Influenza B virus NS2, a nuclear export protein, directly associates with the viral ribonucleoprotein complex

In Influenza A virus and Influenza B virus, the NS2 protein (nuclear export protein) has been proposed to mediate the nucleocytoplasmic trafficking of viral ribonucleoprotein (vRNP) by forming NS2-vRNP complexes. While the binding interactions of NS2 in these complexes have been well characterized for Influenza A virus, much less is known about Influenza B virus NS2 (B/NS2). In this report, we developed a specific antiserum against B/NS2 protein and demonstrated that B/NS2 was synthesized late in infection and packaged into virions after nucleocytoplasmic transport. Fractionation of detergent-disrupted virions in several conditions showed that B/NS2 remained associated with vRNP after separation of matrix protein M1 from vRNP, whereas Influenza A virus NS2 (A/NS2) was easily separated from vRNP and remained associated with M1, in accord with previous findings that A/NS2 associates with vRNP only through its binding of encapsidated M1. The results indicated that complex formation among vRNP, M1 and NS2 of Influenza B virus was different from that of Influenza A virus, and that B/NS2 associated with vRNP in the absence and presence of M1.