Coronavirus biology and replication: implications for SARS-CoV-2

Abstract

The SARS-CoV-2 pandemic and its unprecedented global societal and economic disruptive impact has marked the third zoonotic introduction of a highly pathogenic coronavirus into the human population. Although the previous coronavirus SARS-CoV and MERS-CoV epidemics raised awareness of the need for clinically available therapeutic or preventive interventions, to date, no treatments with proven efficacy are available. The development of effective intervention strategies relies on the knowledge of molecular and cellular mechanisms of coronavirus infections, which highlights the significance of studying virus-host interactions at the molecular level to identify targets for antiviral intervention and to elucidate critical viral and host determinants that are decisive for the development of severe disease. In this Review, we summarize the first discoveries that shape our current understanding of SARS-CoV-2 infection throughout the intracellular viral life cycle and relate that to our knowledge of coronavirus biology. The elucidation of similarities and differences between SARS-CoV-2 and other coronaviruses will support future preparedness and strategies to combat coronavirus infections.

Fig. 1. The coronavirus virion and life cycle.

a | The coronavirus virion consists of structural proteins, namely spike (S), envelope (E), membrane (M), nucleocapsid (N) and, for some betacoronaviruses, haemagglutinin-esterase (not shown). The positive-sense, single-stranded RNA genome (+ssRNA) is encapsidated by N, whereas M and E ensure its incorporation in the viral particle during the assembly process. S trimers protrude from the host-derived viral envelope and provide specificity for cellular entry receptors. b | Coronavirus particles bind to cellular attachment factors and specific S interactions with the cellular receptors (such as angiotensin-converting enzyme 2 (ACE2)), together with host factors (such as the cell surface serine protease TMPRSS2), promote viral uptake and fusion at the cellular or endosomal membrane. Following entry, the release and uncoating of the incoming genomic RNA subject it to the immediate translation of two large open reading frames, ORF1a and ORF1b. The resulting polyproteins pp1a and pp1ab are co-translationally and post-translationally processed into the individual non-structural proteins (nsps) that form the viral replication and transcription complex. Concordant with the expression of nsps, the biogenesis of viral replication organelles consisting of characteristic perinuclear double-membrane vesicles (DMVs), convoluted membranes (CMs) and small open double-membrane spherules (DMSs) create a protective microenvironment for viral genomic RNA replication and transcription of subgenomic mRNAs (sg mRNAs) comprising the characteristic nested set of coronavirus mRNAs. Translated structural proteins translocate into endoplasmic reticulum (ER) membranes and transit through the ER-to-Golgi intermediate compartment (ERGIC), where interaction with N-encapsidated, newly produced genomic RNA results in budding into the lumen of secretory vesicular compartments. Finally, virions are secreted from the infected cell by exocytosis. Key steps inhibited by compounds that are currently being validated and which represent attractive antiviral targets are highlighted in red. An, 3′ polyA sequence; cap, 5′ cap structure; dsRNA, double-stranded RNA; L, leader sequence; RdRP, RNA-dependent RNA polymerase.

Fig. 2. Severe acute respiratory syndrome-related coronavirus spike sequence variation.

a | Schematic illustration of coronavirus spike, indicating domain 1 and domain 2. The receptor-binding motif (RBM) is located on S1 and the fusion peptide (FP), heptad repeat 1 (HR1), HR2 and the transmembrane (TM) domains are located on S2. The cleavage sites are indicated. The colour code designates conserved spike regions surrounding the angiotensin-converting enzyme 2 (ACE2)-binding domain among severe acute respiratory syndrome-related coronaviruses (SARSr-CoVs) and high amino acid sequence variations within the site of receptor interaction. b | Amino acid alignment of human SARS-CoV-2 (Wuhan-Hu-1) and SARS-CoV (Frankfurt-1), bat (RaTG13, RmYN02, CoVZC45 and CoVZXC21) and pangolin (MP789, P1E) SARSr-CoVs. The spike gene sequence alignment was performed using MUSCLE and using the default settings and codon alignment, then translated into amino acids using MEGA7, version 7.0.26. The alignment was coloured according to percentage amino acid similarity with a Blosum 62 score matrix. The colour code designates conserved spike regions surrounding the ACE2-binding domain among SARSr-CoVs and high amino acid sequence variations within the site of receptor interaction. The insertion of a polybasic cleavage site (PRRAR, amino acids 681 to 685) in Wuhan-Hu-1 is indicated, and similar insertions are depicted in bat SARSr-CoV RmYN02. c | Within the spike sequence, the ACE2 receptor-binding motif (amino acids 437 to 509, black line) is depicted. The spike contact residues for ACE2 interaction are marked with asterisks.

Fig. 3. Coronavirus polyprotein processing and non-structural proteins.

Coronavirus polyprotein processing and domains of non-structural proteins (nsp) are illustrated for severe acute respiratory syndrome-related coronaviruses. Proteolytic cleavage of the polyproteins pp1a and pp1ab is facilitated by viral proteases residing in nsp3 (PL^pro) and nsp5 (M^pro). PL^pro proteolytically releases nsp1, nsp2, nsp3 and the amino terminus of nsp4 from the polyproteins pp1a and pp1ab (indicated by the blue arrows). M^pro proteolytically releases nsp5–16 and the carboxy terminus of nsp4 from the polyproteins pp1a and pp1ab (indicated by the red arrows). Conserved domains and known functions are schematically depicted for nsp1–16 (refs^,,,). DMV, double-membrane vesicle; DPUP, Domain Preceding Ubl2 and PL^pro; EndoU, endoribonuclease; ExoN, exoribonuclease; HEL, helicase; Mac I–III, macrodomains 1–3; M^pro, main protease; NiRAN, nidovirus RdRP-associated nucleotidyltransferase; NMT, guanosine N7-methyltransferase; OMT, ribose 2′-O-methyltransferase; PL^pro, papain-like protease; Pr, primase or 3′-terminal adenylyl-transferase; RdRP, RNA-dependent RNA polymerase; TM, transmembrane domains; Ubl, ubiquitin-like domain; Y, Y and CoV-Y domain; ZBD, zinc-binding domain.

Fig. 4. Coronavirus replication and discontinuous transcription.

Schematic depiction of coronaviral RNA synthesis. Full-length positive-sense genomic RNA is used as a template to produce both full-length negative-sense copies for genome replication and subgenomic negative-sense RNAs (–sgRNA) to produce the subgenomic mRNAs (sg mRNA). The negative strand RNA synthesis involving a template switch from a body transcription regulatory sequences (TRS-B) to the leader TRS (TRS-L) is illustrated to produce one sg mRNA. This process can take place at any TRS-B and will collectively result in the production of the characteristic nested set of coronaviral mRNAs.