What is the Coronavirus Spike Protein?
Coronaviruses are a family of viruses named for their crown-like appearance when imaged (corona being the Latin for crown). The large, type I transmembrane spike glycoprotein accounts for this notable feature. It is a heavily-glycosylated, cell-surface protein which is thought to mediate viral entry into susceptible cells. This spike glycoprotein, called ‘S’, is trimeric in structure. In addition to the S protein, there are three other structural base proteins: the envelope, membrane, and nucleocapsid. The S protein has two distinct functional domains, termed S1 and S2, both of which are necessary for a coronavirus to successfully enter a cell.
Fig 1. Atomic-level structure of the SARS-CoV-2 spike glycoprotein. The receptor binding domain is shown in green. (Adapted from UT Austin, McLellan Lab, Link)
S1 is responsible for the first stages of viral entry, and contains the receptor binding domain. S2 acts in the later-stage fusion of the cell and viral membranes, and contains amino acid sequences necessary for continuing infiltration. In order for fusion to take place, the S protein needs to be cleaved by proteases found in the cell. This step is critical, as it allows for the fusion sequences to be exposed. This cleavage is generally mediated by furin, a protein convertase.
Fig 2. The structural proteins of a coronavirus, including the glycoprotein spike, the nucleocapsid phosphoprotein, envelope glycoprotein, and membrane glycoprotein (Adapted from NEJM, Link)
What is Furin?
Furin is an enzyme in the proprotein convertase family which cleaves precursor proteins and facilitates their conversion to a biologically active state. Furin is a member of the subtilisin-like proprotein convertase family, which includes proteases that process protein and peptide precursors trafficking through multiple branches of the secretory pathway. It is a type 1 membrane-bound protease that is expressed in multiple tissues.
Fig 3. A schematic of the crystal structure of the proprotein convertase furin. (Link)
Furin is expressed in significant concentrations in the lungs. Thus, viruses in the respiratory tract can make use of this enzyme to convert and activate their own surface glycoproteins. This makes their role in viral protein processing noteworthy.
Why is a Furin Cleavage Site Important?
As a result of furin’s ability to cleave important cell surface proteins, it and other proprotein convertases are the target of considerable research interest. The S1/S2 cleavage site is the target for furin during infection. It is important to note that other beta coronaviruses do not contain this cleavage site. SARS-CoV, which is closely related to the newest SARS-CoV-2 strain, does not bear a cleavage site.
Some of the most pathogenic forms of influenza have similar cleavage sites, which can be acted upon by furin and other cellular proteases. The ubiquitous expression of cellular proteases across cell types increases the potential for the virus to successfully infiltrate the host.
How ACE2, Furin, and the SARS-CoV2 Spike Interact
The modified S glycoprotein can interact with the cell surface receptor ACE2, angiotensin I converting enzyme 2. The S1 subunit, which contains the receptor binding domain, makes contact with ACE2 which is facilitated by furin cleavage.
Recent studies have found that the S protein of SARS-CoV-2 is between 10 and 20 times more likely to bind to human ACE2 than the S protein of the early 2000s SARS-CoV strain. The heightened affinity for a prevalent cellular receptor may be a factor which increases transmission.
Vaccines, Antibodies, & Future COVID-19 Therapeutics
Fig 4. SARS-CoV-2 model, adapted from the U.S. CDC
Antibodies are in development to target the S1 and S2 subunits of the spike protein. Previously, these subunits have been identified as targets of the immune system in past cases of SARS-CoV. Although the sequences and structures of the S proteins are similar between SARS-CoV and SARS-CoV-2, antibodies against SARS-CoV’s spike protein did not bind to the SARS-CoV-2 S protein. This implies that potential therapeutics and vaccines will need to be tailored to the new viral spike protein.
Current research is identifying target epitopes of the SARS-CoV-2 spike protein. Antibodies from patients who have recovered from COVID-19 could be isolated by utilising the novel S protein structure. Once their structure is elucidated, producing these antibodies in large quantities could allow for treatment of new infections before a specific SARS-CoV-2 vaccine is made widely available.
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