Essential component of the RNA polymerase transcription and replication complex. Binds the viral ribonucleocapsid and positions the L polymerase on the template. May act as a chaperone for newly synthesized free N protein, so-called N(0). Plays a role in virion assembly. Homotrimer. This trimer is stabilized by binding to the L protein. Binds N(0), and N in ribonucleocapsid. Phosphorylated by host kinases. Phosphorylation play an important role in facilitating trimerization and possibly P-L complex formation.