Protease and helicase domains are related to the temperature sensitivity of wild-type rubella viruses

Replication capacity and adaptability of a severe fever with thrombocytopenia syndrome virus at different temperatures

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging disease caused by the SFTS virus (SFTSV). Although fever and thrombocytopenia are the typical manifestations of SFTS, a specific SFTS case with no fever was observed in Zhejiang, China. In this report, we aimed to explore the probable reason for the absence of fever by analyzing the genetic characteristics and temperature sensitivity (ts) of the SFTSV strain ZJ2013-06, which was isolated from the specific case. Phylogenetically, different clusters of SFTSV strains circulated in Zhejiang. ZJ2013-06 was farthest from ZJ2014-02, an isolate belonging to a Chinese dominant cluster, and nearest to the coastal strain NB24/CHN/2013. Ts tests, performed on Vero cells at 37°C and 39°C, indicated that ZJ2013-06 had restricted replication at 39°C. Its viral loads were substantially reduced at 39°C compared with that at 37°C (approximately 100-fold reduction) and were significantly lower than that of ZJ2014-02 at 39°C (P < 0.01). By adaptive culture at 39°C, the induced strain ZJ2013-06-P7 was obtained. Owing to a reverse mutation (S1616), ZJ2013-06-P7 lost the ts of the original strain, displaying similar replication processes with NB24/CHN/2013. The results indicated that the amino acid residue 1616 was related to the ts characteristics of ZJ2013-06. Our study revealed that ZJ2013-06 was temperature-sensitive and may be related to the absence of fever in our case.

Mechanisms of temperature sensitivity of attenuated Urabe mumps virus

Temperature sensitivity is a phenotype often associated with attenuation of viruses. Previously, we purified several mumps variants from an incompletely attenuated Urabe strain live attenuated vaccine. Here we characterize one isolate that is sensitive to growth at high temperature. This virus was attenuated in a small animal model of mumps virulence, and we identified unique coding substitutions in the hemagglutinin-neuraminidase (HN), the viral polymerase (L) gene, and a non-coding substitution close to the anti-genome promoter sequences. At the non-permissive temperature, transcription of viral mRNAs and production of the replication intermediate were reduced compared to events at the permissive temperature and to a non-ts virulent Urabe virus. As well, synthesis of viral proteins was also reduced at the higher temperature. While the actual sequence substitutions in the ts virus were unique, the pattern of substitutions in HN, L and genome end sequences is similar to another attenuated Urabe virus previously described by us.

Short self-interacting N-terminal region of rubella virus capsid protein is essential for cooperative actions of capsid and nonstructural p150 proteins

Nucleocapsid formation is a primary function of the rubella virus capsid protein, which also promotes viral RNA synthesis via an unknown mechanism. The present study demonstrates that in infected cells, the capsid protein is associated with the nonstructural p150 protein via the short self-interacting N-terminal region of the capsid protein. Mutational analyses indicated that hydrophobic amino acids in this N-terminal region are essential for its N-terminal self-interaction, which is critical for the capsid-p150 association. An analysis based on a subgenomic replicon system demonstrated that the self-interacting N-terminal region of the capsid protein plays a key role in promoting viral gene expression. Analyses using a virus-like particle (VLP) system also showed that the self-interacting N-terminal region of the capsid protein is not essential for VLP production but is critical for VLP infectivity. These results demonstrate that the close cooperative actions of the capsid protein and p150 require the short self-interacting N-terminal region of the capsid protein during the life cycle of the rubella virus.

IMPORTANCE

The capsid protein of rubella virus promotes viral RNA replication via an unknown mechanism. This protein interacts with the nonstructural protein p150, but the importance of this interaction is unclear. In this study, we demonstrate that the short N-terminal region of the capsid protein forms a homo-oligomer that is critical for the capsid-p150 interaction. These interactions are required for the viral-gene-expression-promoting activity of the capsid protein, allowing efficient viral growth. These findings provide information about the mechanisms underlying the regulation of rubella virus RNA replication via the cooperative actions of the capsid protein and p150.

[Virology of the family Togaviridae]

Many pathogens important for medicine, veterinary medicine or public health belong to the genera alphavirus and rubivirus within the family Togaviridae. 29 species of alphaviruses have been reported, and most of them are arboviruses. Chikungnya virus re-emerged in Kenya in 2004 and the epidemics spread to the Indian Ocean islands and many countries in South Asia, South-East Asia and Europe. On the other hand, rubella virus, a sole member of the genus rubivirus, is the causative agent of rubella and congenital rubella syndrome (CRS). Because human is only a natural host of the virus and effective live attenuated vaccines are available, immunization activities are strengthened globally to eliminate rubella and CRS, together with measles.