Innate immunity plays a critical role in the control of viral infections. The induction of innate immune responses requires activation of transcription factors. In particular, NF-kappaB plays an essential role in activating the expression of cytokines involved in innate immunity such as beta interferon (IFN-beta) and interleukin-6 (IL-6). However, the mechanisms by which viruses activate NF-kappaB are poorly defined. Infection by parainfluenza virus 5 (PIV5), a prototypical member of the Paramyxoviridae family of Mononegavirales, has been shown to activate the expression of IFN-beta and IL-6. To examine how PIV5 induces this expression, we have examined the activation of NF-kappaB by PIV5 proteins. We have found that expression of PIV5 L protein alone is sufficient to activate NF-kappaB. The L protein of PIV5, the catalytic component of the viral RNA-dependent RNA polymerase, contains six domains that are conserved among all negative-stranded nonsegmented RNA viruses. We have mapped the region that activates NF-kappaB to the second domain, which is thought to be involved in RNA synthesis. The activation of NF-kappaB by L requires AKT1, a serine/threonine kinase, since AKT1 small interfering RNA, an AKT inhibitor as well as a dominant-negative mutant of AKT1, blocks this activation. Furthermore, we have found that L interacts with AKT1 and enhances its phosphorylation. We speculate that L may encode AKT1 kinase activity.