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- PDB-6xm5: Structure of SARS-CoV-2 spike at pH 5.5, all RBDs down -

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Database: PDB / ID: 6xm5
TitleStructure of SARS-CoV-2 spike at pH 5.5, all RBDs down
ComponentsSpike glycoprotein
KeywordsVIRAL PROTEIN / SARS-CoV-2 spike / COVID19
Function / homology
Function and homology information

host cell endoplasmic reticulum-Golgi intermediate compartment membrane / host cell surface receptor binding / viral entry into host cell / fusion of virus membrane with host endosome membrane / viral envelope / virion attachment to host cell / pathogenesis / host cell plasma membrane / virion membrane / integral component of membrane
Spike glycoprotein S2, coronavirus / Spike receptor binding domain, betacoronavirus / Spike glycoprotein, heptad repeat 2, coronavirus / Spike receptor binding domain superfamily, coronavirus
Spike glycoprotein
Biological speciesSevere acute respiratory syndrome coronavirus 2
MethodELECTRON MICROSCOPY / single particle reconstruction / cryo EM / Resolution: 3.1 Å
AuthorsZhou, T. / Tsybovsky, Y. / Olia, A. / Kwong, P.D.
CitationJournal: bioRxiv / Year: 2020
Title: A pH-dependent switch mediates conformational masking of SARS-CoV-2 spike.
Abstract: SARS-CoV-2 has emerged as a global pathogen, sparking urgent vaccine development efforts with the trimeric spike. However, the inability of antibodies like CR3022, which binds a cryptic spike epitope ...SARS-CoV-2 has emerged as a global pathogen, sparking urgent vaccine development efforts with the trimeric spike. However, the inability of antibodies like CR3022, which binds a cryptic spike epitope with nanomolar affinity, to neutralize virus, suggests a spike-based means of neutralization escape. Here, we show the SARS-CoV-2 spike to have 10% the unfolding enthalpy of a globular protein at physiological pH, where it is recognized by antibodies like CR3022, and up to 10-times more unfolding enthalpy at endosomal pH, where it sheds such antibodies, suggesting that the spike evades potentially neutralizing antibody through a pH-dependent mechanism of conformational masking. To understand the compatibility of this mechanism with ACE2-receptor interactions, we carried out binding measurements and determined cryo-EM structures of the spike recognizing up to three ACE2 molecules at both physiological and endosomal pH. In the absence of ACE2, cryo-EM analyses indicated lower pH to reduce conformational heterogeneity. Single-receptor binding domain (RBD)-up conformations dominated at pH 5.5, resolving into a locked all-down conformation at lower pH through lowering of RBD and refolding of a pH-dependent switch. Notably, the emerging Asp614Gly strain partially destabilizes the switch that locks RBD down, thereby enhancing functional interactions with ACE2 while reducing evasion by conformational masking.
Validation Report
SummaryFull reportAbout validation report
DepositionJun 29, 2020Deposition site: RCSB / Processing site: RCSB
Revision 1.0Jul 29, 2020Provider: repository / Type: Initial release
Revision 1.1Aug 12, 2020Group: Database references / Structure summary / Category: chem_comp / citation / citation_author
Item: _chem_comp.pdbx_synonyms / _citation.title / _citation_author.name

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Deposited unit
A: Spike glycoprotein
B: Spike glycoprotein
C: Spike glycoprotein
hetero molecules

Theoretical massNumber of molelcules
Total (without water)435,39945

TypeNameSymmetry operationNumber
identity operation1_5551


#1: Protein Spike glycoprotein / S glycoprotein / E2 / Peplomer protein

Mass: 140952.531 Da / Num. of mol.: 3 / Mutation: R682G, R683S, R685S, K986P, V987P
Source method: isolated from a genetically manipulated source
Source: (gene. exp.) Severe acute respiratory syndrome coronavirus 2
Gene: S, 2 / Cell line (production host): Freestyle 293F / Production host: Homo sapiens (human) / References: UniProt: P0DTC2
#2: Polysaccharide




