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 (PubMed:19386701, PubMed:22675274). Transcription anti-terminator that enhances readthrough of intergenic junctions during viral transcription (PubMed:10364337, PubMed:27194388, PubMed:8552680, PubMed:9420254). Preferentially binds to poly(A)-rich sequences (PubMed:24434552). Plays a role in the association of the matrix protein with the nucleocapsid, which initiates assembly and budding (PubMed:18579594). Also, can activate host NF-kappa-B through association with host RELA (PubMed:15629770). Homotetramer (PubMed:19386701, PubMed:29372904, PubMed:32697936). The homotetramer interacts with RNA (PubMed:32697936, PubMed:22675274). Interacts with the phosphoprotein (P); this interaction is required for protein M2-1 function, localization in host inclusion bodies (PubMed:12970453, PubMed:17098979, PubMed:19386701, PubMed:22675274, PubMed:29489893, PubMed:22978633). Formation of a complex host PP1/M2-1/P allows P to target host PP1 phosphatase to the M2-1 substrate (PubMed:29489893). Interacts with the nucleoprotein (N) (PubMed:10846068, PubMed:22675274). 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 (PubMed:18579594). Interacts with host RELA (PubMed:15629770). Interacts with host PABPC1 (via C-terminus) (PubMed:31649314). Localizes in cytoplasmic inclusion bodies substructures called inclusion bodies associated granules (IBAGs) (PubMed:22675274, PubMed:28916773, PubMed:29489893, PubMed:31649314). Forms a layer between the matrix and nucleocapsid (PubMed:23776214, PubMed:24760890). Down-regulated by cyclopamine (PubMed:27194388). Expressed early in the viral replication cycle (PubMed:10846068). Contains a zinc-finger domain on its N-terminus essential for its anti-termination function (PubMed:24434552). Contains an oligomerization domain (PubMed:22675274). The central globular core is responsible for binding to RNA and phosphoprotein (PubMed:22675274). Phosphorylated by host in infected cells (PubMed:11711610, PubMed:3339328, PubMed:10846068, PubMed:29489893). Only dephosphorylated M2-1 is competent for viral mRNA binding (PubMed:29489893). Cyclic turnover of phosphorylation-dephosphorylation of M2-1 is required for efficient viral transcription (PubMed:29489893). Belongs to the pneumoviridae M2-1 protein family.