Characterisation of an avian influenza virus nucleoprotein expressed in E. coli and in insect cells

Infectious salmon anemia virus (ISAV) genomic segment 3 encodes the viral nucleoprotein (NP), an RNA-binding protein with two monopartite nuclear localization signals (NLS)

Infectious salmon anemia virus (ISAV) is the type species of the genus Isavirus belonging to the Orthomyxoviridae, and causes serious disease in Atlantic salmon (Salmo salar). This study presents the expression and functional analysis of the ISAV genome segment 3, and provides further evidence that it encodes the viral nucleoprotein (NP). The encoded protein was expressed in a baculovirus system, and Western blot analysis showed that it corresponds to the 66-71 kDa structural protein previously found in purified ISAV preparations. RNA-binding activity was established by the interaction of viral and recombinant NP with single-stranded RNA transcribed in vitro. Immunofluorescence studies of infected cells showed the ISAV NP to be an early protein. It locates to the nucleus of infected cells before it is transported to the cytoplasm prior to virus assembly. A similar localization pattern was observed in cells transfected with the NP gene, confirming that the encoded protein has an intrinsic ability to be imported into the nucleus. Two monopartite nuclear localization signals (NLS) at amino acids (230)RPKR(233) and (473)KPKK(476) were identified by computer analysis, and validated by site-directed mutagenesis. In contrast to other orthomyxovirus-NPs, that have several NLSs that function independent of each other, both NLSs had to be present for the ISAV NP protein to be transported into the nucleus, indicating that these motifs cooperate to target the protein to the nucleus.

Expression of influenza neuraminidase in baculovirus-infected cells

Recombinant influenza neuraminidase (NA, subtype 2, A/NT/60/68) was produced by recombinant baculovirus-infected insect cells. The recombinant NA retained enzyme activity and was located on the cell surface. Enzyme activity was both cell-associated and in the cell free supernatant; maximal NA activity was found in the supernatant. Recombinant NA was recognised by polyclonal antisera and by three monoclonal antibodies specific for NA (subtype 2). Enzyme activity was also neutralised by polyclonal antisera. Recombinant NA thus retains most of the immunological and activity properties of authentic influenza NA. Immunoprecipitation of [35S]Methionine-labelled cells and supernatant and partial purification of NA indicated that a approximately 50-kDa form of NA was present in the supernatant, whilst the expected size (approximately 67-kDa) was cell-associated. Purified recombinant extracellular virus was also enzymatically active, and contained the 67-kDa NA which was located on the membrane capsule of the virus. This suggests that the virus had acquired the cell-associated form of recombinant NA during the budding process from the cell.