Vaccinia Virus Defective Particles Lacking the F17 Protein Do Not Inhibit Protein Synthesis: F17, a Double-Edged Sword for Protein Synthesis?

Vaccinia virus (Orthopoxvirus) F17 protein is a major virion structural phosphoprotein having a molecular weight of 11 kDa. Recently, it was shown that F17 synthesised in infected cells interacts with mTOR subunits to evade cell immunity and stimulate late viral protein synthesis. Several years back, we purified an 11 kDa protein that inhibited protein synthesis in reticulocyte lysate from virions, and that possesses all physico-chemical properties of F17 protein. To investigate this discrepancy, we used defective vaccinia virus particles devoid of the F17 protein (designated iF17(-) particles) to assess their ability to inhibit protein synthesis. To this aim, we purified iF17(-) particles from cells infected with a vaccinia virus mutant which expresses F17 only in the presence of IPTG. The SDS-PAGE protein profiles of iF17(-) particles or derived particles, obtained by solubilisation of the viral membrane, were similar to that of infectious iF17 particles. As expected, the profiles of full iF17(-) particles and those lacking the viral membrane were missing the 11 kDa F17 band. The iF17(-) particles did attach to cells and injected their viral DNA into the cytoplasm. Co-infection of the non-permissive BSC40 cells with a modified vaccinia Ankara (MVA) virus, expressing an mCherry protein, and iF17(-) particles, induced a strong mCherry fluorescence. Altogether, these experiments confirmed that the iF17(-) particles can inject their content into cells. We measured the rate of protein synthesis as a function of the multiplicity of infection (MOI), in the presence of puromycin as a label. We showed that iF17(-) particles did not inhibit protein synthesis at high MOI, by contrast to the infectious iF17 mutant. Furthermore, the measured efficiency to inhibit protein synthesis by the iF17 mutant virus generated in the presence of IPTG, was threefold to eightfold lower than that of the wild-type WR virus. The iF17 mutant contained about threefold less F17 protein than wild-type WR. Altogether these results strongly suggest that virion-associated F17 protein is essential to mediate a stoichiometric inhibition of protein synthesis, in contrast to the late synthesised F17. It is possible that this discrepancy is due to different phosphorylation states of the free and virion-associated F17 protein.