1.A.117.1.2
Membrane glycoprotein of 222 aas and 3 or 4 TMSs. It is a component of the viral envelope that plays a central role in virus morphogenesis and assembly via its interactions with other viral proteins. However, it may also function a viroporin, Orf3a (Barrantes 2021). Variants of the M protein arise with high frequency, suggesting that these mutants are more biologically fit, perhaps related to glucose uptake during viral replication (Shen et al. 2021). Zhang et al. 2022 reported the cryo-EM structure of the SARS-CoV-2 M protein in two different conformations. M protein forms a mushroom-shaped dimer, composed of two transmembrane domain-swapped three-helix bundles and two intravirion domains. It also assembles into higher-order oligomers. A highly conserved hinge region is key for conformational changes. The M protein dimer is similar to SARS-CoV-2 ORF3a, a viroporin (TC# 1.A.57.1.5). Interaction analyses of M protein with nucleocapsid protein (N) and RNA suggest that the M protein mediates the concerted recruitment of these components through the positively charged intravirion domain (Zhang et al. 2022).