Essential for viral replication in vivo (PubMed:15890897). 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 (PubMed:25855728). Contains an oligomerization domain (By similarity). The central globular core is responsible for binding to RNA and phosphoprotein (By similarity). Phosphorylated by host in infected cells (PubMed:27252537). Phosphorylation is not essential for zinc binding activity and oligomerization, but zinc binding activity is necessary for the phosphorylation and oligomerization (PubMed:27252537). Phosphorylation up-regulates viral RNA synthesis, replication, and pathogenesis in vivo (PubMed:27252537). Belongs to the pneumoviridae M2-1 protein family.