Glycosylation of SARS-CoV-2: structural and functional insights

Abstract

The COVID-19 pandemic is caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Similar to other coronaviruses, its particles are composed of four structural proteins: spike (S), envelope (E), membrane (M), and nucleocapsid (N) proteins. S, E, and M proteins are glycosylated, and the N protein is phosphorylated. The S protein is involved in the interaction with the host receptor human angiotensin-converting enzyme 2 (hACE2), which is also heavily glycosylated. Recent studies have revealed several other potential host receptors or factors that can increase or modulate the SARS-CoV-2 infection. Interestingly, most of these molecules bear carbohydrate residues. While glycans acquired by the viruses through the hijacking of the host machinery help the viruses in their infectivity, they also play roles in immune evasion or modulation. Glycans play complex roles in viral pathobiology, both on their own and in association with carrier biomolecules, such as proteins or glycosaminoglycans (GAGs). Understanding these roles in detail can help in developing suitable strategies for prevention and therapy of COVID-19. In this review, we sought to emphasize the interplay of SARS-CoV-2 glycosylated proteins and their host receptors in viral attachment, entry, replication, and infection. Moreover, the implications for future therapeutic interventions targeting these glycosylated biomolecules are also discussed in detail.

Keywords: Envelope protein glycosylation; Glycosaminoglycan SARS-CoV-2; Human ACE2 SARS-CoV-2; Membrane protein glycosylation; SARS-CoV-2 glycosylation; Spike protein glycosylation.

Fig. 1

Different processes during SARS-CoV-2 infection. Viral binding: the receptor-binding domain (RBD) of spike (S) protein interacts with the host cell surface receptors such as ACE2, GAGs, and other potential receptors; fusion: host proteases such as TMPRSS2, cathepsins, and furin cleave the S1 and S2 subunits, and the S2 subunit mediates the viral fusion; and entry: the virus enter the host cell by endocytosis or membrane fusion. Once inside the host cell, the RNA uses the host machinery to translate the viral proteins. The post-translational modifications happen on structural proteins by hijacking the host system and viral budding occurs at endoplasmic reticulum-Golgi intermediate compartments (ERGIC). Finally, the viral assembly occurs, and the virus is released by exocytosis

Fig. 2

The glycosylation of SARS-CoV-2 spike (S) protein. A 3D model of SARS-CoV-2 spike protein trimer showing the RBD region and glycosylation sites (only labelled on one monomer). B The site-specific N- and O-glycosylation of S protein [10, 11]

Fig. 3

Distribution of N- (A) and O- (B) glycosylation on specific sites of SARS-CoV-2 spike (S) protein expressed in HEK293 cells [10]

Fig. 4

3D model of human ACE2 showing the S protein binding region and the distribution of N- and O-glycosylation sites (A) and (B) are showing different orientations

Fig. 5

A, B Distribution of N- and O-glycosylation on specific sites of human ACE2 expressed in HEK293 cells [60]

Fig. 6

Kim et al. proposed model of SARS-CoV-2 host cell entry. A Virion binds to heparan sulfate. B Cell surface protease digests S protein, initiating viral-host cell membrane fusion via conformational change by host cell receptor binding to heparan sulfate and ACE2. C Virion enters host cell and experiences further proteolytic processing. Reprinted with permission from Elsevier [50]

Fig. 7

Clausen et al. proposed mechanism of SARS-CoV-2 viral attachment facilitated by host cell heparan sulfate. Reprinted with permission from Elsevier [69]

Fig. 8

Alignment of E protein amino acid sequences comparing SARS-CoV-2 to six other human coronaviruses. Gray boxes highlight predicted transmembrane segments. SARS-CoV-2 native predicted glycosylation acceptor sites are shown bolded, with + or – symbols depicting charge. Orange highlighted residues are conserved; yellow highlighted boxes display differences between SARS-CoV-2 and SARS-CoV-1. Reprinted with permission from Royal Society Publishing [95]