Mechanism of inactivation of NF-κB by a viral homologue of IκBα -: Signal-induced release of IκBα results in binding of the viral homologue to NF-κB

Activation of the nuclear factor kappaB plays a key role in viral pathogenesis, resulting in inflammation and modulation of the immune response. We have previously shown that A238L, an open reading frame from African swine fever virus (ASFV), encoding a protein with 40% homology to porcine I kappaB alpha exerts a potent anti-inflammatory effect in host, macrophages, where it down-regulates NF-kappaB-dependent gene transcription and proinflammatory cytokine production. This paper reveals the mechanism of suppression of NF-kappaB activity by A238Lp. A238Lp is synthesized throughout infection as two molecular mass forms of 28 and 32 kDa, and vaccinia-mediated expression of A238L demonstrated that both proteins are produced from a single gene. Significantly, the higher 32-kDa form of A238L, but not the 28-kDa form, interacts directly with RelA, the 65-kDa subunit of NF-kappaB, indicating that the binding is dependent on a posttranslational modification. Immunoprecipitation analysis shows the NF-kappaB p65-A238L p32 heterodimer is a separate complex from NF-kappaB-I kappaB alpha, and it resides in the cytoplasm. Moreover, we show that ASFV infection stimulates the NF kappaB signal transduction pathway, which results in the rapid degradation of endogenous I kappaB alpha, although both forms of A238Lp are resistant to stimulus-induced degradation. Using the proteasome inhibitor MG132, we show that when degradation of I kappaB alpha is inhibited, A238Lp binding to NF-kappaB p65 is reduced. The results suggest that the virus exploits its activation of the NF-kappaB pathway to enable its own I kappaB homologue to bind to NF-kappaB p65. Last, we show that synthesis of I kappaB alpha is increased during ASFV infection, indicating RelA-independent transcription of the I kappaB alpha gene.