Plays a crucial role in virus assembly into filaments and budding. Early in infection, localizes in the nucleus where it may inhibit host cell transcription. Later in infection, traffics to the cytoplasm to associate with inclusion bodies, the site of viral transcription and replication. During virus assembly and budding, acts as a bridge between the nucleocapsid and the lipid bilayer. Forms dimers. Forms higher-order oligomers. Interacts with glycoprotein G (via N-terminus). Interacts with protein M2-1; this interaction directs the matrix protein localization to cytoplasmic inclusions comprising viral proteins L, N, P, and M2-1 and mediates the matrix protein association with the nucleocapsid. In the cytoplasm, associates with inclusion bodies. During bud formation, associates at the inner side of the plasma membrane of infected cells. Belongs to the pneumovirinae M protein family.