The Role of the Deubiquitylase MYSM1 During Alphavirus Infection Amer Nubgan The members of the genus Alphavirus are positive-sense RNA viruses and it is one of two within the family Togaviridae. Most alphaviruses are predominantly transmitted to susceptible vertebrates by a mosquito vector. Alphavirus disease in humans can be severely debilitating, and depending on the particular viral species, infection may result in encephalitis and possibly life threatening symptoms. Chikungunya virus (CHIKV) is the aetiological agent represents a substantial health burden to affected populations, with clinical symptoms that include severe joint and muscle pain, rashes, and fever, as well as prolonged periods of disability in some patients. In recent years, CHIKV has received significant attention from public health authorities as a consequence of the dramatic emergence infections in the Indian Ocean islands and the Caribbean as well as the recent emergence of CHIKV in the Americas. Infections have also been reported around Europe such as in Italy, France and Greece. Currently, no safe, approved or effective vaccine or treatment exists for CHIKV infection. The ubiquitin-proteasome system (UPS), the major intracellular proteolytic pathway, mediates different kinds of cellular processes, which may be targeted by viruses to aid their replication within cells. In recent years it has been well established that both the forward reaction of ubiquitination, and the reverse reaction of deubiquitination are targeted during virus infection to enhance their replication, either by targeting of cellular proteins or encoding viral homologues of key pathway proteins. The reverse reaction is undertaken by a large family of enzymes termed deubiquitylases or DUBs, and many of these have been shown to play a crucial role, not only in virus replication but also in the regulation of the immune system and vesicle trafficking. The DUBs are attractive drug targets and have increasingly been implicated in cellular processes germane to malignancy which makes the continued characterisation of the role of DUBs during virus infection a worthwhile objective. In on-going experiments in the research group a DUB siRNA pools library screen identified 12 DUBs (USP1, USP4, USP5, USP34, USP45, USP46, OTUD6A, UCHL1, JOSD2, BRCC3 and MYSM1). Depletion of these hits in HeLa cells lead to an increase in cell viability following Semiliki Forest Virus (SFV) infection (and predicted to be pro-viral) and thus could potential be candidate antiviral targets. Inroads into understanding the role of the DUB hits during the alphavirus infection, focusing initial on the BSL2 model virus SFV, and extending this to CHIKV (at BSL3). In the present study, further screening focused on the deconvolution siRNA pools for the DUB hits. Investigation of the subsequent follow up experiments with one strong candidate DUB from this list, MYSM1. Two different approaches were taken. Firstly, the effect of depletion of MYSM1 by siRNA treatment was further investigated in HeLa cells. Secondly, the analysis was extended to investigate the role of MYSM1 in fibroblasts utilising MYSM1 genetic knockout murine embryo fibroblasts. Results from this study indicate that depletion of MYSM1 in HeLa cells by siRNAs resulted in a reduction in both SFV and CHIKV replication, as assayed by measuring RNA levels and plaque formation. It was also found that MYSM1 genetic knockout in MEF cells lead to increase in both SFV and CHIKV replication. In addition, depletion of MYSM1 by siRNAs in MRC-5 cells lead to increase in SFV replication. In conclusion, MYSM1 generated interesting data, implying a role during virus infection that appeared to depend on the cell type being infected. Up to now it is unclear what the effector mechanisms are that contribute to these observations, subject to further mechanistic and functional studies, may increase the options available for targeting this vital DUB during Alphavirus infections.