The double stranded RNA-dependent protein kinase (PKR), in addition to its
role as a translational controlling factor, is a key transcriptional regula
tor exerting antiviral and antitumoral activities. We have previously shown
that induction of NF-kappaB by PKR is involved in apoptosis commitment and
this process is mediated through activation of the IKK complex. To gain in
sights into the mechanism of activation of NF-kappaB by PKR, we have analys
ed the domains of PKR involved in IKK activation and subsequent NF-kappaB i
nduction. In PKR0/0 cells infected with a collection of vaccinia virus (VV)
recombinants expressing different mutant forms of PKR, we found that only
PKR forms conserving the catalytic activity are able to activate NF-kappaB.
An inactive PKR mutant (K296R), was unable to induce NF-kappaB activation
despite full expression of the protein in a wide range of concentrations, a
s defined by Western blot, EMSA, IKK kinase activity and NF-kappaB transact
ivation assays. Moreover, the mutant PKR (K296R) acts as a dominant negativ
e of PKR-induced eIF-2 alpha phosphorylation and NF-kappaB activation. Howe
ver, PKR mutants unable to activate NF-kappaB still retain their ability to
associate with the IKK complex, as confirmed by immunoprecipitation analys
is. We conclude that the catalytic activity of PKR and not only a protein-p
rotein interaction with the IKK complex, is needed for activation of the tr
anscription factor NF-kappaB.