Oligomerization paths of the nucleoprotein of influenza A virus

The influenza viruses contain a segmented, negative strand RNA genome. Each RNA segment is covered by multiple copies of the nucleoprotein (NP) and is associated with the polymerase complex into ribonucleoprotein (RNP) particles. Despite its importance in the virus life cycle, the interactions between the NP and the genome are not well understood. Here, we studied the assembly process of NP-RNA oligomers and analyzed how the oligomeric/monomeric status of RNA-free NP affects RNA binding and oligomerization. Recombinant wild-type NP purified in low salt concentrations and a derived mutant engineered for oligomerization deficiency (R416A) were mainly monomeric in RNA-free solutions as shown by biochemical and electron microscopy techniques. NP monomer formed with RNA a fast 1/1 complex characterized by surface plasmon resonance. In a subsequent and slow process that depended on the RNA length, oligomerization of NP was mediated by RNA binding. In contrast, preparations of wild-type NP purified in high salt concentrations as well as mutant Y148A engineered for deficiency in nucleic acid binding were partly or totally oligomeric in RNA-free solutions. These trimer/tetramer NP oligomers bind directly as oligomers to RNA with a higher affinity than that of the monomers. Both oligomerization routes we characterized could be exploited by cellular or viral factors to modulate or control viral RNA encapsidation by NP.