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, GN and GC, and at least two accessory proteins, NSm, a 14-kDa cytosolic protein, and P78/NSm-GN, 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.

The Envelope Proteins of the Bunyavirales

“…In all cases, the first cleavage is carried out by signalase in the ER. The M segment of the phlebovirus prototype, RVFV (fourth row), is unusual because it contains several translational start codons that are alternatively used to produce NSm, Gn and Gc, or p78 and Gc, where p78 encompasses the NSm-Gn fusion and has a determinant role in virus dissemination within mosquitoes (Kreher et al, 2014). Tick-borne phleboviruses (third row) do not produce NSm, have a single start-codon and their Gn corresponds to RVFV p78.…”

Bunyaviruses: From Transmission by Arthropods to Virus Entry into the Mammalian Host First-Target Cells

“…The expression of this protein, also referred to as P78/NSm-G N , actually results from the translation of an alternative ORF overlapping both sequences of NSm and G N in the M segment [126,127]. While its function remains unclear, P78/NSm-G N appears critical for virus production in insect cells and dispensable in mammalian cells [129]. This makes P78/NSm-G N a distinct determinant of virus propagation in insect vectors and mammalian hosts.…”

Interplay between the Virus and Host in Rift Valley Fever Pathogenesis

“…Although phagocytes can actively intake any invaders, RVFV is able to replicate in macrophages in vitro and in vivo [6, 13, 14, 15, 16, 17, 18]. In human monocyte derived macrophages, secretion of tumor necrosis factor (TNF)-α, interferon (IFN)-β, and IFN-α2 was induced by infection of a virus lacking NSs, but not by wild-type RVFV, indicating that NSs inhibited the secretion of these proteins in wild-type RVFV-infected macrophages [17].…”

“…These 5 AUGs are highly conserved among RVFV strains [47]. A recent study revealed that a truncated NSm protein called NSm' is expressed from the 3 rd AUG; NSm' lacks the N-terminal 38 amino acids of NSm and is predominantly expressed compared to NSm in infected Vero E6 cells, when both the 2 nd and 3 rd AUG are intact [15]. The C-terminal region of NSm, which contains a basic amino acid cluster and a putative transmembrane domain, targets the protein to the mitochondorial outer membrane [48].…”

“…The C-terminal region of NSm, which contains a basic amino acid cluster and a putative transmembrane domain, targets the protein to the mitochondorial outer membrane [48]. Since the C-terminal region is common in NSm and NSm', NSm' is also localized to mitochondria [15]. …”