Noncanonical NF-κB Activation by the Oncoprotein Tio Occurs Through a Nonconserved TRAF3-Binding Motif

Abstract

Members of the nuclear factor κB (NF-κB) family of transcription factors regulate many cellular functions. Activation of NF-κB signaling is commonly classified as occurring through canonical or noncanonical pathways. Most NF-κB-inducing stimuli, including the viral oncoprotein Tio, lead to a concerted activation of both NF-κB pathways; however, extensive crosstalk at multiple levels between these signaling cascades restricts the ability to discriminate between the canonical and the noncanonical effects. We showed that noncanonical NF-κB activation by Tio depends on a distinct sequence motif that directly recruits tumor necrosis factor receptor-associated factor 3 (TRAF3). Through its TRAF3-binding motif, Tio triggered a ubiquitin-independent depletion of TRAF3 from the cytosol, which prevented TRAF3 from inhibiting signaling through the noncanonical NF-κB cascade. Furthermore, the Tio-TRAF3 interaction did not affect components of the canonical NF-κB signaling pathway or the expression of target genes; thus, Tio induced noncanonical NF-κB independently of crosstalk with the canonical pathway. Together, these data identify a distinct molecular mechanism of noncanonical NF-κB activation that should enable studies into the particular functions of this pathway.

The non-canonical NF-κB pathway in immunity and inflammation

“…These include several RNA viruses 40,43–45 , which seem to induce non- canonical NF-κB pathway activation by stimulating the cytoplasmic RNA sensor retinoic acid inducible gene I (RIG- I) 45 . Some viral oncoproteins are capable of directly stimulating non-canonical NF-κB pathway activation; these include the Tax protein of human T cell leukaemia virus type I, Epstein–Barr virus latent membrane protein 1 (LMP1), the Kaposi sarcoma-associated herpesvirus protein vFLIP, Herpesvirus saimiri transforming protein STP-A11 and the Tio protein of Herpesvirus ateles 46–50 . Tio is a transmembrane protein that resembles TNFRs and mediates NIK-dependent non-canonical NF-κB activation by interacting with, and inducing the degradation of, TRAF3 (REF.…”

Roles for TNF-receptor associated factor 3 (TRAF3) in lymphocyte functions

“…In addition to NF-κB activation, Tio expression also induces STAT1,3 and 5 phosphorylation and MAPK activation (122-124). deJong et al showed that Tio-induced NF-κB activation is dependent on TRAF3 binding to a non-conventional TRAF binding site (125). TRAF3 is sequestered by Tio into membrane- and cytoskeleton-associated insoluble fractions.…”

“…Interestingly, TRAF3 sequestration is not diminished by proteasome inhibition or SMAC mimetics, nor by inhibition of lysosomal enzymes. Tio did not enhance but in fact decreased the basal state of ubiquitination of TRAF3, and no ubiquitin activity co-localized with Tio and TRAF3 (125). Though Tio promotes T cell transformation, it has been demonstrated that Src kinase pathways are essential to this process(126).…”

TRAF3 signaling: Competitive binding and evolvability of adaptive viral molecular mimicry

“…If NIK cannot bind to TRAF3, it cannot get ubiquitinated by cIAPs and targeted for degradation. For instance, Tio oncoprotein of Herpesvirus ateles displaces NIK when it binds to TRAF3 and activates non-canonical NF-KB constitutively [43]. Structural details of the interactions elucidate how and why distinct downstream outcomes such as accumulation or degradation of NIK are observed under different conditionsstimulated or unstimulated cellsas in our case here.…”