Cytoplasmic and nuclear virus-specific proteins in influenza virus-infected MDCK cells

Four virus-specific proteins were identified in nuclei purified from influenza virusinfected cells: the nucleocapsid protein (NP); two nonstructural proteins of 24,500 daltons (NS₁) and 11,000 daltons (NS₂) molecular weight; and the largest virion protein, protein P. The nucleocapsid protein was found to be distributed approximately equally between the nucleoplasmic and nucleolar fractions, whereas most of the other virus-specific proteins were associated with the nucleolar fraction. In the nucleoplasm, most, if not all, of the nucleocapsid protein was in the form of viral ribonucleoproteins.In addition to being in the nucleus, NS₁ has been found concentrated in cytoplasmic fractions containing polyribosomes and ribosomes (Pons, 1972; Compans, 1973). This suggested that NS₁ might be associated with ribosomes as a structural ribosomal protein and that, therefore, NS₁ was in the nucleolus for the purpose of being incorporated into newly assembled ribosomal subunits. Our studies show that in cytoplasmic extracts of infected cells, NS₁ was actually associated with ribosomes, but not as a structural protein; it was simply adsorbed to both the large and the small ribosomal subunits by salt-dissociable bonds. Consequently, NS₁ in the nucleolus is not serving as precursor to a ribosomal structural protein.