Poxvirus Targeting of E3 Ligase β-TrCP by Molecular Mimicry: A Mechanism to Inhibit NF-κB Activation and Promote Immune Evasion and Virulence

The transcription factor NF-kappa B is essential for immune responses against pathogens and its activation requires the phosphorylation, ubiquitination and proteasomal degradation of I kappa B alpha. Here we describe an inhibitor of NF-kappa B from vaccinia virus that has a closely related counterpart in variola virus, the cause of smallpox, and mechanistic similarity with the HIV protein Vpu. Protein A49 blocks NF-kappa B activation by molecular mimicry and contains a motif conserved in I kappa B alpha which, in I kappa B alpha, is phosphorylated by IKK beta causing ubiquitination and degradation. Like I kappa B alpha, A49 binds the E3 ligase beta-TrCP, thereby preventing ubiquitination and degradation of I kappa B alpha. Consequently, A49 stabilised phosphorylated I kappa B alpha (p-I kappa B alpha) and its interaction with p65, so preventing p65 nuclear translocation. Serine-to-alanine mutagenesis within the I kappa B alpha-like motif of A49 abolished b-TrCP binding, stabilisation of p-I kappa B alpha and inhibition of NF-kappa B activation. Remarkably, despite encoding nine other inhibitors of NF-kappa B, a VACV lacking A49 showed reduced virulence in vivo.