Rift valley fever virus L protein forms a biologically active oligomer

Rift Valley fever virus (RVFV) (genus Phlebovirus, family Bunyaviridae) causes mosquito-borne epidemic diseases in humans and livestock. The virus carries three RNA segments, L, M, and S, of negative or ambisense polarity. L protein, an RNA-dependent RNA polymerase, encoded in the L segment, and N protein, encoded in the S segment, exert viral RNA replication and transcription. Coexpression of N, hemagglutinin (HA)-tagged L, and viral minigenome resulted in minigenome replication and transcription, a finding that demonstrated HA-tagged L was biologically active. Likewise L tagged with green fluorescent protein (GFP) was biologically competent. Coimmunoprecipitation analysis using extracts from cells coexpressing HA-tagged L and GFP-tagged L showed the formation of an L oligomer. Bimolecular fluorescence complementation analysis and coimmunoprecipitation studies demonstrated the formation of an intermolecular L-L interaction through its N-terminal and C-terminal regions and also suggested an intramolecular association between the N-terminal and C-terminal regions of L protein. A biologically inactive L mutant, in which the conserved signature SDD motif was replaced by the amino acid residues GNN, exhibited a dominant negative phenotype when coexpressed with wild-type L in the minigenome assay system. Expression of this mutant L also inhibited viral gene expression in virus-infected cells. These data provided compelling evidence for the importance of oligomerization of RVFV L protein for its polymerase activity.