Structure and dynamics of the nucleocapsid-binding domain of the Sendai virus phosphoprotein in solution

The RNA-dependent RNA polymerase of the Sendai virus (SeV) consists of the large protein (L) and the phosphoprotein (P). P plays a crucial role in the enzyme by positioning L (which carries the polymerase activity) onto the matrix for transcription and replication formed by the RNA and the nucleoprotein, the N-RNA. P has a modular structure with distinct functional domains: an N-terminal domain involved in binding to N degrees (N that is not yet bound to RNA) and a C-terminal domain that carries the oligomerisation domain, the N-RNA binding domain and the L binding domain and that, combined with L, is active in transcription. Structural data have previously been obtained on the N-terminal domain and on the oligomerisation domain of P, but not yet on its N-RNA binding domain (also-called the X protein). Here we present an NMR and a small angle neutron scattering study of the SeV X protein. We show that this molecule presents two subdomains linked by an 11-residue linker, with the N-subdomain lacking a well-defined conformation. The 3D structure of the C-subdomain consists of three alpha-helices revealing an asymmetric charge distribution that may be important for binding to RNA-bound nucleoprotein. The structure of the entire C-terminal domain of P is modelled from its constituent parts in combination with small angle scattering data on this domain.