Plays a crucial role in virus assembly into filaments and budding (PubMed:25673702, PubMed:27654298, PubMed:23903836, PubMed:24672034). Early in infection, localizes in the nucleus where it inhibits host cell transcription through direct binding to host chromatin (PubMed:12827470, PubMed:34685766). 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 (PubMed:19297465, PubMed:23903836). During virus assembly and budding, acts as a bridge between the nucleocapsid and the lipid bilayer (PubMed:11907323, PubMed:12350355, PubMed:15958665, PubMed:18579594). Forms dimers (PubMed:29510513, PubMed:25673702). Forms higher-order oligomers (PubMed:24672034). Interacts with glycoprotein G (via N-terminus) (PubMed:15958665). 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 (PubMed:18579594). Interacts with host importin KPNB1; this interaction mediates nuclear import of the matrix protein early during infection (PubMed:16171404). Interacts with host AP3M1; this interaction plays an essential role in trafficking the matrix protein in host cells (PubMed:29028839). Interacts with host CAV1; this interaction probably facilitates viral budding (PubMed:25556234). Interacts with host CFL1; this interaction probably facilitates viral replication (PubMed:25556234). Interacts with host ZNF502; this interaction probably facilitates viral release (PubMed:25556234). In the cytoplasm, associates with inclusion bodies (PubMed:23903836). 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.