An investigation into the subcellular localisation of non-structural protein NS3 of African horsesickness virus

Abstract

African horsesickness virus (AHSV) is a double-stranded RNA virus belonging to the Orbivirus genus in the Reoviridae family (Bremer et al., 1990; Calisher and Mertens, 1998). The virus is highly pathogenic and its mortality rate in horses, the most susceptible species, may be as high as 95% (House, 1993). S10, the smallest genome segment of AHSV, codes for two proteins (NS3 and NS3A) from in-phase overlapping reading frames. The C-terminal sequences of these proteins are identical, but NS3A lacks the first 10 amino acids present on the N-terminal of NS3 (Van Staden and Huismans, 1991). Nonstructural protein NS3 is a membrane protein, associated with both smooth intracellular membranes and the plasma membrane. NS3 has pleiotropic roles in the viral life cycle including the transport and release of mature virions and viroporin-like alteration of cell membrane permeability. NS3 is cytotoxic when expressed in bacterial or insect cells, and is speculated to play a vital role in viral virulence and disease pathogenesis (Stoltz et al., 1996; Van Staden et al., 1995). A number of different domains that could mediate the membrane interaction or intracellular trafficking of NS3 have been identified. The relative contributions of these domains in insect and mammalian cells are not known, but could differ, as there are distinct differences in NS3 expression levels, cytopathic effects and virus release mechanisms in these two cell types. In order to investigate the subcellular localisation of NS3, a number of full-length, truncated or mutant versions of AHSV-3 NS3 were constructed as C-terminal eGFP (enhanced green fluorescent protein) fusion proteins. These proteins were used to generate recombinant baculoviruses for expression in Spodoptera frugiperda (Sf9) insect cells and were compared in terms of their subcellular localisation by conventional fluorescence microscopy. Confocal laser microscopy was used to investigate co-localisation with the nucleus, the Golgi apparatus and the Endoplasmic Reticulum (ER). Subcellular fractionations and membrane flotation analyses were used to confirm membrane interactions and to identify detergent-resistant membrane fractions. NS3 as well as a C-terminal deletion of NS3 targeting a putative dileucine motif both localised to cellular/nuclear membrane components. In contrast, site-specific mutations to either of the transmembrane domains abolished membrane association and resulted in cytoplasmic localisation. NS3A showed mixed results, displaying both membrane localisation and a cytoplasmic distribution. The 11 amino acid region unique to NS3 and absent from NS3A, which has been shown to bind to cellular exocytosis proteins in bluetongue virus (Beaton et al., 2002), did not display membrane interaction. These results indicate that both of the hydrophobic domains as well as the N11 region are required to be present for NS3 to be properly targeted to the plasma/nuclear membrane. In addition, NS3 was shown to be present in detergent-resistant membrane fractions, indicative of a possible localisation within lipid rafts. The above results indicate that the NS3 protein contains specific signals involved in membrane targeting, confirming a potential role for NS3 in viral localisation and release in the AHSV replication cycle.