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- PDB-9w3u: Structure of Csm6 from Actinomyces procaprae -

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Basic information

Entry
Database: PDB / ID: 9w3u
TitleStructure of Csm6 from Actinomyces procaprae
ComponentsCsm6
KeywordsIMMUNE SYSTEM / Csm6 / Ancillary Nuclease / Cyclic Oligoadenylate / Type III CRISPR
Biological speciesActinomyces procaprae (bacteria)
MethodELECTRON MICROSCOPY / single particle reconstruction / cryo EM / Resolution: 2.53 Å
AuthorsLin, Z. / Gao, H. / Shi, R. / Yang, M. / Liu, Y.
Funding support China, 1items
OrganizationGrant numberCountry
National Natural Science Foundation of China (NSFC) China
CitationJournal: EMBO J / Year: 2026
Title: Mechanistic basis for selective Csm6-2 activation by cyclic penta-adenylate in a type III CRISPR-Cas system.
Authors: Ruyi Shi / Mengquan Yang / Yusong Liu / Haishan Gao / Zhonghui Lin /
Abstract: Type III CRISPR systems generate cyclic oligoadenylate (cOA, 3 to 6 AMPs) messengers upon detecting viral RNA, activating downstream effectors to defend against viral infection. Although cOA- ...Type III CRISPR systems generate cyclic oligoadenylate (cOA, 3 to 6 AMPs) messengers upon detecting viral RNA, activating downstream effectors to defend against viral infection. Although cOA-activated effectors have been extensively characterized, the effectors specific to cA5-one of the most abundant cOA species produced during phage infection-have remained unexplored. Here, we report that the CRISPR ribonuclease Csm6 (Csm6-2) from Actinomyces procaprae selectively employs cA5 as its activator. Csm6-2 utilizes its HEPN domain, rather than the CARF domain, to mediate self-limiting cleavage of cOA activators. Cryo-EM structural analyses reveal that Csm6-2 functions as a homotetramer, and disruption of tetramer formation significantly reduces its ribonuclease activity. Although cA6 and cA5 bind Csm6-2 with comparable affinity, only cA5 induces CARF domain closure, stabilizes the tetramer, and remodels the active site in the HEPN domain. In contrast, the sixth AMP of cA6 imposes significant steric hindrance on CARF domain movement, preventing its closure and subsequent allosteric activation. These findings expand our understanding of the cOA signaling diversity and specific cOA recognition mechanisms in type III CRISPR immunity.
History
DepositionJul 30, 2025Deposition site: PDBJ / Processing site: PDBC
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Structure visualization

Structure viewerMolecule:
MolmilJmol/JSmol

Downloads & links

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Assembly

Deposited unit
A: Csm6
B: Csm6
C: Csm6
D: Csm6


Theoretical massNumber of molelcules
Total (without water)342,1504
Polymers342,1504
Non-polymers00
Water00
1


  • Idetical with deposited unit
  • defined by author
  • Evidence: electron microscopy, not applicable
TypeNameSymmetry operationNumber
identity operation1_5551

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Components

#1: Protein
Csm6


Mass: 85537.391 Da / Num. of mol.: 4
Source method: isolated from a genetically manipulated source
Source: (gene. exp.) Actinomyces procaprae (bacteria) / Production host: Escherichia coli (E. coli)
Has protein modificationN

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Experimental details

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Experiment

ExperimentMethod: ELECTRON MICROSCOPY
EM experimentAggregation state: PARTICLE / 3D reconstruction method: single particle reconstruction

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Sample preparation

ComponentName: Csm6 / Type: COMPLEX / Entity ID: all / Source: RECOMBINANT
Molecular weightExperimental value: NO
Source (natural)Organism: Actinomyces procaprae (bacteria)
Source (recombinant)Organism: Escherichia coli (E. coli)
Buffer solutionpH: 7.5
Buffer component
IDConc.NameFormulaBuffer-ID
1150 mMsodium chlorideNaCl1
250 mMHydroxymethyl Aminomethane HydrochlorideTris-HCl1
SpecimenEmbedding applied: NO / Shadowing applied: NO / Staining applied: NO / Vitrification applied: YES
VitrificationCryogen name: ETHANE

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Electron microscopy imaging

MicroscopyModel: TFS TITAN THEMIS
Electron gunElectron source: FIELD EMISSION GUN / Accelerating voltage: 300 kV / Illumination mode: FLOOD BEAM
Electron lensMode: BRIGHT FIELD / Nominal defocus max: 2000 nm / Nominal defocus min: 1000 nm
Image recordingElectron dose: 50 e/Å2 / Film or detector model: FEI FALCON IV (4k x 4k)

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Processing

EM software
IDNameVersionCategory
1cryoSPARCparticle selection
2PHENIX1.21.2_5419:model refinement
13cryoSPARC3D reconstruction
CTF correctionType: NONE
3D reconstructionResolution: 2.53 Å / Resolution method: FSC 0.143 CUT-OFF / Num. of particles: 667978 / Symmetry type: POINT

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