The Measles Virus Phosphoprotein: An Intrinsically Disordered Chaperone that Regulates Nucleocapsid Assembly

Measles virus, a negative strand RNA virus, packages its genome into large, helical superstructures formed by the nucleoprotein (N) that assembles on the RNA genome. Although a vital step in viral replication, the assembly process of N into nucleocapsids (NC) has largely remained obscure since overexpression of N in eukaryotic and prokaryotic expression systems leads to NC formation within the expression host. We now show how the presence of a short 50 residue, intrinsically disordered, peptide from the measles phosphoprotein (P) can prevent premature assembly during expression by forming a monomeric N0P construct that self-assembles into NC-like particles upon addition of RNA in vitro. This approach allowed us, for the first time, to study NC assembly and its kinetics by NMR and fluorescence spectroscopies, and revealed a remarkable dependence on RNA sequence despite the fact that N has to encapsidate the entire viral genome. An integrated structural analysis of N0P comprising the full intrinsically disordered tails of P and N of more than 400 residues in length allowed us to delineate the importance of PTAIL within the assembly process beyond its first 50 residues combining information from NMR and fluorescence spectroscopies, as well as electron microscopy and small angle X-ray scattering. Since other negative strand RNA viruses share similar protein architectures, these results promise to have implications on a large number of important human pathogens.