Studies on the intranuclear localization of influenza virus-specific proteins.

The major influenza virus-specific proteins appearing in the nucleus of infected cells are the virion nucleocapsid protein (NP) and a putative nonstructural protein (NS₁) of approximately 25,000 daltons. In the present study, we have determined the intranuclear localization of NP and NS₁ in two cell lines, canine kidney cells (MDCK cells) and HeLa cells. Nuclear fractionation was monitored by using ribosomal precursor RNA and heterogeneous nuclear RNA as markers for nucleolar and nucleoplasmic components, respectively; and nucleoli were purified by equilibrium centrifugation in Renografin gradients.The purified nucleoli contained essentially no NP protein, indicating that this protein is mostly, if not totally, nucleoplasmic in location. Some of the nuclear NS₁ remained associated with the purified nucleoli. Of the NS₁ in the nucleus of infected HeLa cells, 35% remained with the nucleoli, whereas in MDCK cells only 6% remained. Reconstruction experiments further suggest that the NS₁ found tightly associated with nucleoli did not arise as an artifact of nuclear fractionation.