8hj4 | pdb_00008hj4
CryoEM structure of an anti-CRISPR protein AcrIIC5 bound to Nme1Cas9-sgRNA complex
Structural highlights
FunctionCAS9_NEIM8 CRISPR (clustered regularly interspaced short palindromic repeat) is an adaptive immune system that provides protection against mobile genetic elements (viruses, transposable elements and conjugative plasmids). CRISPR clusters contain spacers, sequences complementary to antecedent mobile elements, and target invading nucleic acids. CRISPR clusters are transcribed and processed into CRISPR RNA (crRNA). In type II CRISPR systems correct processing of pre-crRNA requires a trans-encoded small RNA (tracrRNA), endogenous ribonuclease 3 (rnc) and this protein, although RNase 3 is not required for 5'-processing of crRNA in this strain. Cas9/crRNA/tracrRNA endonucleolytically cleaves linear or circular dsDNA target complementary to the spacer; Cas9 is inactive in the absence of the 2 guide RNAs (gRNA, PubMed:23940360). Cas9 recognizes the protospacer adjacent motif (PAM) in the CRISPR repeat sequences to help distinguish self versus nonself, as targets within the bacterial CRISPR locus do not have PAMs. PAM recognition is also required for catalytic activity. Plasmids containing sequences homologous to endogenous spacer elements and that have flanking PAM consensus sequences cannot transform this strain unless the cas9 gene is disrupted or critical residues of Cas9 are mutated.[1] [2] Publication Abstract from PubMedAnti-CRISPR proteins are encoded by phages to inhibit the CRISPR-Cas systems of the hosts. AcrIIC5 inhibits several naturally high-fidelity type II-C Cas9 enzymes, including orthologs from Neisseria meningitidis (Nme1Cas9) and Simonsiella muelleri (SmuCas9). Here, we solve the structure of AcrIIC5 in complex with Nme1Cas9 and sgRNA. We show that AcrIIC5 adopts a novel fold to mimic the size and charge distribution of double-stranded DNA, and uses its negatively charged grooves to bind and occlude the protospacer adjacent motif (PAM) binding site in the target DNA cleft of Cas9. AcrIIC5 is positioned into the crevice between the WED and PI domains of Cas9, and one end of the anti-CRISPR interacts with the phosphate lock loop and a linker between the RuvC and BH domains. We employ biochemical and mutational analyses to build a model for AcrIIC5's mechanism of action, and identify residues on both the anti-CRISPR and Cas9 that are important for their interaction and inhibition. Together, the structure and mechanism of AcrIIC5 reveal convergent evolution among disparate anti-CRISPR proteins that use a DNA-mimic strategy to inhibit diverse CRISPR-Cas surveillance complexes, and provide new insights into a tool for potent inhibition of type II-C Cas9 orthologs. Anti-CRISPR AcrIIC5 is a dsDNA mimic that inhibits type II-C Cas9 effectors by blocking PAM recognition.,Sun W, Zhao X, Wang J, Yang X, Cheng Z, Liu S, Wang J, Sheng G, Wang Y Nucleic Acids Res. 2023 Feb 28;51(4):1984-1995. doi: 10.1093/nar/gkad052. PMID:36744495[3] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. See AlsoReferences
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