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- PDB-9xko: High-resolution cryo-EM structure of Maltose Binding Protein -

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Basic information

Entry
Database: PDB / ID: 9xko
TitleHigh-resolution cryo-EM structure of Maltose Binding Protein
ComponentsMaltose/maltodextrin-binding periplasmic protein
KeywordsSUGAR BINDING PROTEIN / small protein-ligand complex
Function / homology
Function and homology information


detection of maltose stimulus / maltose transport complex / carbohydrate transport / carbohydrate transmembrane transporter activity / maltose binding / maltose transport / maltodextrin transmembrane transport / ATP-binding cassette (ABC) transporter complex, substrate-binding subunit-containing / ATP-binding cassette (ABC) transporter complex / cell chemotaxis ...detection of maltose stimulus / maltose transport complex / carbohydrate transport / carbohydrate transmembrane transporter activity / maltose binding / maltose transport / maltodextrin transmembrane transport / ATP-binding cassette (ABC) transporter complex, substrate-binding subunit-containing / ATP-binding cassette (ABC) transporter complex / cell chemotaxis / outer membrane-bounded periplasmic space / periplasmic space / DNA damage response / membrane
Similarity search - Function
Maltose/Cyclodextrin ABC transporter, substrate-binding protein / Solute-binding family 1, conserved site / Bacterial extracellular solute-binding proteins, family 1 signature. / Bacterial extracellular solute-binding protein / Bacterial extracellular solute-binding protein
Similarity search - Domain/homology
alpha-maltose / Maltose/maltodextrin-binding periplasmic protein
Similarity search - Component
Biological speciesEscherichia coli (E. coli)
MethodELECTRON MICROSCOPY / single particle reconstruction / cryo EM / Resolution: 2.35 Å
AuthorsPark, K. / Yoo, Y. / Jeon, H. / Choi, K. / Kwon, E. / Lim, H. / Kim, D.Y. / No, K.T.
Funding support1items
OrganizationGrant numberCountry
Not funded
CitationJournal: Nat Commun / Year: 2026
Title: High-resolution cryo-EM structures of small protein-ligand complexes near the theoretical size limit.
Authors: Kunwoong Park / Youngki Yoo / Hyunbum Jeon / Kiju Choi / Hanseong Kim / Eunju Kwon / Hyun-Ho Lim / Dong Young Kim / Kyoung Tai No /
Abstract: Cryo-electron microscopy (cryo-EM) is a widely used technique for determining macromolecular structures at near-atomic resolution. The theoretical lower limit of particle sizes suitable for cryo-EM ...Cryo-electron microscopy (cryo-EM) is a widely used technique for determining macromolecular structures at near-atomic resolution. The theoretical lower limit of particle sizes suitable for cryo-EM structural analysis is estimated to be 38 kDa; typical constraints involve factors such as image contrast and particle alignment accuracy. In this study, we present cryo-EM structures of two protein-ligand complexes near this lower size threshold. First, the structure of the maltose-binding protein complexed with maltose, with a structurally ordered mass of 40.8 kDa, was determined at a resolution of 2.4 Å; both the maltose and water molecules were clearly identified in this structure. The second structure was the kinase domain of human PLK1 complexed with onvansertib, with a structurally ordered mass of 31.6 kDa, below the theoretical 38 kDa limit; this domain was determined at a resolution of 3.4 Å using a gold-supported grid in the presence of β-octyl-glucoside. The density map clearly shows the backbone of PLK1 secondary structure, and the onvansertib. These results demonstrate that cryo-EM can be effectively employed to determine structures of small proteins or domains, and to perform structure-based drug screening for small proteins, without requiring structural fiducials for particle alignment.
History
DepositionNov 6, 2025Deposition site: PDBJ / Processing site: PDBJ
Revision 1.0Apr 29, 2026Provider: repository / Type: Initial release
Revision 1.0Apr 29, 2026Data content type: EM metadata / Data content type: EM metadata / Provider: repository / Type: Initial release
Revision 1.0Apr 29, 2026Data content type: FSC / Data content type: FSC / Provider: repository / Type: Initial release
Revision 1.0Apr 29, 2026Data content type: Half map / Part number: 1 / Data content type: Half map / Provider: repository / Type: Initial release
Revision 1.0Apr 29, 2026Data content type: Half map / Part number: 2 / Data content type: Half map / Provider: repository / Type: Initial release
Revision 1.0Apr 29, 2026Data content type: Image / Data content type: Image / Provider: repository / Type: Initial release
Revision 1.0Apr 29, 2026Data content type: Mask / Part number: 1 / Data content type: Mask / Provider: repository / Type: Initial release
Revision 1.0Apr 29, 2026Data content type: Primary map / Data content type: Primary map / Provider: repository / Type: Initial release

