S glycoprotein
Summary
The S (spike) glycoprotein is the surface protein of coronaviruses that binds the host receptor and mediates membrane fusion, giving the virion its crown-like appearance on electron microscopy. It determines host range and tissue tropism and is the antigen targeted by COVID-19 vaccines.
Detail
The spike is a trimeric class I fusion protein cleaved into S1, which contains the receptor binding domain, and S2, which drives fusion. Receptor usage differs by virus and explains their epidemiology: SARS-CoV and SARS-CoV-2 bind angiotensin-converting enzyme 2, expressed on type II pneumocytes, enterocytes, and vascular endothelium, whereas MERS-CoV binds dipeptidyl peptidase 4. Priming by host proteases, notably TMPRSS2 and furin, is required for fusion, and the presence of a polybasic furin cleavage site in SARS-CoV-2 enhances infectivity. Because it is surface-exposed and essential, the spike is the dominant target of neutralizing antibodies and the immunogen in mRNA and viral-vector COVID-19 vaccines, typically stabilized in the prefusion conformation. It is also the site of the mutations that define variants and drive immune escape, which is why vaccine composition is periodically updated.
Sources
- Levinson Medical Microbiology and Immunology
- Harrison's Principles of Internal Medicine
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