Essential for viral replication in vivo (By similarity). Plays a role in the association of the matrix protein with the nucleocapsid, which initiates assembly and budding (By similarity). Homotetramer. The homotetramer interacts with RNA. Interacts with the phosphoprotein (P); this interaction is required for protein M2-1 function, localization in host inclusion bodies. Interacts with the nucleoprotein (N). Interacts with the matrix protein (M); this interaction directs M localization to cytoplasmic inclusions comprising viral proteins L, N, P, and M2-1 and mediates M association with the nucleocapsid. Localizes in cytoplasmic inclusion bodies substructures called inclusion bodies associated granules (IBAGs). Forms a layer between the matrix and nucleocapsid. Contains a zinc-finger domain on its N-terminus essential for its function (By similarity). Contains an oligomerization domain. The central globular core is responsible for binding to RNA and phosphoprotein (By similarity). Phosphorylated by host in infected cells. Phosphorylation is not essential for zinc binding activity and oligomerization, but zinc binding activity is necessary for the phosphorylation and oligomerization. Phosphorylation up-regulates viral RNA synthesis, replication, and pathogenesis in vivo. Belongs to the pneumoviridae M2-1 protein family.