Coronavirus membrane protein

The membrane (M) protein (previously called E1, sometimes also matrix protein) is an integral membrane protein that is the most abundant of the four major structural proteins found in coronaviruses. The M protein organizes the assembly of coronavirus virions through protein-protein interactions with other M protein molecules as well as with the other three structural proteins, the envelope (E), spike (S), and nucleocapsid (N) proteins. The M protein is a transmembrane protein with three transmembrane domains and is around 230 amino acid residues long. In SARS-CoV-2, the causative agent of COVID-19, the M protein is 222 residues long. Its membrane topology orients the C-terminus toward the cytosolic face of the membrane and thus into the interior of the virion. It has a short N-terminal segment and a larger C-terminal domain. Although the protein sequence is not well conserved across all coronavirus groups, there is a conserved amphipathic region near the C-terminal end of the third transmembrane segment. M functions as a homodimer. Studies of the M protein in multiple coronaviruses by cryo-electron microscopy have identified two distinct functional protein conformations, thought to have different roles in forming protein-protein interactions with other structural proteins. M protein of SARS-CoV-2 is homologous to the prokaryotic sugar transport protein SemiSWEET. 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. There are some exceptions; for example, in SARS-CoV, a betacoronavirus, the M protein has one N-glycosylation site. Glycosylation state does not appear to have a measurable effect on viral growth. No other post-translational modifications have been described for the M protein. The gene encoding the M protein is located toward the 3' end of the virus's positive-sense RNA genome, along with the genes for the other three structural proteins and various virus-specific accessory proteins.