Type: oligosaccharide / Mass: 424.401 Da / Num. of mol.: 16
Source method: isolated from a genetically manipulated source
DGlcpNAcb1-4DGlcpNAcb1-Glycam Condensed SequenceGMML 1.0
WURCS=2.0/1,2,1/[a2122h-1b_1-5_2*NCC/3=O]/1-1/a4-b1WURCSPDB2Glycan 1.1.0
#3: Sugar...
ChemComp-NAG / 2-acetamido-2-deoxy-beta-D-glucopyranose / N-acetyl-beta-D-glucosamine / 2-acetamido-2-deoxy-beta-D-glucose / 2-acetamido-2-deoxy-D-glucose / 2-acetamido-2-deoxy-glucose / N-ACETYL-D-GLUCOSAMINE / N-Acetylglucosamine

Type: D-saccharide, beta linking / Mass: 221.208 Da / Num. of mol.: 26
Source method: isolated from a genetically manipulated source
Formula: C8H15NO6
N-acetyl-b-D-glucopyranosamineCOMMON NAMEGMML 1.0
Has ligand of interestN

Experimental details


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

Sample preparation

ComponentName: SARS-CoV-2 Spike / Type: COMPLEX / Entity ID: 1 / Source: RECOMBINANT
Molecular weightValue: 0.423 MDa / Experimental value: NO
Source (natural)Organism: Severe acute respiratory syndrome coronavirus 2
Source (recombinant)Organism: Homo sapiens (human) / Cell: Freestyle 293F
Buffer solutionpH: 5.5
Buffer componentConc.: 0.1 M / Name: Acetate
SpecimenConc.: 0.5 mg/ml / Embedding applied: NO / Shadowing applied: NO / Staining applied: NO / Vitrification applied: YES
Specimen supportGrid material: GOLD / Grid mesh size: 200 divisions/in. / Grid type: Quantifoil R2/2
VitrificationInstrument: FEI VITROBOT MARK IV / Cryogen name: ETHANE / Humidity: 95 % / Chamber temperature: 277 K

Electron microscopy imaging

Experimental equipment
Model: Titan Krios / Image courtesy: FEI Company
MicroscopyModel: FEI TITAN KRIOS
Electron gunElectron source: FIELD EMISSION GUN / Accelerating voltage: 300 kV / Illumination mode: FLOOD BEAM
Electron lensMode: BRIGHT FIELDBright-field microscopy / Nominal magnification: 105000 X / Nominal defocus max: 2500 nm / Nominal defocus min: 1250 nm / Cs: 2.7 mm / C2 aperture diameter: 100 µm
Image recordingAverage exposure time: 2.18 sec. / Electron dose: 40 e/Å2 / Film or detector model: GATAN K3 BIOQUANTUM (6k x 4k) / Num. of grids imaged: 1 / Num. of real images: 9816
EM imaging opticsEnergyfilter name: GIF Bioquantum / Energyfilter slit width: 20 eV
Image scansSampling size: 5 µm / Width: 5760 / Height: 4092


SoftwareName: PHENIX / Version: 1.14_3260: / Classification: refinement
EM software
2Latitudeimage acquisition
4CTFFIND4.1.13CTF correction
7UCSF Chimera1.14model fitting
8Coot0.8.9.2model fitting
10PHENIX1.14model refinement
11RELION3.0.8initial Euler assignment
12RELION3.0.8final Euler assignment
14RELION3.0.83D reconstruction
Particle selectionNum. of particles selected: 2113090 / Details: Automated picking
SymmetryPoint symmetry: C1 (asymmetric)
3D reconstructionResolution: 3.1 Å / Resolution method: FSC 0.143 CUT-OFF / Num. of particles: 108053 / Algorithm: FOURIER SPACE / Num. of class averages: 1 / Symmetry type: POINT
Atomic model buildingProtocol: OTHER / Space: REAL
Atomic model buildingPDB-ID: 6XM0
RefinementStereochemistry target values: CDL v1.2
Refine LS restraints
Refinement-IDTypeDev idealNumber
ELECTRON MICROSCOPYf_bond_d0.006326051
ELECTRON MICROSCOPYf_angle_d0.823535583
ELECTRON MICROSCOPYf_chiral_restr0.0564195
ELECTRON MICROSCOPYf_plane_restr0.00754538
ELECTRON MICROSCOPYf_dihedral_angle_d6.658215221

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