase. In the presence of 3',3',3'-cyclic AMP-AMP-AMP (cAAA) and to a lesser extent cyclic-di-AMP (c-di-AMP), endonucleolytically degrades dsDNA (Probable). Binds one cAAA in a pocket on one surface of the trimer; cAAA binding promotes hexamerization which is probably necessary for nuclease activation (PubMed:31932164). The nuclease digests dsDNA to about 50 bp lengths. DNA has been modeled to contact a pair of juxtaposed active sites (one from each layer of the hexamer), accounting for cleavage on both strands (By similarity).[UniProtKB:D7Y2H5][3] [4] Publication Abstract from PubMedBacteria possess an array of defenses against foreign invaders, including a broadly distributed bacteriophage defense system termed CBASS (cyclic oligonucleotide-based anti-phage signaling system). In CBASS systems, a cGAS/DncV-like nucleotidyltransferase synthesizes cyclic di- or tri-nucleotide second messengers in response to infection, and these molecules activate diverse effectors to mediate bacteriophage immunity via abortive infection. Here, we show that the CBASS effector NucC is related to restriction enzymes but uniquely assembles into a homotrimer. Binding of NucC trimers to a cyclic tri-adenylate second messenger promotes assembly of a NucC homohexamer competent for non-specific double-strand DNA cleavage. In infected cells, NucC activation leads to complete destruction of the bacterial chromosome, causing cell death prior to completion of phage replication. In addition to CBASS systems, we identify NucC homologs in over 30 type III CRISPR/Cas systems, where they likely function as accessory nucleases activated by cyclic oligoadenylate second messengers synthesized by these systems' effector complexes. Structure and Mechanism of a Cyclic Trinucleotide-Activated Bacterial Endonuclease Mediating Bacteriophage Immunity.,Lau RK, Ye Q, Birkholz EA, Berg KR, Patel L, Mathews IT, Watrous JD, Ego K, Whiteley AT, Lowey B, Mekalanos JJ, Kranzusch PJ, Jain M, Pogliano J, Corbett KD Mol Cell. 2020 Jan 6. pii: S1097-2765(19)30923-2. doi:, 10.1016/j.molcel.2019.12.010. PMID:31932164[5] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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