Spike-heparan sulfate interactions in SARS-CoV-2 infection

Abstract

Recent biochemical, biophysical, and genetic studies have shown that heparan sulfate, a major component of the cellular glycocalyx, participates in infection of SARS-CoV-2 by facilitating the so-called open conformation of the spike protein, which is required for binding to ACE2. This review highlights the involvement of heparan sulfate in the SARS-CoV-2 infection cycle and argues that there is a high degree of coordination between host cell heparan sulfate and asparagine-linked glycans on the spike in enabling ACE2 binding and subsequent infection. The discovery that spike protein binding and infection depends on both viral and host glycans provides insights into the evolution, spread and potential therapies for SARS-CoV-2 and its variants.

Graphical abstract

Figure 1

Cartoon illustration depicting SARS-CoV-2 viral attachment on a host cell via initial binding to heparan sulfate proteoglycans (HSPGs), followed by ternary complex formation between SARS-CoV-2 spike protein, HSPG and ACE2 (highlighted with a rectangular box).

Figure 2

Representative example of heparan sulfate (HS) structure, sulfation pattern and 3D spatial geometry. Top panel: The SNFG representation of an HS 18-mer connected to the tetrasaccharide (glucuronic acid-galactose-galactose-xylose) linker that is attached to HSPG core protein (HSPG core protein shown in purple) [19, 19]. A potential binding site for a fibroblast growth factor receptor and the high affinity binding site for antithrombin is indicated. Bottom panel: a molecular model, shown in stick form and in a biologically relevant conformation, reflecting the oligosaccharide in the top panel.

Figure 3

Spike/heparan sulfate binding. ACE2 and the SARS-CoV-2 spike protein are displayed with yellow and cyan surfaces, respectively. The RBD is depicted with a transparent cyan surface. Spike protein and ACE2 are surrounded by a glycan shield generated by N-linked glycans illustrated with blue and dark gold sticks, respectively. HS is depicted with a per-atom-colored, stick representation: oxygen, red; hydrogen, white; nitrogen, blue; sulfur, yellow. Carbon atoms of N-acetyl-d-glucosamine residues are colored in blue, and carbon atoms of l-iduronic acid residues are colored in brown. a–b, Top and side view of the SARS-CoV-2 spike protein bound to HS. c, Electrostatic potential projected onto the RBD HS-binding patch with heparin bound. The surface is colored from red (negative) to blue (positive), representing electrostatic potential values of −4 k_bT/e to +4 k_bT/e. d, HS contributes to the formation of a ternary complex with the SARS-CoV-2 spike and ACE2.

Figure 4

Interplay between N-glycans and heparan sulfate in priming the spike for infection. In all panels, ACE2 and the SARS-CoV-2 spike protein are displayed with yellow and cyan surfaces, respectively. The RBD is depicted with a transparent cyan surface. Glycans contributing to the glycan shield are illustrated with blue sticks. Hydrogen atoms have been hidden for clarity. a, The N-glycan at N343, highlighted in magenta, facilitates RBD opening, acting as a molecular crowbar. b, N-glycans at N165 and N234, depicted with orange and red sticks, respectively, stabilize the RBD in the “up” state, locking-and-loading the RBD for infection. c, N-glycans at N90 and N322 of ACE2, highlighted with yellow sticks, stabilize ACE2-RBD binding. d, HS, represented with per-atom-colored space filling spheres, modulate RBD opening and stabilization together with N-linked glycans (oxygen, red; hydrogen, white; nitrogen, blue; sulfur, yellow; carbons of N-acetyl-d-glucosamine residues, blue; and carbons of l-iduronic acid residues, brown).