Phosphorylation of the Nucleoprotein of an Avian Influenza Virus

High resolution polyacrylamide gel electrophoresis (PAGE) of chick embryo fibroblast cells infected with the avian influenza virus FPV-Rostock revealed two distinct polypeptides migrating in the region of the nucleoprotein (NP). One-dimensional fingerprinting of these polypeptides showed that they were both nucleoprotein, and [32P]orthophosphate labelling revealed that they differed with respect to their state of phosphorylation. Pulse-chase studies using [35S]methionine indicated that phosphorylation of a certain proportion of NP occurs rapidly after synthesis and is associated with transport to the nucleus. Nucleoprotein which remained in the cytoplasm was predominantly non-phosphorylated. Both the phosphorylated and the non-phosphorylated types of NP were found in ribonucleoprotein complexes (RNPs) of different densities isolated on renografin gradients, but RNPs isolated from the nucleus contained much more phosphorylated NP than those from the cytoplasm. The kinase responsible for nucleoprotein phosphorylation appears to be influenced by temperature of incubation of the infected cells.