1. Classification

SARS-related coronavirus is a member of the genus Betacoronavirus (group 2) and subgenus Sarbecovirus (subgroup B).^[12] Sarbecoviruses, unlike embecoviruses or alphacoronaviruses, have only one papain-like proteinase (PLpro) instead of two in the open reading frame ORF1.^[13] SARSr-CoV was determined to be an early split-off from the betacoronaviruses based on a set of conserved domains that it shares with the group.^[14][15]

Bats serve as the main host reservoir species for the SARS-related coronaviruses like SARS-CoV-1 and SARS-CoV-2. The virus has coevolved in the bat host reservoir over a long period of time.^[16] Only recently have strains of SARS-related coronavirus evolved and made the cross-species jump from bats to humans, as in the case of the strains SARS-CoV and SARS-CoV-2.^[17][4] Both of these strains descended from a single ancestor but made the cross-species jump into humans separately. SARS-CoV-2 is not a direct descendant of SARS-CoV.^[7]

2. Genome

Figure 1. Genome organization and viral proteins of SARS-CoV. By Xianchun Tang, Gang Li, Nikos Vasilakis, Yuan Zhang, Zhengli Shi, Yang Zhong, Lin-Fa Wang, and Shuyi Zhang - Tang, X., Li, G., Vasilakis, N. et al. Differential stepwise evolution of SARS coronavirus functional proteins in different host species. BMC Evolutionary Biology 9, 52 (2009). https://doi.org/10.1186/1471-2148-9-52https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2676248/, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=87776328

The SARS-related coronavirus is an enveloped, positive-sense, single-stranded RNA virus. Its genome is about 30 kb, which is one of the largest among RNA viruses. The virus has 14 open reading frames which overlap in some cases.^[18] The genome has the usual 5′ methylated cap and a 3′ polyadenylated tail.^[19] There are 265 nucleotides in the 5'UTR and 342 nucleotides in the 3'UTR.^[18]

The 5' methylated cap and 3' polyadenylated tail allows the positive-sense RNA genome to be directly translated by the host cell's ribosome on viral entry.^[20] SARSr-CoV is similar to other coronaviruses in that its genome expression starts with translation by the host cell's ribosomes of its initial two large overlapping open reading frames (ORFs), 1a and 1b, both of which produce polyproteins.^[18]

Table 1. Function of SARS-CoV genome proteins (orf1a to orf9b)

The functions of several of the viral proteins are known.^[24] ORFs 1a and 1b encode the replicase/transcriptase polyprotein, and later ORFs 2, 4, 5, and 9a encode, respectively, the four major structural proteins: spike, envelope, membrane, and nucleocapsid.^[25] The later ORFs also encode for eight unique proteins (orf3a to orf9b), known as the accessory proteins, many with no known homologues. The different functions of the accessory proteins are not well understood.^[24]

3. Morphology

 

Figure 2. Illustration of SARSr-CoV virion. By SPQR10 - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=88349537

The morphology of the SARS-related coronavirus is characteristic of the coronavirus family as a whole. The viruses are large pleomorphic spherical particles with bulbous surface projections that form a corona around the particles in electron micrographs.^[26] The size of the virus particles is in the 80–90 nm range. The envelope of the virus in electron micrographs appears as a distinct pair of electron dense shells.^[27]

The viral envelope consists of a lipid bilayer where the membrane (M), envelope (E) and spike (S) proteins are anchored.^[28] The spike proteins provide the virus with its bulbous surface projections. The spike protein's interaction with its complement host cell receptor is central in determining the tissue tropism, infectivity, and species range of the virus.^[29][30]

Inside the envelope, there is the nucleocapsid, which is formed from multiple copies of the nucleocapsid (N) protein, which are bound to the positive-sense single-stranded (~30 kb) RNA genome in a continuous beads-on-a-string type conformation.^[31][32] The lipid bilayer envelope, membrane proteins, and nucleocapsid protect the virus when it is outside the host.^[33]

4. Life Cycle

SARS-related coronavirus follows the replication strategy typical of all coronaviruses.^[19][34]

4.1. Attachment and entry

Figure 3. Coronavirus replication cycle. By Yvonne Xinyi Lim, Yan Ling Ng, James P. Tam, and Ding Xiang Liu - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5456285/, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=87764851

The attachment of the SARS-related coronavirus to the host cell is mediated by the spike protein and its receptor.^[35] The spike protein receptor binding domain (RBD) recognizes and attaches to the angiotensin-converting enzyme 2 (ACE2) receptor.^[4] Following attachment, the virus can enter the host cell by two different paths. The path the virus takes depends on the host protease available to cleave and activate the receptor-attached spike protein.^[36]

The first path the SARS coronavirus can take to enter the host cell is by endocytosis and uptake of the virus in an endosome. The receptor-attached spike protein is then activated by the host's pH-dependent cysteine protease cathepsin L. Activation of the receptor-attached spike protein causes a conformational change, and the subsequent fusion of the viral envelope with the endosomal wall.^[36]

Alternatively, the virus can enter the host cell directly by proteolytic cleavage of the receptor-attached spike protein by the host's TMPRSS2 or TMPRSS11D serine proteases at the cell surface.^[37][38] In the SARS coronavirus, the activation of the C-terminal part of the spike protein triggers the fusion of the viral envelope with the host cell membrane by inducing conformational changes which are not fully understood.^[39]

Table 2. Genome translation

After fusion the nucleocapsid passes into the cytoplasm, where the viral genome is released.^[35] The genome acts as a messenger RNA, and the cell's ribosome translates two-thirds of the genome, which corresponds to the open reading frame ORF1a and ORF1b, into two large overlapping polyproteins, pp1a and pp1ab.

