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

Background: Kaposi's sarcoma associated herpesvirus encoded viral FLICE inhibitory protein (vFLIP) K13 activates the NF-kappa B pathway by binding to the NEMO/IKK gamma subunit of the I kappa B 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-kappa B 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 (Sharpin(cpdm/ 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-kappa B activity. K13-induced NF-kappa B 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/IKK alpha and IKK2/IKK beta, which resulted in their activation by T Loop'' phosphorylation. Conclusions/Significance: Our results demonstrate that K13 activates the NF-kappa B pathway by binding to NEMO which results in the recruitment of IKK1/IKK alpha and IKK2/IKK beta and their subsequent activation by phosphorylation. Thus, K13 activates NF-kappa B via a mechanism distinct from that utilized by inflammatory cytokines. These results have important implications for the development of therapeutic agents targeting K13-induced NF-kappa B for the treatment of KSHV-associated malignancies.