11/04/2020
COVID-19 and the Flu part Two
Genome
Coronaviruses and influenza viruses are both enveloped, single-stranded RNA viruses, and both are encapsidated by nucleoprotein. However, the genomes of these 2 viruses differ in polarity and segmentation. Influenza virus is comprised of 8 single-stranded negative-sense, viral RNA segments. SARS-CoV-2 has single-stranded, non-segmented, positive-sense, viral RNA.
Surface Proteins
Both viruses possess distinguishing surface proteins that serve as important virulence factors for infection. SARS-CoV-2 is covered in spike (S) proteins that facilitate invasion of host cells. S proteins bind to the host cell receptor, angiotensin-converting enzyme 2 (ACE2), which regulates blood pressure and fluid-salt balances and is expressed by multiple organ systems throughout the body, including the lungs, heart, kidneys, liver, intestines, brain and adipose tissues. Upon binding, SARS-CoV-2 injects its RNA into the infected cell and uses host cell machinery to replicate its genome. Newly synthesized virus particles are then released to infect additional host cells.
Influenza viruses rely on the collaborative functions of 2 viral surface proteins, haemagglutinin (HA) and neuraminidase (NA) to enter and exit host cells. The host cell receptor for influenza viruses is sialic acid, a sugar chain that is fairly ubiquitous and attached to surface lipids and proteins of most host cells, as well as soluble proteins. HA preferentially binds to sialic acid on the surface of respiratory epithelial cells, and mediates entry of the virus to host cells. Once inside, influenza virus also releases its RNA to be copied and synthesized into new virus particles. However, as long as HA remains bound to sialic acid on cell surfaces, newly synthesized virus particles are unable to exit the infected cells. NA cleaves sialic acid from the cell surface, which releases HA and allows progeny viruses to exit infected cells and continue spreading.