Comparative localisation and trafficking of wild-type and mutant NS3 proteins of African horsesickness virus

Abstract

African horsesickness (AHS) is a non-contagious, infectious illness of equids, caused by African horsesickness virus (AHSV). The disease causes symptoms ranging from mild fever to severe infection and death in its mammalian host, whilst the insect vector that transmits the virus remains unaffected. The AHSV protein believed to play an important role in cytotoxicity and pathogenesis is the non-structural protein NS3. In Bluetongue virus (BTV) it has been shown that the N- and C-terminal domains of NS3 interacts with VP2 on the outer capsid of newly formed virions, whilst the C-terminal interacts with host cell proteins in order to establish trafficking to the plasma membrane. Once at the cell membrane, virions are released from host cells either through viral budding or by means of cell lysis depending on the host cell type. Previous studies have identified a number of conserved regions in the NS3 protein. In this study, various mutant AHSV NS3 constructs were compared through membrane association assays and subcellular localisation and trafficking. Some mutants contained substitutions in the conserved regions of the protein to determine the role of these regions in protein trafficking whilst other NS3 constructs contained an eGFP tag on either the N- or C-terminus of the protein to establish the effects of the reporter molecule on localisation and trafficking. All of the mutant proteins showed altered localisation patterns, confirming that these regions play an important functional role in NS3 protein trafficking. Surprisingly, not all the eGFP proteins localised the same as their non-eGFP counterparts suggesting that caution should be taken when studying localisation with reporter molecules. Colocalisation assays were performed with NS3 and a number of cell organelle markers in order to determine if NS3 uses the cell secretory pathway for trafficking to the plasma membrane. In this pathway, membrane proteins are inserted into the lipid bilayer of the endoplasmic reticulum and buds from one organelle to the next until the protein reaches its target destination (plasma membrane). It was found that NS3 partially colocalises with the Golgi apparatus which suggest that NS3 traffics through this organelle toward its target destination. By means of pathway inhibition of the ER to Golgi transport, NS3 trafficking to the cell membrane was repressed confirming that NS3 uses the cell secretory pathway. This was found in both insect and mammalian cell lines. Copyright