SARS-CoV-1 Infection

SARS-CoV-1 infection begins with the binding of viral S (spike) protein to cell surface angiotensin converting enzyme 2 (ACE2) and endocytosis of the bound virion. Within the endocytic vesicle, host proteases mediate cleavage of S protein into S1 and S2 fragments, leading to S2-mediated fusion of the viral and host endosome membranes and release of the viral capsid into the host cell cytosol. The capsid is uncoated to free the viral genomic RNA, whose cap-dependent translation produces polyprotein pp1a and, by means of a 1-base frameshift, polyprotein pp1ab. Autoproteolytic cleavage of pp1a and pp1ab generates 15 or 16 nonstructural proteins (nsps) with various functions. Importantly, the RNA dependent RNA polymerase (RdRP) activity is encoded in nsp12. Nsp3, 4, and 6 induce rearrangement of the cellular endoplasmic reticulum membrane to form cytosolic double membrane vesicles (DMVs) where the viral replication transcription complex is assembled and anchored. With viral genomic RNA as a template, viral replicase-transcriptase synthesizes a full length negative sense antigenome, which in turn serves as a template for the synthesis of new genomic RNA. The replicase-transcriptase can also switch template during discontinuous transcription of the genome at transcription regulated sequences to produce a nested set of negative-sense subgenomic (sg) RNAs, which are used as templates for the synthesis of positive-sense sgRNAs that are translated to generate viral proteins. Finally, viral particle assembly occurs in the ER Golgi intermediate compartment (ERGIC). Viral M protein provides the scaffold for virion morphogenesis.