Structure-function relationship in the 'termination upstream ribosomal binding site' of the calicivirus rabbit hemorrhagic disease virus

Caliciviruses use a termination/reinitiation mechanism for translation of their minor capsid protein VP2. A sequence element of about 80 nucleotides denoted ‘termination upstream ribosomal binding site’ (TURBS) is crucial for reinitiation. RNA secondary structure probing and computer aided secondary structure prediction revealed a rather low degree of secondary structure determinants for the TURBS of the rabbit hermorrhagic disease virus. Mutation analysis showed that prevention of duplex formation had major impact on the VP2 expression levels. Restoration of complementarity of the respective sequences by reciprocal mutation at least partially restored reinitiating rates. Synthetic TURBS structures preserving only the secondary structure forming sequences and the known short motifs important for TURBS function were found to drive reinitiation when the altered sequence could be predicted to allow establishment of the crucial secondary structures of the TURBS.

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

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.

Role of the cytosolic tails of Rift Valley fever virus envelope glycoproteins in viral morphogenesis

The correct folding, heterodimerization and trafficking of Gn/Gc envelope glycoproteins of Rift Valley fever virus, RVFV (Bunyaviridae and Phlebovirus genus) are essential for Golgi assembly and budding of viral particles. The Gn and Gc carboxy-terminus contain a Golgi targeting and an ER-retrieval signal, respectively. We generated RVFV-like particles with mutations in the cytosolic tails of Gn or Gc and identified regions important for release of infectious particles. The role of specific amino-acids in these regions was further investigated by creating recombinant mutant viruses by reverse-genetics. Residues outside the suspected Golgi targeting motif, i.e. the di-lysine K29-K30 motif and the N43, R44 and I46 residues of the Gn cytosolic domain, appeared important for Golgi localization and RNP packaging. Concerning the Gc tail, replacement of K2 or K3 in the di-lysine motif, had a drastic impact on Gn trafficking and induced an important organelle redistribution and cell remodeling, greatly affecting particle formation and release.