Plays critical roles in regulating RNA replication and transcription through its interactions with multiple proteins (PubMed:26474524, PubMed:25568210). Tethers the RNA-directed RNA polymerase L to the nucleoprotein-RNA complex (PubMed:26474524). Recruits the M2-1 protein, a processivity factor that is required for efficient transcription of viral RNA (PubMed:26474524). Acts as a chaperone for neo-synthesized nucleoprotein by forming an N-P complex that preserves N in a monomeric and RNA-free state and prevents the association of nascent N with host cell RNAs (PubMed:25568210). Recruits the host phosphatase PP1 to inclusion bodies to regulate viral transcription (PubMed:29489893). Together with the nucleoprotein, sequesters host NF-kappa-B in inclusion bodies (IBs) thereby inhibiting this host defense pathway (By similarity). Homotetramer (PubMed:22978633, PubMed:15166449). Interacts with protein M2-1; the interaction between the two tetramers is required for the anti-termination and elongation transcriptional activities of protein M2-1 (PubMed:17098979, PubMed:22978633, PubMed:29489893, PubMed:26474524, PubMed:12970453). Interacts with host phosphatase PP1; this interaction recruits PP1 to the inclusion bodies (PubMed:29489893). Formation of a complex PP1/M2-1/P allows P to target host PP1 phosphatase to the M2-1 substrate (PubMed:29489893). Interacts (via C-terminus) with the nucleoprotein N (via N-terminus); the phosphorylated phosphoprotein P binds to N-RNA complex (PubMed:26474524, PubMed:17170452, PubMed:22623798, PubMed:25407889, PubMed:11861854, PubMed:12368320). Interacts (via N-terminus) with the monomeric RNA-free nucleoprotein N (PubMed:25568210, PubMed:28031463, PubMed:30626736). Interacts (via C-terminus) with RNA-directed RNA polymerase L; the association of P and L forms the polymerase complex (PubMed:26474524, PubMed:25653447, PubMed:31953395). Localizes in cytoplasmic inclusion bodies. The N-terminus is important for viral particle assembly (PubMed:31009855). The oligomerization region is central (PubMed:15166449). The C-terminus part contains binding regions for the RNA-directed RNA polymerase L and the nucleoprotein (PubMed:26474524). Constitutively phosphorylated by host (PubMed:17098979). Phosphorylation at S-116, S-117, S-119, S-232 and S-237 is required for transcription inhibition by M2-2 and viral particle egress (PubMed:26474524). Phosphorylation at S-232 and S-237 increases the affinity of the binding to the nucleoprotein (PubMed:25407889). Belongs to the pneumoviridae phosphoprotein P family.