We investigated the mechanisms by which influenza virus prevents shutoff of
protein synthesis by a cellular
protein kinase normally activated during
infection. Earlier work has shown that influenza virus
superinfection of cells previously infected by the
adenovirus VAI RNA-negative mutant dl331 resulted in selective translation of influenza virus mRNAs and suppression of the elevated
protein kinase levels normally found in cells infected by the mutant alone (M. G. Katze, B. M. Detjen, B. Safer, and R. M. Krug, Mol. Cell. Biol. 6:1741-1750, 1986). We elucidated the mechanisms of this
kinase repression and can now report that influenza virus encodes a gene product which functions to directly block the autophosphorylation and activity of the
interferon-induced,
double-stranded-RNA-activated
protein kinase, P68. Suppressed P68 activity was found not only in doubly infected cells but also in cells infected by influenza virus alone. Moreover, the decrease in P68 activity correlated with a decrease in the endogenous levels of phosphorylation of the alpha subunit of the eucaryotic
initiation factor eIF-2, the natural substrate of the
protein kinase. Suppression of P68 activity occurred as early
as 2 h after influenza virus
infection and required viral gene expression beyond the level of primary
mRNA transcription to take place. We confirmed our in vivo observations with in vitro mixing experiments which showed that the influenza virus inhibitor can act in trans to block P68 activity. Combined repression of P68 function and
eIF-2 alpha phosphorylation during influenza virus
infection is essential for continued catalytic recycling of
eIF-2 and efficient mRNA translation.