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Semliki Forest Virus (SFV), a member of the Alphavirus genus in the Togaviridae family, is a small-enveloped, positive-sense single-stranded RNA (+ssRNA) virus. The virus is spread by mosquitos and can infect humans, resulting in mild febrile disease with symptoms that include fever, myalgia, arthralgia, persistent headaches and asthenia.  Virulent strains of SFV in mice cause lethal encephalitis by infecting neurons in the central nervous system. In on-going experiments in the research group using a focused siRNA screen we have investigated the role of deubiquitylases (DUBs) during SFV infection (as a model alphavirus) and monitored the effect of DUB depletion on cell viability after infection. We identified a group of DUBs that have a pro-viral effect. The DUB, USP5, from this screen was validated to determine its effect upon viral replication. Here, we show that depleted USP5 in HeLa cells resulted in SFV RNA and viral yield at 8 h post-infection being significantly reduced. In the multi-step viral growth curve assay, in the absence of USP5, similar yields of SFV were determined at 2 and 4 h post-infection. However, a significant reduction in the infectious viral particles release at 6, 8, 10 and 12 h post-infection was observed and this could be reversed by direct constraining viral replication. These results raise the potential for USP5 to play a distinct role in the replication of SFV, suggesting that USP5 may be a possible anti-viral therapy for alphavirus infection.

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