Acts as a tetrameric transcription processivity factor that binds in a competitive manner to RNA and the phosphoprotein (P) to prevent premature termination during transcription. Transcription anti-terminator that enhances readthrough of intergenic junctions during viral transcription. Preferentially binds to poly(A)-rich sequences. Plays a role in the association of the matrix protein with the nucleocapsid, which initiates assembly and budding. Also, can activate host NF-kappa-B through association with host RELA. 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. Formation of a complex host PP1/M2-1/P allows P to target host PP1 phosphatase to the M2-1 substrate. 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. Interacts with host RELA. Interacts with host PABPC1 (via C-terminus). 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 anti-termination function. Contains an oligomerization domain. The central globular core is responsible for binding to RNA and phosphoprotein. Phosphorylated by host in infected cells. Only dephosphorylated M2-1 is competent for viral mRNA binding. Cyclic turnover of phosphorylation-dephosphorylation of M2-1 is required for efficient viral transcription. Belongs to the pneumoviridae M2-1 protein family.