The larger polyprotein pp1ab is a result of a -1 ribosomal frameshift caused by a slippery sequence (UUUAAAC) and a downstream RNA pseudoknot at the end of open reading frame ORF1a.^[41] The ribosomal frameshift allows for the continuous translation of ORF1a followed by ORF1b.^[42]

The polyproteins contain their own proteases, PLpro and 3CLpro, which cleave the polyproteins at different specific sites. The cleavage of polyprotein pp1ab yields 16 nonstructural proteins (nsp1 to nsp16). Product proteins include various replication proteins such as RNA-dependent RNA polymerase (RdRp), RNA helicase, and exoribonuclease (ExoN).^[42]

The two SARS-CoV-2 proteases (PLpro and 3CLpro) also interfere with the immune system response to the viral infection by cleaving three immune system proteins. PLpro cleaves IRF3 and 3CLpro cleaves both NLRP12 and TAB1. "Direct cleavage of IRF3 by NSP3 could explain the blunted Type-I IFN response seen during SARS-CoV-2 infections while NSP5 mediated cleavage of NLRP12 and TAB1 point to a molecular mechanism for enhanced production of IL-6 and inflammatory response observed in COVID-19 patients."^[43]

4.2. Replication and transcription

Figure 4. Model of the replicase-transcriptase complex of a coronavirus. RdRp for replication (red), ExoN for proofreading (dark blue), ExoN cofactor (yellow), RBPs to avoid secondary structure (light blue), RNA sliding clamp for processivity and primase domain for priming (green/orange), and a helicase to unwind RNA (downstream). By Everett Clinton Smith, Mark R. Denison - https://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1003760, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=87850815

A number of the nonstructural replication proteins coalesce to form a multi-protein replicase-transcriptase complex (RTC).^[42] The main replicase-transcriptase protein is the RNA-dependent RNA polymerase (RdRp). It is directly involved in the replication and transcription of RNA from an RNA strand. The other nonstructural proteins in the complex assist in the replication and transcription process.^[40]

The protein nsp14 is a 3'-5' exoribonuclease which provides extra fidelity to the replication process. The exoribonuclease provides a proofreading function to the complex which the RNA-dependent RNA polymerase lacks. Similarly, proteins nsp7 and nsp8 form a hexadecameric sliding clamp as part of the complex which greatly increases the processivity of the RNA-dependent RNA polymerase.^[40] The coronaviruses require the increased fidelity and processivity during RNA synthesis because of the relatively large genome size in comparison to other RNA viruses.^[44]

One of the main functions of the replicase-transcriptase complex is to transcribe the viral genome. RdRp directly mediates the synthesis of negative-sense subgenomic RNA molecules from the positive-sense genomic RNA. This is followed by the transcription of these negative-sense subgenomic RNA molecules to their corresponding positive-sense mRNAs.^[45]

The other important function of the replicase-transcriptase complex is to replicate the viral genome. RdRp directly mediates the synthesis of negative-sense genomic RNA from the positive-sense genomic RNA. This is followed by the replication of positive-sense genomic RNA from the negative-sense genomic RNA.^[45]

The replicated positive-sense genomic RNA becomes the genome of the progeny viruses. The various smaller mRNAs are transcripts from the last third of the virus genome which follows the reading frames ORF1a and ORF1b. These mRNAs are translated into the four structural proteins (S, E, M, and N) that will become part of the progeny virus particles and also eight other accessory proteins (orf3 to orf9b) which assist the virus.^[46]

4.3. Recombination

When two SARS-CoV genomes are present in a host cell, they may interact with each other to form recombinant genomes that can be transmitted to progeny viruses. Recombination likely occurs during genome replication when the RNA polymerase switches from one template to another (copy choice recombination).^[47] Human SARS-CoV appears to have had a complex history of recombination between ancestral coronaviruses that were hosted in several different animal groups.^[48][47]

4.4. Assembly and release

RNA translation occurs inside the endoplasmic reticulum. The viral structural proteins S, E and M move along the secretory pathway into the Golgi intermediate compartment. There, the M proteins direct most protein-protein interactions required for assembly of viruses following its binding to the nucleocapsid.^[49]

Progeny viruses are released from the host cell by exocytosis through secretory vesicles.^[49]