Component of the viral envelope that plays a central role in virus morphogenesis and assembly via its interactions with other viral proteins (By similarity). Regulates the localization of S protein at cis-Golgi, the place of virus budding (PubMed:33229438). May act by binding cytoplasmic c-terminus of S (PubMed:33229438). Homomultimer. Interacts with envelope E protein in the budding compartment of the host cell, which is located between endoplasmic reticulum and the Golgi complex (By similarity). Forms a complex with S proteins (PubMed:33229438). Interacts with nucleocapsid N protein. This interaction probably participates in RNA packaging into the virus (By similarity). Interacts with the accessory proteins 3a and 7a (By similarity). Largely embedded in the lipid bilayer. Glycosylated at N-terminus. Variant Delta/B.1.617.2 belongs to a lineage first isolated in India (October 2020) and is also called G/478K.V1. It has an estimated 97% increase of transmissibility. Variant Omicron/BA.1 and BA.2 belong to a lineage first isolated in South Africa (November 2021). Belongs to the betacoronaviruses M protein family.