SARS VIRUS: STRUCTURE AND LIFE CYCLE

Severe acute respiratory syndrome (SARS) is caused by a previously unknown virus from the Coronaviridae family.

Coronaviruses are large, enveloped, positive-stranded RNA viruses. The two major proteins are located on the envelope: the spike glycoprotein, responsible for binding to host cells, and the transmembrane glycoprotein, which is involved in budding and envelope formation and may also connect to the nucleocapsid, forming the core. The viral genome is associated with the nucleocapsid glycoprotein.

Coronaviruses (CoVs) typically spread via aerosols. In the case of SARS, however, transmission may not be via aerosols of respiratory secretion but by close direct contact with an infected individual, or less probably by contaminated objects. SARS-CoV primarily infects the lower respiratory tract, but the upper tract can also be infected, resulting in severe, acute viral pneumonia.

The virion preferentially targets epithelial cells, but it can also infect macrophages. It binds to cell surface receptors using its projecting spikes and then enters the cell via membrane fusion or receptor-mediated endocytosis.

One hypothesized mechanism of replication is that genomic RNA creates negative strand copies of itself, that are used as templates to produce new genomic RNA and mRNA.

Viral proteins are encoded from mRNA. N proteins join the new genomic RNA to form ribonucleoproteins. Proteins M, S, E and HE attach to the membrane to form a vesicle.

Ribonucleoproteins bud into the lumen of the vesicle and the newly formed immature virion lies free inside. The particle then makes its way to the periphery of the Golgi, maturing into a denser and more icosahedral form.

The new virus particles collect forming large vesicles and are finally released onto the cell surface. The cycle then begins again.

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