Component of the viral envelope that plays a central role in virus morphogenesis and assembly via its interactions with other viral proteins. Interacts with many viral proteins in infected cells. Interacts with envelope E protein in the budding compartment of the host cell, which is located between endoplasmic reticulum and the Golgi complex. Forms a complex with HE and S proteins. Interacts with nucleocapsid N protein. This interaction probably participates in the viral mRNA packaging into the virus (By similarity). Belongs to the torovirinae M protein family.