Subcellular localization and interactions of Infectious Salmon Anemia Virus (ISAV) M1 and NEP as well as host Hsc70

BACKGROUND: Infectious salmon anemia virus (ISAV) is an important fish pathogen that causes high mortality in farmed Atlantic salmon. The ISAV genome consists of eight single-stranded, negative-sense RNA segments. The six largest segments contain one open reading frame (ORF) each, and encode three polymerase proteins, nucleoprotein, fusion protein, and hemagglutinin esterase protein. The two smallest segments contain more than one ORF each. The segment 7 encodes non-structural protein 1 (NS1) and nuclear export protein (NEP), while segment 8 encodes matrix protein 1 and 2 (M1 and M2). NS1 and M2 have been well known as antagonist of type I interferon. However, little is known about the characterization of M1 or NEP. In addition, heat shock cognate 70 (Hsc70) has been reported to interact with M1 and NEP of influenza viruses for the export of viral ribonucleoprotein (vRNP) via vRNP-M1-NEP complex, the goal of this study therefore was to characterize the subcellular localization and interactions of ISAV M1 and NEP as well as cellular Hsc70. RESULTS: When M1, NEP, and Hsc70 were individually expressed in the stripped snakehead (SSN-1) cells, we found that M1 protein was localized in both cytosol and nucleus of the cells, NEP was localized only in the cytosol and accumulated adjacent to the nucleus, while Hsc70 was localized throughout the cytosol, but not in the nucleus. However, when two of them were co-expressed, we found that both M1 and Hsc70 were co-localized with NEP in the cytosol and accumulated adjacent to the nucleus, while M1 and Hsc70 were still localized as they were expressed individually. Furthermore, pull-down assay was performed and showed that NEP could interact with both M1 and Hsc70, and M1-Hsc70 interaction was also observed although the interaction was weaker than that of NEP-Hsc70. CONCLUSION: Our study characterized the subcellular localization and interactions of three proteins including M1 and NEP of ISAV, and Hsc70. These data will help towards a better understanding of the life cycle of ISAV, especially the process of vRNP export.