The Rift Valley fever accessory proteins NSm and P78/NSm-G_N are distinct determinants of virus propagation in vertebrate and invertebrate hosts

Abstract

Rift Valley fever virus (RVFV) is an enzootic virus circulating in Africa that is transmitted to its vertebrate host by a mosquito vector and causes severe clinical manifestations in humans and ruminants. RVFV has a tripartite genome of negative or ambisense polarity. The M segment contains five in-frame AUG codons that are alternatively used for the synthesis of two major structural glycoproteins, G_N and G_C, and at least two accessory proteins, NSm, a 14-kDa cytosolic protein, and P78/NSm-G_N, a 78-kDa glycoprotein. To determine the relative contribution of P78 and NSm to RVFV infectivity, AUG codons were knocked out to generate mutant viruses expressing various sets of the M-encoded proteins. We found that, in the absence of the second AUG codon used to express NSm, a 13-kDa protein corresponding to an N-terminally truncated form of NSm, named NSm′, was synthesized from AUG 3. None of the individual accessory proteins had any significant impact on RVFV virulence in mice. However, a mutant virus lacking both NSm and NSm′ was strongly attenuated in mice and grew to reduced titers in murine macrophages, a major target cell type of RVFV. In contrast, P78 was not associated with reduced viral virulence in mice, yet it appeared as a major determinant of virus dissemination in mosquitoes. This study demonstrates how related accessory proteins differentially contribute to RVFV propagation in mammalian and arthropod hosts.

• arbovirus
• attenuation
• bunyavirus
• transmission
• minor structural protein
• non-structural protein
• vector
• virulence factor

The authors acknowledge Agnès Billecocq and Xavier Carnec (Molecular Genetics of Bunyaviruses, Institut Pasteur, France) for their initial help in using the reverse genetics system. The authors also wish to thank Félix Rey (Head of the Structural Virology Unit, Institut Pasteur, France), François Rougeon (Professor Emeritus, Institut Pasteur) and Xavier Montagutelli (Head of the Central Animal Facilities, Institut Pasteur, France) for their support and critical reading of the manuscript. This work was funded by the Agence Nationale de la Recherche (grant NO 11-BSV3-007 01).

Note:Supplementary Information for this article can be found on Emerging Microbes and Infections' website (http://www.nature.com/emi/).