Human herpes virus 8 (HHV8), 1 also known as Kaposi's sarcoma-associated herpes virus, is a ␥-2 herpes virus that was originally isolated from Kaposi's sarcoma and has been subsequently associated with several lymphoproliferative disorders, including primary effusion lymphoma or body cavity lymphoma, multicentric Castleman's disease, angioimmunoblastic lymphadenopathy, and some cases of reactive lymphadenopathies (1-3). Although HHV8 encodes for homologs of cytokines, chemokines, and their receptors, none of them is expressed in latently infected primary effusion lymphoma cell lines or Kaposi's sarcoma spindle cells (1). HHV8 also encodes for a protein called K13 (or orf71), which is one of the few viral proteins known to be expressed in cells latently infected with HHV8 and therefore is a prime candidate for causing the cellular transformation associated with infection by this virus (1, 4 -6).HHV8 K13 protein contains two homologous copies of a death effector domain and thus resembles the prodomain of caspase 8 (also known as FLICE). Proteins with two death effector domains have been encountered in other viruses as well and include MC159L and MC169 from the molluscum contagiosum virus and E8 from equine herpes virus 2 (7-9). These virally encoded death effector domain-containing proteins are believed to protect virally infected cells from death receptorinduced apoptosis by blocking the recruitment and/or activation of caspase 8/FLICE (7-9). As a result, these proteins are also collectively known as vFLIPs (viral FLICE inhibitory protein) (7).We have previously demonstrated that HHV8 vFLIP K13 possesses the unique ability to activate the NF-B pathway that is not shared by the vFLIP E8 from equine herpes virus 2 and the vFLIP MC159L from molluscum contagiosum virus (10). We recently demonstrated that vFLIP K13 can co-immunoprecipitate with several protein kinases known to be involved in NF-B activation, such as RIP, NIK, IKK1 (or IKK␣), IKK2 (or IKK), and NEMO (or IKK␥) (10, 11). Furthermore, vFLIP K13 was found to physically associate with a ϳ700-kDa I B kinase (IKK) signalsome complex consisting of IKK1, IKK2, and NEMO, which is known to play a crucial role in cytokine-induced NF-B activation (11). In the present study, we have focused on the functional contribution of the above interactions of vFLIP K13 to NF-B activation.
MATERIALS AND METHODSPlasmids and Cell Lines-Plasmids containing IKK1, IKK1 kinase mutant, IKK2, IKK2 kinase mutant, and pcDNA3-K13-FLAG have been described previously (10). Wild type, IKK1Ϫ/Ϫ, IKK2Ϫ/Ϫ, and NEMOϪ/Ϫ murine embryonic fibroblast (MEF) cells were obtained from Drs. Inder Verma and Richard Gaynor. Wild type and NIKϪ/Ϫ MEFs have been described previously (12) and were obtained from Dr. Robert Schreiber. These cell lines were cultured in Dulbecco's modified Eagle's medium with 10% fetal calf serum. Wild type and RIP-deficient Jurkat cells were obtained from Dr. Brian Seed and were cultured in RPMI medium supplemented with 10% fetal calf serum (13). Both of these media were supplem...