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Spike (S) glycoprotein (sometimes also called spike protein, [2] formerly known as E2 [3]) is the largest of the four major structural proteins found in coronaviruses. [4] The spike protein assembles into trimers that form large structures, called spikes or peplomers, [3] that project from the surface of the virion.
M is a glycoprotein whose glycosylation varies according to coronavirus subgroup; N-linked glycosylation is typically found in the alpha and gamma groups while O-linked glycosylation is typically found in the beta group. [8] [9] There are some exceptions; for example, in SARS-CoV, a betacoronavirus, the M protein has one N-glycosylation site.
For this reason the spike protein has been the focus of development for COVID-19 vaccines in response to the COVID-19 pandemic caused by the virus SARS-CoV-2. [11] [12] A subgenus of the betacoronaviruses, known as embecoviruses (not including SARS-like coronaviruses), have an additional shorter surface protein known as hemagglutinin esterase. [13]
The envelope (E) protein is the smallest and least well-characterized of the four major structural proteins found in coronavirus virions. [2] [3] [4] It is an integral membrane protein less than 110 amino acid residues long; [2] in SARS-CoV-2, the causative agent of Covid-19, the E protein is 75 residues long. [5]
Structure of a coronavirus. Coronaviruses are large, roughly spherical particles with unique surface projections. [43] Their size is highly variable with average diameters of 80 to 120 nm. Extreme sizes are known from 50 to 200 nm in diameter. [44] The total molecular mass is on average 40,000 kDa.
SARS-CoV-2 is the seventh known coronavirus to infect people, after 229E, NL63, OC43, HKU1, MERS-CoV, and the original SARS-CoV. [105] Like the SARS-related coronavirus implicated in the 2003 SARS outbreak, SARS‑CoV‑2 is a member of the subgenus Sarbecovirus (beta-CoV lineage B). [106] [107] Coronaviruses undergo frequent recombination. [108]
Replication cycle of a coronavirus. The 5' and 3' ends of the genome have a cap and poly(A) tract, respectively.The viral envelope, obtained by budding through membranes of the endoplasmic reticulum (ER) or Golgi apparatus, invariably contains two virus-specified glycoprotein species, known as the spike (S) and membrane (M) proteins.
Throughout the COVID-19 pandemic, the genome of SARS-CoV-2 viruses has been sequenced many times, resulting in identification of thousands of distinct variants. In a World Health Organization analysis from July 2020, ORF1ab was the most frequently mutated gene, followed by the S gene encoding the spike protein .