Cryo-EM structure of the 2019-nCoV spike in the prefusion conformation

Abstract

The outbreak of a novel coronavirus (2019-nCoV) represents a pandemic threat that has been declared a public health emergency of international concern. The CoV spike (S) glycoprotein is a key target for vaccines, therapeutic antibodies, and diagnostics. To facilitate medical countermeasure development, we determined a 3.5-angstrom-resolution cryo-electron microscopy structure of the 2019-nCoV S trimer in the prefusion conformation. The predominant state of the trimer has one of the three receptor-binding domains (RBDs) rotated up in a receptor-accessible conformation. We also provide biophysical and structural evidence that the 2019-nCoV S protein binds angiotensin-converting enzyme 2 (ACE2) with higher affinity than does severe acute respiratory syndrome (SARS)-CoV S. Additionally, we tested several published SARS-CoV RBD-specific monoclonal antibodies and found that they do not have appreciable binding to 2019-nCoV S, suggesting that antibody cross-reactivity may be limited between the two RBDs. The structure of 2019-nCoV S should enable the rapid development and evaluation of medical countermeasures to address the ongoing public health crisis.

Fig. 1. Structure of 2019-nCoV S in the prefusion conformation.

(A) Schematic of 2019-nCoV S primary structure colored by domain. Domains that were excluded from the ectodomain expression construct or could not be visualized in the final map are colored white. SS, signal sequence; S2′, S2′ protease cleavage site; FP, fusion peptide; HR1, heptad repeat 1; CH, central helix; CD, connector domain; HR2, heptad repeat 2; TM, transmembrane domain; CT, cytoplasmic tail. Arrows denote protease cleavage sites. (B) Side and top views of the prefusion structure of the 2019-nCoV S protein with a single RBD in the up conformation. The two RBD down protomers are shown as cryo-EM density in either white or gray and the RBD up protomer is shown in ribbons colored corresponding to the schematic in (A).

Fig. 2. Structural comparison between 2019-nCoV S and SARS-CoV S.

(A) Single protomer of 2019-nCoV S with the RBD in the down conformation (left) is shown in ribbons colored according to Fig. 1. A protomer of 2019-nCoV S in the RBD up conformation is shown (center) next to a protomer of SARS-CoV S in the RBD up conformation (right), displayed as ribbons and colored white (PDB ID: 6CRZ). (B) RBDs of 2019-nCoV and SARS-CoV aligned based on the position of the adjacent NTD from the neighboring protomer. The 2019-nCoV RBD is colored green and the SARS-CoV RBD is colored white. The 2019-nCoV NTD is colored blue. (C) Structural domains from 2019-nCoV S have been aligned to their counterparts from SARS-CoV S as follows: NTD (top left), RBD (top right), SD1 and SD2 (bottom left), and S2 (bottom right).

Fig. 3. 2019-nCoV S binds human ACE2 with high affinity.

(A) Surface plasmon resonance sensorgram showing the binding kinetics for human ACE2 and immobilized 2019-nCoV S. Data are shown as black lines, and the best fit of the data to a 1:1 binding model is shown in red. (B) Negative-stain EM 2D class averages of 2019-nCoV S bound by ACE2. Averages have been rotated so that ACE2 is positioned above the 2019-nCoV S protein with respect to the viral membrane. A diagram depicting the ACE2-bound 2019-nCoV S protein is shown (right) with ACE2 in blue and S protein protomers colored tan, pink, and green.

Fig. 4. Antigenicity of the 2019-nCoV RBD.

(A) SARS-CoV RBD shown as a white molecular surface (PDB ID: 2AJF), with residues that vary in the 2019-nCoV RBD colored red. The ACE2-binding site is outlined with a black dashed line. (B) Biolayer interferometry sensorgram showing binding to ACE2 by the 2019-nCoV RBD-SD1. Binding data are shown as a black line, and the best fit of the data to a 1:1 binding model is shown in red. (C) Biolayer interferometry to measure cross-reactivity of the SARS-CoV RBD-directed antibodies S230, m396, and 80R. Sensor tips with immobilized antibodies were dipped into wells containing 2019-nCoV RBD-SD1, and the resulting data are shown as a black line.