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.

Most human coronaviruses cause mild upper respiratory tract disease but may be associated with more severe pulmonary disease in immunocompromised individuals. However, SARS coronavirus caused severe lower respiratory disease with nearly 10% mortality and evidence of systemic spread. Recently, another coronavirus (human coronavirus-Erasmus Medical Center (hCoV-EMC)) was identified in patients with severe and sometimes lethal lower respiratory tract infection. Viral genome analysis revealed close relatedness to coronaviruses found in bats. Here we identify dipeptidyl peptidase 4 (DPP4; also known as CD26) as a functional receptor for hCoV-EMC. DPP4 specifically co-purified with the receptor-binding S1 domain of the hCoV-EMC spike protein from lysates of susceptible Huh-7 cells. Antibodies directed against DPP4 inhibited hCoV-EMC infection of primary human bronchial epithelial cells and Huh-7 cells. Expression of human and bat (Pipistrellus pipistrellus) DPP4 in non-susceptible COS-7 cells enabled infection by hCoV-EMC. The use of the evolutionarily conserved DPP4 protein from different species as a functional receptor provides clues about the host range potential of hCoV-EMC. In addition, it will contribute critically to our understanding of the pathogenesis and epidemiology of this emerging human coronavirus, and may facilitate the development of intervention strategies.

The genome organization and expression strategy of the newly identified severe acute respiratory syndrome coronavirus (SARS-CoV) were predicted using recently published genome sequences. Fourteen putative open reading frames were identified, 12 of which were predicted to be expressed from a nested set of eight subgenomic mRNAs. The synthesis of these mRNAs in SARS-CoV-infected cells was confirmed experimentally. The 4382- and 7073 amino acid residue SARS-CoV replicase polyproteins are predicted to be cleaved into 16 subunits by two viral proteinases (bringing the total number of SARS-CoV proteins to 28). A phylogenetic analysis of the replicase gene, using a distantly related torovirus as an outgroup, demonstrated that, despite a number of unique features, SARS-CoV is most closely related to group 2 coronaviruses. Distant homologs of cellular RNA processing enzymes were identified in group 2 coronaviruses, with four of them being conserved in SARS-CoV. These newly recognized viral enzymes place the mechanism of coronavirus RNA synthesis in a completely new perspective. Furthermore, together with previously described viral enzymes, they will be important targets for the design of antiviral strategies aimed at controlling the further spread of SARS-CoV.

A novel coronavirus is the causative agent of the current epidemic of severe acute respiratory syndrome (SARS). Coronaviruses are exceptionally large RNA viruses and employ complex regulatory mechanisms to express their genomes. Here, we determined the sequence of SARS coronavirus (SARS-CoV), isolate Frankfurt 1, and characterized key RNA elements and protein functions involved in viral genome expression. Important regulatory mechanisms, such as the (discontinuous) synthesis of eight subgenomic mRNAs, ribosomal frameshifting and posttranslational proteolytic processing, were addressed. Activities of three SARS coronavirus enzymes, the helicase and two cysteine proteinases, which are known to be critically involved in replication, transcription and/or post-translational polyprotein processing, were characterized. The availability of recombinant forms of key replicative enzymes of SARS coronavirus should pave the way for high-throughput screening approaches to identify candidate inhibitors in compound libraries. INTRODUCTIONSevere acute respiratory syndrome (SARS) is a lifethreatening form of pneumonia (Peiris et al., 2003a). In the course of a few months in 2003, an epidemic emerged that has spread from its likely origin in Guangdong Province, China, to 32 countries. By 11 June 2003 more than 8400 cases and 789 deaths had been recorded by the World Health Organization. The rapid transmission by aerosols (and probably also the faecal-oral route) and the high mortality rate make SARS a global threat for which no efficacious therapy is available. There is now clear evidence that SARS is caused by a previously unknown coronavirus, provisionally termed SARS coronavirus (SARS-CoV) (Peiris et al., 2003b;Drosten et al., 2003;Ksiazek et al., 2003;Fouchier et al., 2003). Genome sequences of SARS-CoV isolates obtained from a number of index patients have been published recently and provide important information on the organization, phylogeny and variability of the 29?7 kb positive-strand RNA genome of SARS-CoV (Rota et al., 2003;Marra et al., 2003;Ruan et al., 2003). By analogy with other coronaviruses (Lai & Holmes, 2001;Gorbalenya, 2001), SARS-CoV gene expression is expected to involve complex transcriptional, translational and post-translational regulatory mechanisms, whose molecular details remain to be determined. SARS-CoV genome expression starts with the translation of two large replicative polyproteins, pp1a (486 kDa) and pp1ab (790 kDa), which are encoded by the viral replicase gene (21 221 nt) that comprises ORFs 1a and 1b (Fig. 1). Expression of the ORF1b-encoded region of pp1ab is predicted to involve ribosomal frameshifting into the 21 frame just upstream of the ORF1a translation termination codon (Brierley et al., 1989). The pp1a and pp1ab polyproteins are processed by viral proteinases to yield the functional components of the membrane-bound replicase complex (Ziebuhr et al., 2000). In contrast to most other coronaviruses, which use three proteinase activities for replicase polyprotein processing (Ziebuhr et ...