The spike protein is very large, often 1200 to 1400 amino acid residues long it is 1273 residues in SARS-CoV-2. The black bar at the bottom indicates the position of the viral membrane. Helices show in orange and cyan form parts of S2 that will undergo conformational changes during fusion. The S1 NTD is shown in blue and the S1 CTD (which serves as the receptor-binding domain) is shown in pink. ![]() Structure Cryo-electron microscopy structure of a SARS-CoV-2 spike protein trimer in the pre-fusion conformation, with a single monomer highlighted. Spike glycoprotein is not the main determinant of pathogenicity, which are other proteins that remain to be identified, but its mutations do confer the virus the ability to evade immunity. Most COVID-19 vaccine development efforts in response to the COVID-19 pandemic aim to activate the immune system against the spike protein. Neutralizing antibodies target epitopes on the receptor-binding domain. Antibodies against spike glycoprotein are found in patients recovered from SARS and COVID-19. Spike glycoprotein is highly immunogenic. Spike glycoprotein determines the virus' host range (which organisms it can infect) and cell tropism (which cells or tissues it can infect within an organism). The S2 region contains the fusion peptide and other fusion infrastructure necessary for membrane fusion with the host cell, a required step for infection and viral replication. Coronaviruses use a very diverse range of receptors SARS-CoV (which causes SARS) and SARS-CoV-2 (which causes COVID-19) both interact with angiotensin-converting enzyme 2 (ACE2). The S1 region contains the receptor-binding domain that binds to receptors on the cell surface. Spike glycoprotein is a class I fusion protein that contains two regions, known as S1 and S2, responsible for these two functions. The function of the spike glycoprotein is to mediate viral entry into the host cell by first interacting with molecules on the exterior cell surface and then fusing the viral and cellular membranes. ![]() The distinctive appearance of these spikes when visualized using negative stain transmission electron microscopy, "recalling the solar corona", gives the virus family its main name. The spike protein assembles into trimers that form large structures, called spikes or peplomers, that project from the surface of the virion. ![]() Spike (S) glycoprotein (sometimes also called spike protein, formerly known as E2 ) is the largest of the four major structural proteins found in coronaviruses. Model of the external structure of the SARS-CoV-2 virion.
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