Kaposi’s Sarcoma Associated Herpesvirus Encoded Viral FLICE Inhibitory Protein K13 Activates NF-kB Pathway Independent of TRAF6, TAK1 and LUBAC

Background: Kaposi’s sarcoma associated herpesvirus encoded viral FLICE inhibitory protein (vFLIP) K13 activates the NF-kB pathway by binding to the NEMO/IKKc subunit of the IkB kinase (IKK) complex. However, it has remained enigmatic how K13-NEMO interaction results in the activation of the IKK complex. Recent studies have implicated TRAF6, TAK1 and linear ubiquitin chains assembled by a linear ubiquitin chain assembly complex (LUBAC) consisting of HOIL-1, HOIP and SHARPIN in IKK activation by proinflammatory cytokines. Methodology/Principal Findings: Here we demonstrate that K13-induced NF-kB DNA binding and transcriptional activities are not impaired in cells derived from mice with targeted disruption of TRAF6, TAK1 and HOIL-1 genes and in cells derived from mice with chronic proliferative dermatitis (cpdm), which have mutation in the Sharpin gene (Sharpincpdm/cpdm). Furthermore, reconstitution of NEMO-deficient murine embryonic fibroblast cells with NEMO mutants that are incapable of binding to linear ubiquitin chains supported K13-induced NF-kB activity. K13-induced NF-kB activity was not blocked by CYLD, a deubiquitylating enzyme that can cleave linear and Lys63-linked ubiquitin chains. On the other hand, NEMO was required for interaction of K13 with IKK1/IKKa and IKK2/IKKb, which resulted in their activation by ‘‘T Loop’ ’ phosphorylation. Conclusions/Significance: Our results demonstrate that K13 activates the NF-kB pathway by binding to NEMO which results in the recruitment of IKK1/IKKa and IKK2/IKKb and their subsequent activation by phosphorylation. Thus, K13 activates NF