The N terminus of Rift Valley fever virus nucleoprotein is essential for dimerization

Rift Valley fever virus (RVFV) is a Phlebovirus in the Bunyaviridae family. The nucleoprotein N is the most abundant component of the virion; numerous copies of N associate with the viral RNA genome and form pseudohelicoidal ribonucleoproteins (RNPs) circularized by a panhandle structure formed by the base-paired RNA sequences at the 3' and 5' termini. These structures play a central role in transcription and replication. We investigated the intermolecular interactions of the RVFV N protein and found that after chemical cross-linking treatment, the nucleoprotein from purified RNPs migrates mainly as dimers. The N-N interaction was studied using the yeast two-hybrid system, the GST pull-down method, and mutational analysis. We demonstrated that the N terminus from residue 1 to 71, and particularly Tyr 4 and Phe 11, which are conserved among phlebovirus N sequences, are involved in the interaction. The C-terminal region did not seem to be essential for the N-N interaction. Moreover, we showed that N(TOS), the N protein of the related Toscana phlebovirus, interacts with itself and forms heterodimers with N(RVF), suggesting that the dimeric form of N may be a conserved feature in phlebovirus RNPs.