Non catalytic polymerase cofactor and regulatory protein that plays a role in viral transcription and replication. Stabilizes the RNA polymerase L to the N-RNA template and binds the soluble protein N, preventing it from encapsidating non-genomic RNA (By similarity). Homotrimer when phosphorylated. This trimer is stabilized by binding to the L protein. Binds soluble protein N, and ribonucleocapsid (By similarity). Phosphorylated by host kinases. Belongs to the nucleorhabdovirus protein P family.