Activation of the
nuclear factor kappa B 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 kappa B alpha exerts a potent anti-inflammatory effect in host macrophages, where it down-regulates
NF-kappa B-dependent gene transcription and proinflammatory
cytokine production. This paper reveals the mechanism of suppression of
NF-kappa B 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-kappa B, indicating that the binding is dependent on a post-translational modification. Immunoprecipitation analysis shows the
NF-kappa B p65-A238L p32 heterodimer is a separate complex from
NF-kappa B-
I kappa B alpha, and it resides in the cytoplasm. Moreover, we show that ASFV
infection stimulates the
NF kappa B signal transduction pathway, which results in the rapid degradation of endogenous
I kappa B alpha, although both forms of A238Lp are resistant to stimulus-induced degradation. Using the
proteasome inhibitor MG132, we show that when degradation of
I kappa B alpha is inhibited, A238Lp binding to
NF-kappa B p65 is reduced. The results suggest that the virus exploits its activation of the
NF-kappa B pathway to enable its own
I kappa B homologue to bind to
NF-kappa B p65. Last, we show that synthesis of
I kappa B alpha is increased during ASFV
infection, indicating RelA-independent transcription of the
I kappa B alpha gene.