Localization of the influenza virus nucleoprotein: cell-associated and extracellular non-virion forms

Both the supernatant of influenza virus-infected chick embryo cells and allantoic fluid containing influenza virus were shown to contain non-virion nucleoprotein (NP), which reacted readily with anti-NP monoclonal antibodies. Adsorption onto erythrocytes and centrifugation at 70,000 g for 2 h resulted in the removal of about 20% of the extracellular NP, whereas centrifugation at 100,000 g for 4 h eliminated about 50%, and practically all [3H]uridine-labelled virions. These results suggest that of the extracellular NP about 30% exists in the form of ribonucleoprotein, about 20% is precipitated with virions and about 50% occurs as free molecules. Comparative analysis of the kinetics of the accumulation of NP in the supernatant of infected cells, on the cell surface and inside the cells in relation to virus production, showed that there is a significant correlation between them.