Plays a crucial role in virus assembly into filaments and budding. Early in infection, localizes in the nucleus where it inhibits host cell transcription through direct binding to host chromatin. Later in infection, traffics to the cytoplasm through the action of host CRM1 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. Interacts with host KPNB1; this interaction mediates nuclear import of the matrix protein early during infection. Interacts with host AP3M1; this interaction plays an essential role in trafficking the matrix protein in host cells. Interacts with host CAV1; this interaction probably facilitates viral budding. Interacts with host CFL1; this interaction probably facilitates viral replication. Interacts with host ZNF502; this interaction probably facilitates viral release. In the cytoplasm, associates with inclusion bodies. During bud formation, associates at the inner side of the plasma membrane of infected cells. Phosphorylation is important for oligomerization. Belongs to the pneumovirinae M protein family.