Sequence of events in measles virus replication: Role of phosphoprotein-nucleocapsid interactions

The genome of nonsegmented negative-strand RNA viruses is tightly embedded within a nucleocapsid made of a nucleoprotein (N) homopolymer. To ensure processive RNA synthesis, the viral polymerase L in complex with its cofactor phosphoprotein (P) binds the nucleocapsid that constitutes the functional template. Measles virus P and N interact through two binding sites. While binding of the P amino terminus with the core of N (N_CORE) prevents illegitimate encapsidation of cellular RNA, the interaction between their C-terminal domains, P_XD and N_TAIL is required for viral RNA synthesis. To investigate the binding dynamics between the two latter domains, the P_XD F497 residue that makes multiple hydrophobic intramolecular interactions was mutated. Using a quantitative mammalian protein complementation assay and recombinant viruses, we found that an increase in P_XD-to- N_TAIL binding strength is associated with a slower transcript accumulation rate and that abolishing the interaction renders the polymerase nonfunctional. The use of a newly developed system allowing conditional expression of wild-type or mutated P genes, revealed that the loss of the P_XD-N_TAIL interaction results in reduced transcription by preformed transcriptases, suggesting reduced engagement on the genomic template. These intracellular data indicate that the viral polymerase entry into and progression along its genomic template relies on a protein-protein interaction that serves as a tightly controlled dynamic anchor.