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Structure visualization

Structure viewerMolecule:
MolmilJmol/JSmol

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Assembly

Deposited unit
A: Maltose/maltodextrin-binding periplasmic protein
hetero molecules


Theoretical massNumber of molelcules
Total (without water)44,1622
Polymers43,8191
Non-polymers3421
Water3,729207
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 Maltose/maltodextrin-binding periplasmic protein / MMBP / Maltodextrin-binding protein / Maltose-binding protein / MBP


Mass: 43819.270 Da / Num. of mol.: 1
Source method: isolated from a genetically manipulated source
Details: A313V substitution reflects the MBP sequence encoded in the pMAL-c5X expression vector backbone
Source: (gene. exp.) Escherichia coli (E. coli) / Gene: malE, b4034, JW3994 / Production host: Escherichia coli (E. coli) / References: UniProt: P0AEX9
#2: Polysaccharide alpha-D-glucopyranose-(1-4)-alpha-D-glucopyranose


Type: oligosaccharide, Oligosaccharide / Class: Nutrient / Mass: 342.297 Da / Num. of mol.: 1
Source method: isolated from a genetically manipulated source
Details: oligosaccharide / References: alpha-maltose
DescriptorTypeProgram
DGlcpa1-4DGlcpa1-ROHGlycam Condensed SequenceGMML 1.0
WURCS=2.0/1,2,1/[a2122h-1a_1-5]/1-1/a4-b1WURCSPDB2Glycan 1.1.0
[][D-1-deoxy-Glcp]{[(4+1)][a-D-Glcp]{}}LINUCSPDB-CARE
#3: Water ChemComp-HOH / water


Mass: 18.015 Da / Num. of mol.: 207 / Source method: isolated from a natural source / Formula: H2O
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: High-resolution cryo-EM structure of Maltosee Binding Protein
Type: COMPLEX / Entity ID: #1 / Source: RECOMBINANT
Molecular weightExperimental value: NO
Source (natural)Organism: Escherichia coli (E. coli)
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: BRIGHT FIELD / Nominal defocus max: 1800 nm / Nominal defocus min: 400 nm / Alignment procedure: ZEMLIN TABLEAU
Specimen holderCryogen: NITROGEN
Image recordingElectron dose: 60 e/Å2 / Film or detector model: FEI FALCON IV (4k x 4k)

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Processing

EM software
IDNameVersionCategory
1cryoSPARCparticle selection
7PHENIX1.20.1model fitting
9PHENIX1.20.1model refinement
10Coot0.9.8.1model refinement
14cryoSPARC3D reconstruction
CTF correctionType: PHASE FLIPPING ONLY
Particle selectionNum. of particles selected: 8352733
3D reconstructionResolution: 2.35 Å / Resolution method: FSC 0.143 CUT-OFF / Num. of particles: 509884 / Symmetry type: POINT
Atomic model buildingProtocol: FLEXIBLE FIT
Atomic model buildingPDB-ID: 1FQC

1fqc


Accession code: 1FQC / Source name: PDB / Type: experimental model
RefinementHighest resolution: 2.35 Å
Stereochemistry target values: REAL-SPACE (WEIGHTED MAP SUM AT ATOM CENTERS)
Refine LS restraints
Refine-IDTypeDev idealNumber
ELECTRON MICROSCOPYf_bond_d0.0022945
ELECTRON MICROSCOPYf_angle_d0.4913999
ELECTRON MICROSCOPYf_dihedral_angle_d4.522389
ELECTRON MICROSCOPYf_chiral_restr0.04440
ELECTRON MICROSCOPYf_plane_restr0.004515

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