General information on M protein
M protein is a virulence factor located in viruses, parasites and certain species of Streptococcus. The protein counteracts the host’s defense and help the microorganisms gain entry into the target cells. M protein is anti-phagocytic. The protein binds to serum factor H which is a regulator of complement activation. The binding destroys C3-convertase and prevents opsonization by C3b. M protein defines the shape of the coronavirus (CoVs) viral envelope. CoVs cause enzootic infections in birds and mammals and CoVs strain is responsible for 2019 novel coronavirus disease in humans (COVID-2019). CoVs are non-segmented positive-sense RNA viruses in the Coronaviridae family. They have a high mutation rate.
M protein is a glycoprotein and consists of three domains: a C-terminal endodomain, a triple-spanning transmembrane domain and a short N terminal ectodomain. It’s the most abundant structural protein and is essential for CoV assembly. The homotypic interactions between M proteins are essential for virion envelope formation as the protein alone is not sufficient. The protein interacts with all other major CoV structural proteins which included envelope proteins, E and S, and the nucleocapsid. The nucleocapsid consists of N protein and genomic RNA. M protein interacts with N protein in a pre-Golgi compartment in infected cells. The M protein also interacted with nucleocapsid mRNA 1. The interaction between M and N protein stabilizes the nucleocapsid, the CoVs internal core and promotes completion of viral assembly. M Protein also interacts with S protein. The latter is responsible for forming a 180/90-kDa peplomers that bind to receptors the target cell and induce cell fusion. The M-S protein interaction is necessary to keep S protein in the ER-Golgi intermediate compartment.
E protein, similar to M protein also contributes to viral envelope. The interaction between E and M protein interaction is sufficient and necessary for the production and release of coronavirus-like particles (VLPs).