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

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-1-408(0) 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, P1-48, to the C terminus of an N-21-408(0) 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 angstrom. 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.