Membrane binding proteins of coronaviruses

J Alsaadi, Entedar A; Jones, Ian M

The CoV membrane protein (M) is a type III transmembrane glycoprotein and is the most abundant glycoprotein in the CoV particle. Despite variability in the primary M protein sequence the predicted secondary structures of M proteins are maintained [58] . The M protein is approximately 230 amino acids in length and is composed of three parts: a short N-terminal domain situated outside the virion membrane, three transmembrane domains and a carboxy-terminal domain situated inside the particle [59, 60] . An amphipathic region situated at the end of the third transmembrane domain is well conserved in almost all Coronaviridae members [58] . CoV M proteins are characterized by N-linked glycosylation in the α and δ CoVs and O-linked glycosylation in the β CoVs [61, 62] and study of chimeric M proteins has shown that the type of glycosylation is not critical for virus assembly or growth at 37 • C [50] . It seems more likely that, as for many virus glycoproteins, glycosylation has a more general significance in maintaining bioactive conformation and antigenic character [63, 64] . M is located among the S proteins in the virus envelope along with small amounts of E and is the primary driver of the virus budding process [51] . During assembly of the authentic virion M interacts with itself, with the nucleocapsid protein N, with E and with the S protein [44, 58, 65] . M protein is present as a dimer in the virion and high resolution imaging has suggested that it presents as two conformations, long and compact (M LONG and M COMPACT ), which together induce membrane curvature as well as binding to the nucleocapsid [66, 67] .