IKKγ Mediates the Interaction of Cellular IκB Kinases with the Tax Transforming Protein of Human T Cell Leukemia Virus Type 1

Abstract

The Tax oncoprotein of human T cell leukemia virus type 1 constitutively activates transcription factor NF-B by a mechanism involving Tax-induced phosphorylation of IB␣, a labile cytoplasmic inhibitor of NF-B. To trigger this signaling cascade, Tax associates stably with and persistently activates a cellular IB kinase (IKK) containing both catalytic (IKK␣ and IKK␤) and noncatalytic (IKK␥) subunits. We now demonstrate that IKK␥ enables Tax to dock with the IKK␤ catalytic subunit, resulting in chronic IB kinase activation. Mutations in either IKK␥ or Tax that prevent formation of these higher order Tax⅐IKK complexes also interfere with the ability of Tax to induce IKK␤ catalytic function in vivo. Deletion mapping studies indicate that amino acids 1-100 of IKK␥ are required for this Tax targeting function. Together, these findings identify IKK␥ as an adaptor protein that directs the stable formation of pathologic Tax⅐IKK complexes in virally infected T cells.During an adaptive immune response, antigen-stimulated CD4 ϩ T lymphocytes become committed to an activation program that triggers a transient phase of clonal expansion (1). In contrast, infection with human T cell leukemia virus type 1 (HTLV-1) 1 can lead to the loss of cell cycle control and development of an aggressive malignancy called adult T cell leukemia (2). The Tax oncoprotein encoded by HTLV-1 stimulates the constitutive nuclear expression of transcription factor NF-B, which regulates antigen-directed T cell proliferation (3, 4).Studies with Tax-transgenic mice suggest that this viral/host interaction is required to maintain the transformed phenotype of HTLV-1-infected cells (5).In quiescent T cells, the activity of NF-B is controlled from the cytoplasmic compartment by virtue of its signal-dependent interaction with inhibitors, including IB␣ (6). Recent studies have identified two cytokine-inducible IB kinases (IKKs), termed IKK␣ and IKK␤, that target IB␣ for degradation via phosphorylation at Ser-32 and Ser-36 (7). These two kinases form heterodimers and function as catalytic subunits within a 700 -900-kDa multicomponent complex (8). Whereas IKK␣ and IKK␤ are activated transiently in cells treated with the cytokine tumor necrosis factor-␣ (TNF) (8 -10), Tax induces their constitutive expression in HTLV-1-infected T cells (11,12). We have recently found that Tax-induced activation of both IKK and NF-B requires the formation of Tax⅐IKK complexes (12). However, the precise mechanism of Tax action on IKKs remains unclear.Here we provide several lines of experimental evidence indicating that Tax-directed IKK activation is mediated by IKK␥ (also called NEMO, IKKAP1, or FIP-3), a recently identified subunit of TNF-responsive IKKs whose precise signaling function is unknown (13-16). First, interference with IKK␥ expression in T cell transfectants inhibits Tax-mediated activation of NF-B. Second, IKK␥ and Tax interact stably in the context of a high molecular mass IB kinase derived from HTLV-1-infected T cells. Third, overexpression of IKK␥ in vivo is suffic...

NF-κB-related genetic diseases

“…It has been shown that NEMO interacts with IKK subunits through the N-terminal part of its CC1, whereas the C-terminal part of this domain provides a binding site for Tax or RIP. 7,8 In addition, the CC2/ LZ part of NEMO represents the minimal oligomerization domain of the molecule. 9 Finally, the NEMO ZF appears required for IKK activation in response to TNF, LPS or IL-1 10 although its exact function remains unclear.…”

Molecular mechanisms of cellular transformation by HTLV-1 Tax

“…Recently, evidence suggests that the assembly of a transcriptionally competent 'nuclear' Tax-NF-kB-CBP complex likely first occurs in the cytoplasm (Azran et al, 2005). One prevailing view is that Tax binds the IKKg/ NEMO molecule in the cytoplasm to influence the activity of the IKKa/IKKb/IKKg complex (Chu et al, 1999;Harhaj and Sun, 1999;Jin et al, 1999a). Activated IKKa/ IKKb/IKKg complexes then phosphorylate IKKa/b leading to a cascade of events (reviewed elsewhere in this issue by Sun and Yamaoka) which result in nuclear migration of NF-kB (Iha et al, 2003).…”

Activation of NF-κB by HTLV-I and implications for cell transformation

“…In HTLV-I-transformed T cells, Tax is stably assembled into the canonical IKK complex, in which IKK remains chronically phosphorylated and activated (Chu et al, 1998;Carter et al, 2001). The Tax/ IKK complex formation relies on physical interaction between Tax and the IKK regulatory subunit, IKKg (Chu et al, 1999;Jin et al, 1999). Tax appears to directly target IKKg, since these two proteins interact in both mammalian cells (Chu et al, 1999;Jin et al, 1999) and yeast cells (Jin et al, 1999).…”

“…The Tax/ IKK complex formation relies on physical interaction between Tax and the IKK regulatory subunit, IKKg (Chu et al, 1999;Jin et al, 1999). Tax appears to directly target IKKg, since these two proteins interact in both mammalian cells (Chu et al, 1999;Jin et al, 1999) and yeast cells (Jin et al, 1999). The Tax/IKKg interaction is required for recruiting Tax to the IKK catalytic subunits and for Tax-mediated activation of IKK .…”