Intracellular distribution and phosphorylation of virus-induced polypeptides in frog virus 3-infected cells

Abstract

Analysis of the intracellular localization of frog virus 3 (FV 3)-infected cell polypeptides (ICPs) showed that although the largest amount of newly synthesized proteins were recovered, at any time postinfection, in the cytoplasmic fraction, most of the early viral-induced ICPs were present in the nuclei and the relative molar ratio of ICPs 90, 42, 31 was higher in the nuclei than in the cytoplasm. After 5–8 hr late polypeptides species were found concentrated in the nucleus (ICPs 70, 63, 12). The absence of viral DNA replication did not prevent the appearance of ICPs in the nucleus. Under abortive conditions produced by the replacement of arginine by canavanine, only a restricted set of ICPs was induced (early polypeptides) and the nuclear concentrations of ICPs 31 and 42 was modified. Newly phosphorylated proteins in infected cells were predominantly found in the nucleus, most species being viral induced. Moreover, following the induction of protein kinase, the specific activity of this enzyme was about 50 times higher in the nuclei than in the cytoplasmic fraction. These results extend the evidence for specific steps of FV 3 replication in the host nucleus, emphasizing the importance of the nucleus as the predominant site of viral polypeptide phosphorylation.