Inhibition of interferon gene activation bydeath-effector domain-containing proteins from the molluscum contagiosum virus

Apoptosis, NF-KB activation, and IRF3 activation are a triad of intrinsic immune responses that play crucial roles in the pathogene-sis of infectious diseases, cancer, and autoimmunity. FLIPs area family of viral and cellular proteins initially found to inhibit ap-optosis and more recently to either up- or down-regulate NF-KB.As such, a broad role for FLIPs in disease regulation is postulated,but exactly how a FLIP performs such multifunctional roles remainsto be established. Here we examine FLIPs (MC159 and MC160)encoded by the molluscum contagiosum virus, a dermatotropicpoxvirus causing skin infections common in children and immuno-compromised individuals, to better understand their roles in viralpathogenesis. While studying their molecular mechanisms responsible for NF-KB inhibition, we discovered that each protein inhibitedIRF3-controlled luciferase activity, identifying a unique function forFLIPs. MC159 and MC160 each inhibited TBK1 phosphorylation, confirming this unique function. Surprisingly, MC159 coimmunoprecipi-tatedwithTBK1 andIKKe but MC160 did not, suggesting that thesehomologs use distinct molecular mechanisms to inhibit IRF3 activation. Equally surprising was the finding that the FLIP regions necessary for TBK1 inhibition were distinct from those MC159 or MC160regions previously defined to inhibit NF-KB or apoptosis. These datareveal previously unappreciated complexities of FLIPs, and that subtle differences within the conserved regions of FLIPs possess distinctmolecular and structural fingerprints that define crucial differencesin biological activities. A future comparison of mechanistic differences between viral FLIP proteins can provide new means of precisely manipulating distinct aspects of intrinsic immune responses.