Updated: Jul 25, 2021
“…prior knowledge has greatly helped researchers in determining the functions of the different proteins present on the viral envelope of the virus, as well as mapping out the mechanism of action…”
Author: Rushil Dua
The coronavirus disease of 2019 (COVID-19) is a disease that has defined 2020, turning what we had all imagined this year to be on its head. This disease is caused by a virus known as SARS-CoV-2 . While many simply know it to be a virus that is transmitted through air droplets, what exactly does that mean? How is the SARS-CoV-2 coronavirus transmitted to the body, and what does it do on a molecular level? To develop a better understanding of the virus, this article will discuss its mode of transmission, structure, and mechanism of action.
Mode of Transmission
Essentially, COVID-19 is transmitted through respiratory droplets . Respiratory droplets are bits of moisture that can occur through sneezing, coughing, etc., and are the means of direct transmission . The virus can also be spread through indirect contact, where fomites (surfaces that are likely to carry infections) can come in contact with a symptomatic patient, harbour the virus, and pass it on to other people . Airborne transmission of the virus may be possible; however, it is only likely to happen in certain medical procedures which generate aerosols . There is still some debate about this, as there have been reports which argue for both sides of the possibility of airborne transmission .
Before discussing how the virus acts on the body, it may be beneficial to understand what the virus looks like. Biological entities are made of proteins, and the novel coronavirus is no different; it is essentially genetic material enveloped in protein . This envelope serves to protect the genetic material, and to bind the other proteins present on the virus together .
On top of the envelope, there are three such proteins: The M, E, and S proteins [3-5]. The first is the membrane (M) protein, and is the most abundant protein present on the envelope [4, 5]. It serves to maintain the structural integrity and shape of the envelope and virus itself [3-5]. When the M protein interacts with the envelope (E) protein, the viral envelope is maintained and virus-like particles (VLPs) may be released . Once the virus is able to release VLPs, the E protein interacts with the spike (S) protein [3-5]. This will then fuse the membranes of the host cell and the virus, allowing the virus to enter the host cell [3-5]. It, as viruses do, will then reprogram the cell to create clones of the virus .
Figure 1: An illustration of the SARS-CoV-2 virus .
Keep in mind that the novel coronavirus belongs to the CoV virus family . The virus that arose in 2002 (SARS) showed a similar structure to this one; however, the 2019 variant is allegedly more aggressive, allowing up to a 10 to 20 times higher success rate of merging with the host cells’ membrane . Since the virus has a higher chance of entering human cells, there is ultimately a higher chance of SARS-CoV-2 (2019) spreading compared to SARS-CoV (2002) .
Mechanism of Action
Within the viral shell lies the nucleocapsid (N) protein, which is vital to the RNA’s replication and transcription . As discussed above, the M protein aids in construction of the viral envelope and virus itself, and the E protein mediates cohesion between the virus and host cell [3-5]. The E protein will also aid the S protein in attaching to the host cell [4, 5].
The SARS-CoV-2 virus attaches to the angiotensin converting enzyme 2 (ACE2) receptor . The ACE2 receptor is commonly found on many human cells, including those in the lungs . The virus acts through a cleavage of the S protein, where two subunits (S1 and S2, responsible for viral and cellular fusion, respectively) are thus produced . Later, the S2 subunit is further cleaved to release the fusion peptide and activates the membrane fusion mechanism .
Figure 2: The mechanism of action of SARS-CoV-2 
Once this occurs, the virus enters the cell through endocytosis . The viral membrane is then broken down and releases the viral RNA into the cytoplasm, to be transcripted and replicated . This process is carried out by the replication-transcription complex (RTC), which is encoded in the virus’ genome . The replicated genome is used to create full length genomes via the nuclear endoplasmic reticulum, and from there, transfers to the Golgi body for packaging .
Virions are then produced, and shipped out through exocytosis to infect other cells . The host cell will eventually die due to the stress it has faced in being hijacked to create copies of a viral genome .
Summary and Medicinal Developments
The SARS-CoV-2 virus is a complex virus with an interesting structure and lengthy mechanism of action. It is important to be mindful of how transmission works with COVID-19 because, as mentioned, it is easier to transmit than previous variations of CoV viruses . Much of the research conducted in relation to COVID-19 draws from previous studies related to SARS, and as such, there is still much about COVID-19 that is not fully known, or that researchers are still unsure of . In spite of that, this prior knowledge has greatly helped researchers in determining the functions of the different proteins present on the viral envelope of the virus, as well as mapping out the mechanism of action.
Currently, there are different types of medical treatments being developed in hopes of curing/preventing COVID-19, whether it be a vaccine or otherwise. For example, some researchers are attempting to target the main protease (shortened to Mpro) present in the virus as they believe this may stop propagation of the virus between cells . Research has shown that proteins synthesised from the Mpro are capable of demonstrating anti-SARS-CoV-2 properties and may prevent infection . There are also many different vaccine trials happening worldwide to tackle this particular strand of the virus.
Being aware of the virus’s rapid mechanism of action within the body, be sure to follow public health guidelines to protect yourself and others from the novel coronavirus.
GlossaryEndocytosis: the taking in of matter by a living cell by forming a cavity of its membrane to form a vacuole which is then accepted into the cell Exocytosis: a process by which the contents of a cell vacuole are released to the exterior through fusion of the vacuole membrane with the cell membrane Protease: an enzyme which breaks down proteins Viral: of relation to a virus; spreading rapidly (i.e viral video, viral infection, etc.) Virion: the infectious form of a virus; an individual viral particle outside the host cell 
Nichole Dai, Winnie Lui, Rhea Verma
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