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Immune evasion function and nucleocytoplasmic trafficking of accessory proteins of new and emerging paramyxoviruses
thesisposted on 22.02.2017, 02:20 by Audsley, Michelle Dianne
The Paramyxovirinae subfamily contains more than 30 viruses, a number of which represent major human health threats, including the pathogenic bat-borne henipaviruses, Nipah virus (NiV) and Hendra virus (HeV). Pathogenicity and host-specificity of paramyxoviruses has been linked to the functions of the viral “accessory” proteins V, W and C, particularly through their inhibition of host type I interferon (IFN) antiviral responses. IFN-antagonist functions of V protein have been characterised in detail for a number of paramyxoviruses, but this has been largely limited to the earliest discovered members of the family and/or members considered medically important. For the majority of new/emerging paramyxoviruses, functional evaluation of the accessory proteins remains superficial or non-existent. This thesis describes the first functional characterisation of accessory proteins from paramyxoviruses of a proposed new genus “Jeilongvirus”. Antagonism of signal transducers and activators of transcription (STAT) 1 and/or STAT2 is a common function of paramyxovirus V proteins. However, while IFN-activated nuclear translocation of STAT1 was inhibited by jeilongvirus infection, this was not attributable to V protein. Thus, it appears that jeilongvirus IFN antagonism strategies diverge from other paramyxoviruses. Nevertheless, the jeilongvirus V proteins were found to interact with melanoma differentiation associated protein 5 to inhibit induction of type I IFN, contributing to the large body of evidence indicating that this is a critical function of V. Nucleocytoplasmic transport of paramyxovirus accessory proteins has been previously suggested to play key roles in IFN antagonism, but understanding of the underlying mechanisms remains incomplete, and is critical to assessing roles in pathogenicity. Thus, a detailed characterisation was undertaken of henipavirus V and W nucleocytoplasmic trafficking mechanisms. It was previously thought that henipavirus V and W proteins localise to the cytoplasm and nucleus, respectively, due to a single nuclear export signal in V and a single nuclear localisation signal in W. The findings of this study substantially alter this model, showing that henipavirus V proteins can undergo importin α1-dependent nuclear import and that henipavirus W proteins can undergo exportin-1-mediated nuclear export. Together this indicates that henipavirus V and W proteins can undergo facilitated nucleocytoplasmic trafficking in both directions, suggestive of active shuttling. To identify the extent to which nucleocytoplasmic trafficking is conserved in paramyxovirus accessory proteins, the functionally divergent jeilongviruses and two rubulaviruses were also assessed. The jeilongvirus and rubulavirus V proteins localised in both the cytoplasm and the nucleus, distinct from henipavirus V proteins that appear excluded from the nucleus at equilibrium. However, similarly to the henipaviruses, the W proteins of the jeilongviruses localised almost exclusively within the nucleus. These results indicate that mechanisms of V and W nucleocytoplasmic trafficking can diverge significantly between genera, but suggests that the capacity to actively traffic between the nucleus and cytoplasm is common to multiple genera. Taken together, these data contribute significantly to understanding the conservation/diversity of paramyxovirus accessory protein functions. The results from this study will inform future work to elucidate roles of accessory proteins in pathogenicity and the potential of targeting their functions for development of antiviral therapeutics.