Inhibition of vaccinia virus replication by nicotinamide: evidence for ADP-ribosylation of viral proteins

Replication of vaccinia virus (VV) in monolayers of BSC40 cells was inhibited 99.9% in the presence of 60 mM nicotinamide (NIC), a competitive inhibitor of ADP-ribosylation reactions. Dot-blot hybridization analysis of infected cell extracts utilizing a VV DNA-specific probe indicated that the drug had only minimal effects on viral DNA synthesis. SDS-polyacrylamide gel electrophoresis of newly synthesized VV proteins pulse-labeled at early (2 h) or late (8 h) times post-infection revealed that although the full spectrum of expected viral polypeptides was evident, quantitative differences in the levels of expression of a distinct subset of viral proteins were observed in the presence of the drug. Velocity sedimentation of virus-infected cell lysates established that no mature particles were assembled in drug treated cells. Additional evidence suggesting that VV morphogenesis was abortive in the presence of NIC was obtained by pulse-chase labeling experiments that demonstrated that the two VV major late core polypeptide precursors P94 and P65, whose proteolytic processing to VP62 and VP60 is intimately associated with viral assembly, were not cleaved in the presence of NIC. Interestingly, growth of VV in the presence of [3H]adenosine resulted in the metabolic labeling of eight proteins that were associated with purified virions. These proteins co-migrated with proteins labeled with [3H]adenosine that were present in extracts of VV-infected, but not uninfected, cells. These analyses also revealed that the [3H]adenosine-labeling of a subset of cellular proteins (MW 18-20 kDa, possibly histones) was increased 4-fold by VV infection. The observed induction of either increased synthesis or hyper-modification of these 18-20 kDa proteins was inhibited by NIC. These results are discussed with respect to whether one or more VV polypeptides are subject to obligatory ADP-ribosylation modification reactions in order to attain their active configuration, and if so, whether the enzymes catalyzing these reactions are specified by the virus or host cell.