Has a crucial role in virus assembly and budding. The matrix interacts with the RNP complex and this association serves two functions: facilitate virion assembly and inhibit the viral transcriptase activity. Early in infection, M is localized to the nucleus and may inhibit host cell transcription. Later on, M can associate with lipid rafts supposely by interacting with the cytoskeleton and with the cytoplasmic tail of glycoprotein G. The binding of M to host membrane is stabilized by the surface expression of the viral glycoproteins. These interactions may allow virus formation by mediating association of the nucleocapsid with the nascent envelope (By similarity). Interacts with glycoprotein G (via N-terminus), and protein N. Interacts with protein M2-1; this interaction mediates the association between proteins M and N (By similarity). During bud formation, associates at the inner side of the plasma membrane of infected cells. Belongs to the paramyxoviruses M protein family.