Structural insights into phosphoprotein chaperoning of nucleoprotein in measles virus

Measles virus is an important, highly contagious, human pathogen. The nucleoprotein N binds only to viral genomic RNA and forms the helical ribonucleocapsid that serves as a template for viral replication. We address how N is regulated by another protein, the phosphoprotein, P, to prevent newly synthesized N from binding to cellular RNA. Here, we pulled down an N⁰_1-408 fragment lacking most of its C-terminal tail domain by several affinity-tagged, N-terminal, P fragments to map the N⁰-binding region of P to the first 48 amino acids. We showed biochemically and using P mutants the importance of the hydrophobic interactions for the binding. We fused an N⁰ binding peptide, P_1-48, to the C-terminus of an N⁰_21-408 fragment lacking both the N-terminal peptide and the C-terminal tail of N protein to reconstitute and crystallize the N⁰-P complex. We solved the X-ray structure of the resulting N⁰-P chimeric protein at 2.7 Å resolution. The structure reveals the molecular details of the conserved N⁰-P interface and explains how P chaperones N⁰ preventing both self-assembly of N⁰ and its binding to RNA. We compare the structure of an N⁰-P complex to atomic model of helical ribonucleocapsid. We thus propose a model how P may help to start viral RNA synthesis. Our results provide a new insight into mechanisms of paramyxovirus replication. New data on the mechanisms of phosphoprotein chaperone action allows better understanding of the virus genome replication and nucleocapsid assembly. We describe a conserved structural interface for the N-P interaction which could be a target for drug development not only to treat measles but also potentially other paramyxovirus diseases.