Crystal Structure of the Measles Virus Nucleoprotein Core in Complex with an N-Terminal Region of Phosphoprotein

ABSTRACT The enveloped negative-stranded RNA virus measles virus (MeV) is an important human pathogen. The nucleoprotein (N 0 ) assembles with the viral RNA into helical ribonucleocapsids (NC) which are, in turn, coated by a helical layer of the matrix protein. The viral polymerase complex uses the NC as its template. The N 0 assembly onto the NC and the activity of the polymerase are regulated by the viral phosphoprotein (P). In this study, we pulled down an N 0 1-408 fragment lacking most of its C-terminal tail domain by several affinity-tagged, N-terminal P fragments to map the N 0 -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 N 0 binding peptide, P 1-48 , to the C terminus of an N 0 21-408 fragment lacking both the N-terminal peptide and the C-terminal tail of N protein to reconstitute and crystallize the N 0 -P complex. We solved the X-ray structure of the resulting N 0 -P chimeric protein at a resolution of 2.7 A. The structure reveals the molecular details of the conserved N 0 -P interface and explains how P chaperones N 0 , preventing both self-assembly of N 0 and its binding to RNA. Finally, we propose a model for a preinitiation complex for RNA polymerization. IMPORTANCE 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. We describe the atomic model of an N-P complex and compare it to helical ribonucleocapsid. We thus provide insight into how P chaperones N and helps 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 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 to treat not only measles but also potentially other paramyxovirus diseases.