Structural studies of bunyavirus interferon antagonist proteins

Abstract

Bunyaviridae is one of the biggest known viral families, and includes many viruses of clinical and economic importance. The major virulence factor of most bunyaviruses is the non-structural protein (NSs). NSs is expressed early in infection and inhibits the innate immune response of the host by blocking several steps in the interferon induction and signalling pathways. Hence, NSs significantly contributes to the establishment of a successful viral infection and replication, persistent infection and the zoonotic capacity of bunyaviruses. Although functions and structures of many viral interferon antagonists are known, no structure of a bunyavirus NSs protein has been solved to date. This strongly limits our understanding of the role and the mechanism of interferon antagonism in this large virus family. In this work the first structure for a bunyavirus interferon antagonist, the core domain crystal structure of NSs from the Rift Valley fever virus (RVFV) is presented. RVFV is one of the most clinically significant members of the Bunyaviridae family, causing recurrent epidemics in Africa and Arabia, often featuring high-mortality haemorrhagic fevers. The structure shows a novel all-helical fold. The unique molecular packing of NSs in the crystal creates stable fibrillar networks, which could correspond to the characteristic fibrillation of NSs observed in vivo in the nuclei of RVFV infected cells. This first NSs structure might be a useful template for future structure-aided design of drugs that target the RVFV interferon antagonism. Attempts at characterising other bunyavirus NSs proteins of other genera were made, but were hampered by problems with obtaining sufficient amounts of soluble and folded protein. The approaches that proved unsuccessful for the solubilisation of these NSs proteins, however, should inform future experiments aimed at obtaining recombinant NSs for structural studies.