The fusogenic avian Orthoreoviruses of Reoviridae family are important pathogens of birds that can cause considerable economic losses in the poultry industry. Avian reoviruses have been associated with a variety of disease conditions in poultry (Jones 2000). The avian reovirions are non-enveloped icosahedral particles of 85 nm external diameter with 10 dsRNA genomic segments (23.5 kb) encased within two concentric protein shells, forming the outer capsid and the core (Zhang et al. 2005). The RNA replication and morphogenesis of reoviruses occurs exclusively within cytoplasmic inclusion bodies, also known as viral factories, or ‘viroplasms’. The viroplasms are formed by non-structural protein μNS in association with non-structural protein σNS (Benavente and Martínez-Costas 2007). The σNS acts as RNA chaperone and destabilizes helical regions of RNAs. The structure is not known. The σNS protein was constructed in order to study the process of the viroplasm formation in details. The σNS is a non-structural protein approximately 41 kDa large and is composed of 367 amino acids. The homology modelling by Phyre2 prediction server estimated a high α- helical structure (Dermody et al. 2013). The SAXS experiments revealed the elongated pear- shaped structure. The σNS protein is homodimer as a biological unit with high probable further hexamerisation. It forms likely octamers in the presence of bound ssRNA in solution by hydrophobic interactions. The σNS rapidly bind ssRNA in a sequence-independent manner and then form large nucleoprotein complex (Borodavka et al. 2015).
Here we report the results of the expression, purification and further crystallization experiments of the σNS protein. Subsequent monitoring of purified samples revealed that protein is quite stable and in presence of ssRNAs forms octamers. In other case, σNS forms pseudocapsids that were observed by EM negative staining.