Subclasses of ribonucleoproteins in influenza virus-infected cells

Abstract

Influenza virus-specific ribonucleoprotein particles (RNPs) are present in the nucleoplasmic and cytoplasmic fractions of infected cells, but are not found in the nucleolar fraction. Virus-specific RNPs are separated into two subclasses by isopycnic centrifugation in renografin density gradients; one species has an average buoyant density of 1.26 g/ml in renografin and a second, more heterogeneous species has an average buoyant density of 1.21 g/ml in renografin. The two subclasses of RNPs are dynamic structures with different polypeptide compositions at different times during the replicative cycle. Both species of RNPs are associated with the major structure unit of virus RNPs, the NP polypeptide; however the relative amounts of the lesser components, P 1, P 2, and P 3 vary during infection. The 1.21 g/ml RNPs are enriched in P polypeptides early in infection, but later the P to NP ratio of these RNPs declines. In contrast, the P polypeptides are not detected in 1.26 g/ml RNPs until after 4 hr postinfection. The 1.26 g/ml RNPs are separated from host cell RNPs in renografin density gradients and, after glutaraldehyde treatment, these RNPs have the same buoyant density in CsCl as virion RNPs. These results suggest that 1.26 g/ml RNPs are free virus-specific RNPs, and the 1.21 g/ml RNPs are associated with complexes. The possible functional role of the subclasses of RNPs in virus replication is discussed.