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- PDB-9liq: CryoEM Structures Uncover the Unexpected Hinges of IscB for Enhan... -

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Basic information

Entry
Database: PDB / ID: 9liq
TitleCryoEM Structures Uncover the Unexpected Hinges of IscB for Enhanced Gene Editing
Components
  • DNA (5'-D(P*GP*GP*CP*C)-3')
  • Iscb
  • RNA (190-MER)
KeywordsRNA BINDING PROTEIN / Iscb / HNH
Function / homologyDNA / RNA / RNA (> 10) / RNA (> 100)
Function and homology information
Biological speciessynthetic construct (others)
MethodELECTRON MICROSCOPY / single particle reconstruction / cryo EM / Resolution: 3.04 Å
AuthorsHu, C.Y. / Wang, F.Z. / Ma, S.S. / Zhang, S.F.
Funding support Singapore, 1items
OrganizationGrant numberCountry
Ministry of Education (MoE, Singapore) Singapore
CitationJournal: Nat Struct Mol Biol / Year: 2026
Title: Structural insight into IscB's RNA-lid-based inactivation mechanism.
Authors: Feizuo Wang / Ruochen Guo / Senfeng Zhang / Yinuo Cui / Junlan Wang / Tao Hu / Kunming Liu / Qi Wang / Yao Liu / Ki Hyun Nam / Ziqing Winston Zhao / Quanquan Ji / Xin Xu / Ercheng Wang / ...Authors: Feizuo Wang / Ruochen Guo / Senfeng Zhang / Yinuo Cui / Junlan Wang / Tao Hu / Kunming Liu / Qi Wang / Yao Liu / Ki Hyun Nam / Ziqing Winston Zhao / Quanquan Ji / Xin Xu / Ercheng Wang / Youyuan Zhu / Yao Yang / Min Luo / Peixiang Ma / Shengsheng Ma / Chunlong Xu / Chunyi Hu /
Abstract: IscB, a compact Cas9 ancestor from the obligate mobile element guided activity system, has attracted growing interest as a programmable genome editor because of its small size and therapeutic ...IscB, a compact Cas9 ancestor from the obligate mobile element guided activity system, has attracted growing interest as a programmable genome editor because of its small size and therapeutic delivery potential. Despite its promise, structural insights into IscB's regulation remain limited, with only a target-bound R-loop structure previously reported. Here, we present the structural trajectory of an engineered IscB, capturing its transition from a resting state to activation. Using cryo-electron microscopy, we resolve four high-resolution structures: the apo resting state, two intermediate complexes with 6-nt and 10-nt guide-target pairing and a fully paired 16-nt primed cleavage state. These structures uncover a dual inactivation mechanism mediated by RNA lids; the ωRNA lid blocks HNH domain access, while the guide RNA lid occludes the RuvC active site. As guide-target pairing progresses, the guide RNA undergoes a stepwise displacement, mimicking a 'car pedal' motion that triggers activation at 11-nt pairing. The HNH domain also contributes to R-loop stabilization through a positively charged R-wedge motif and undergoes a ~90° activation-driven rotation mediated by two hinge regions. In variants IscBHig1 and IscBHig2, engineering these hinge motifs to enhance conformational flexibility notably improved genome-editing efficiency in cells. In summary, our study reveals the molecular basis underlying IscB autoinhibition and activation, identifies previously uncharacterized regulatory features and establishes hinge elements as a target region for engineering compact, efficient genome editors.
History
DepositionJan 14, 2025Deposition site: PDBJ / Processing site: PDBJ
Revision 1.0Jan 21, 2026Provider: repository / Type: Initial release
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Structure visualization

Structure viewerMolecule:
MolmilJmol/JSmol

Downloads & links

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Assembly

Deposited unit
A: Iscb
B: RNA (190-MER)
C: DNA (5'-D(P*GP*GP*CP*C)-3')
hetero molecules


Theoretical massNumber of molelcules
Total (without water)124,8197
Polymers124,6813
Non-polymers1384
Water00
1


  • Idetical with deposited unit
  • defined by author&software
  • Evidence: electron microscopy, not applicable
TypeNameSymmetry operationNumber
identity operation1_555x,y,z1

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Components

#1: Protein Iscb


Mass: 62165.832 Da / Num. of mol.: 1
Source method: isolated from a genetically manipulated source
Source: (gene. exp.) synthetic construct (others) / Production host: Escherichia coli (E. coli)
#2: RNA chain RNA (190-MER)


Mass: 61323.492 Da / Num. of mol.: 1
Source method: isolated from a genetically manipulated source
Source: (gene. exp.) synthetic construct (others) / Production host: Escherichia coli (E. coli)
#3: DNA chain DNA (5'-D(P*GP*GP*CP*C)-3')


Mass: 1191.818 Da / Num. of mol.: 1
Source method: isolated from a genetically manipulated source
Source: (gene. exp.) synthetic construct (others) / Production host: synthetic construct (others)
#4: Chemical ChemComp-ZN / ZINC ION


Mass: 65.409 Da / Num. of mol.: 1 / Source method: obtained synthetically / Formula: Zn / Feature type: SUBJECT OF INVESTIGATION
#5: Chemical ChemComp-MG / MAGNESIUM ION


Mass: 24.305 Da / Num. of mol.: 3 / Source method: obtained synthetically / Formula: Mg / Feature type: SUBJECT OF INVESTIGATION
Has ligand of interestY
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: The binary complex of ISCB and wRNA / Type: COMPLEX / Entity ID: #1-#3 / Source: RECOMBINANT
Source (natural)Organism: synthetic construct (others)
Source (recombinant)Organism: Escherichia coli (E. coli)
Buffer solutionpH: 7.5
SpecimenEmbedding applied: NO / Shadowing applied: NO / Staining applied: NO / Vitrification applied: YES
VitrificationCryogen name: ETHANE / Humidity: 100 %

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Electron microscopy imaging

Experimental equipment
Model: Titan Krios / Image courtesy: FEI Company
MicroscopyModel: TFS KRIOS
Electron gunElectron source: FIELD EMISSION GUN / Accelerating voltage: 300 kV / Illumination mode: FLOOD BEAM
Electron lensMode: DIFFRACTION / Nominal defocus max: 1500 nm / Nominal defocus min: 800 nm
Image recordingElectron dose: 48.84 e/Å2 / Film or detector model: GATAN K3 (6k x 4k)

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Processing

EM software
IDNameVersionCategory
1cryoSPARCparticle selection
2PHENIX1.20.1_4487model refinement
13cryoSPARC3D reconstruction
CTF correctionType: PHASE FLIPPING AND AMPLITUDE CORRECTION
3D reconstructionResolution: 3.04 Å / Resolution method: FSC 0.143 CUT-OFF / Num. of particles: 100520 / Symmetry type: POINT
RefinementHighest resolution: 3.04 Å
Stereochemistry target values: REAL-SPACE (WEIGHTED MAP SUM AT ATOM CENTERS)
Refine LS restraints
Refine-IDTypeDev idealNumber
ELECTRON MICROSCOPYf_bond_d0.0048702
ELECTRON MICROSCOPYf_angle_d0.74712684
ELECTRON MICROSCOPYf_dihedral_angle_d23.1643388
ELECTRON MICROSCOPYf_chiral_restr0.041545
ELECTRON MICROSCOPYf_plane_restr0.006902

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