Investigating two AHSV non-structural proteins : tubule-forming protein NS1 and novel protein NS4

Abstract

African horse sickness is an equid disease caused by African horse sickness virus (AHSV). AHSV produces seven structural proteins that form the virion and four non-structural proteins with various roles during replication. The first part of this study investigated the intracellular distribution and co-localisations of NS1 with other AHSV proteins to facilitate its eventual functional characterisation. Confocal microscopy revealed that NS1 formed small cytoplasmic foci early after infection that gradually converged into large fluorescent NS1 tubule bundles. Tubule bundles were more organised in AHSV-infected cells than in cells expressing NS1 alone, suggesting that tubule bundle formation requires the presence of other AHSV proteins or regulation of NS1 expression rates. NS1 occasionally co-localised with VP7 crystalline structures, independently of other AHSV proteins. However, when NS1-eGFP, a modified NS1 protein that contains enhanced green fluorescent protein (eGFP) near the C-terminus, was co-expressed with VP7, co-localisation between these proteins occurred in most co-infected cells. It is not clear how the addition of eGFP to NS1 induces this co-localisation and further investigation will be required to determine the function of NS1 during viral replication. The second part of the study focused on characterising the novel non-structural AHSV protein NS4. The NS4 open reading frame (ORF) occurs on segment 9, overlapping the VP6 ORF in a different reading frame. In silico analysis of segment 9 nucleotide and NS4 predicted amino acid sequences revealed a large amount of variation between serotypes, and two main types of NS4 were identified based on these analyses. These proteins differed in length and amino acid sequence and were named NS4-I and NS4-II. Immunoblotting confirmed that AHSV NS4 is translated in AHSV infected insect and mammalian cells, and also in Sf9 insect cells infected with recombinant baculoviruses that overexpress the genome segment 9 proteins, VP6 and NS4. Confocal microscopy showed that NS4 localised to both the cytoplasm and nucleus, but not the nucleolus, in AHSV-infected cells and recombinant baculovirus infected Sf9 cells. Nucleic acid protection assays using bacterially expressed purified NS4 showed that both types of NS4 bind dsDNA, but not dsRNA. This was the first study to focus on AHSV NS4. Future work will focus on determining the role of non-structural proteins in viral pathogenesis, and will involve the use of a reverse genetics system for AHSV.