NS2 is also described as nuclear export protein (NEP). In Influenza A virus and Influenza B virus, the NS2 protein (nuclear export protein) is proposed to mediate the nucleocytoplasmic trafficking of viral ribonucleoprotein (vRNP) by forming NS2-vRNP complexes. NS2 protein is a structural component of the viral particle and it associates with the viral matrix M1 protein. New and unexpected roles for NEP during the influenza virus life cycle have started to emerge. These recent studies have shown NEP to be involved in regulating the accumulation of viral genomic vRNA and antigenomic cRNA as well as viral mRNA synthesized by the viral RNA-dependent RNA polymerase.
Nonstructural protein 2 Proteins
NS2/NEP can be divided into a protease-sensitive N-terminal domain (amino acids 1–53) and a protease-resistant C-terminal domain (amino acids 54–121). The NES is proposed to interact with the cellular nuclear export protein Crm1 and is unusual in the sense that three of the five critical hydrophobic residues are methionines rather than the canonical leucine.NEP is phosphorylated during the influenza replication cycle. The phosphorylation of a highly conserved serine-rich motif (S23, S24, and S25) proximal to the NES has recently been demonstrated in virion-associated NEP.
The highly structured C-terminal domain consists of two ahelices C1 (amino acids 64–85) and C2 (amino acids 94–115) that are connected by a short interhelical turn. The two a-helices are comparable in length and interact extensively, forming an almost perfectly antiparallel hairpin. N-terminal domain effectively buries the hydrophobic face of the C-terminal hairpin.
Organisation and structure of influenza virus A NEP
NS2/NEP is proposed to play multiple biologically important roles during the influenza virus life cycle. NS2 is involved in nuclear export of viral ribonucleoprotein complexes. NEP was first thought to be nonstructural in function, however it is now recognised that NEP is resident within influenza virions where it may interact with M1. NS2 mediates the export of vRNPs from the nucleus to the cytoplasm through export signal via its interaction with XPO1, ensuring that the viral genomic segments are available for packaging into daughter virions on the cellular periphery.
Recent studies have suggested that NEP may have more than one function during the influenza virus replication cycle. NEP contributes to the viral budding process through interaction with a cellular ATPase. NEP is capable of regulating the accumulation of viral RNA species, potentially leading to a switch from viral transcription during early viral replication to favour the production of genomic vRNPs.
However, many functions of NS2, in particular its transit through the cytoplasm and its incorporation into the viral particle, are not understood.
• Duncan Paterson.Emerging Roles for the Influenza A Virus Nuclear Export Protein (NEP).PLOS Pathogens.2012
• M. Imai.Influenza B virus NS2, a nuclear export protein, directly associates with the viral ribonucleoprotein complex.Archives of Virology.2013
• Yong Hu.CHD3 facilitates vRNP nuclear export by interacting with NES1 of influenza A virus NS2.Cell. Mol. Life Sci.2015
• Benoıˆt de Chassey.The Interactomes of Influenza Virus NS1 and NS2 Proteins Identify New Host Factors and Provide Insights for ADAR1 Playing a Supportive Role in Virus Replication.PLOS Pathogens.2013