HTLV-1 Tax Induces Formation of the Active Macromolecular IKK Complex by Generating Lys63- and Met1-Linked Hybrid Polyubiquitin Chains.

Yuri Shibata,Satoshi Inoue,Tsukasa Seya,Hiroyasu Nakano,Hirotaka Takahashi,Fuminori Tokunaga,Yasushi Saeki,Jun-Ichiro Inoue,Jin Gohda,Kazuhiro Iwai,Hiroyuki Oshiumi,Yuetsu Tanaka,Ginga Komatsu,Keiji Tanaka,Eiji Goto,Tatsuya Sawasaki,Charles R M Bangham

doi:10.1371/journal.ppat.1006162

Abstract

The Tax protein of human T-cell leukemia virus type 1 (HTLV-1) is crucial for the development of adult T-cell leukemia (ATL), a highly malignant CD4+ T cell neoplasm. Among the multiple aberrant Tax-induced effects on cellular processes, persistent activation of transcription factor NF-κB, which is activated only transiently upon physiological stimulation, is essential for leukemogenesis. We and others have shown that Tax induces activation of the IκB kinase (IKK) complex, which is a critical step in NF-κB activation, by generating Lys63-linked polyubiquitin chains. However, the molecular mechanism underlying Tax-induced IKK activation is controversial and not fully understood. Here, we demonstrate that Tax recruits linear (Met1-linked) ubiquitin chain assembly complex (LUBAC) to the IKK complex and that Tax fails to induce IKK activation in cells that lack LUBAC activity. Mass spectrometric analyses revealed that both Lys63-linked and Met1-linked polyubiquitin chains are associated with the IKK complex. Furthermore, treatment of the IKK-associated polyubiquitin chains with Met1-linked-chain-specific deubiquitinase (OTULIN) resulted in the reduction of high molecular weight polyubiquitin chains and the generation of short Lys63-linked ubiquitin chains, indicating that Tax can induce the generation of Lys63- and Met1-linked hybrid polyubiquitin chains. We also demonstrate that Tax induces formation of the active macromolecular IKK complex and that the blocking of Tax-induced polyubiquitin chain synthesis inhibited formation of the macromolecular complex. Taken together, these results lead us to propose a novel model in which the hybrid-chain-dependent oligomerization of the IKK complex triggered by Tax leads to trans-autophosphorylation-mediated IKK activation.

Highlights

Human T-cell leukemia virus type 1 (HTLV-1) is etiologically associated with adult T-cell leukemia (ATL), an aggressive and lethal malignancy of CD4+ T cells, and with HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) [1, 2]
Infection with the human retrovirus HTLV-1 causes adult Tcell leukemia, and HTLV-1 Tax-mediated persistent nuclear factor-κB (NF-κB) activation is crucial for leukemogenesis
The IKK complex-associated Lys63/Met1-linked hybrid polyubiquitin chains are generated through the Tax-mediated recruitment of linear ubiquitin chain assembly complex (LUBAC) to the IKK complex

Summary

Introduction

Human T-cell leukemia virus type 1 (HTLV-1) is etiologically associated with adult T-cell leukemia (ATL), an aggressive and lethal malignancy of CD4+ T cells, and with HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) [1, 2]. Tax aberrantly activates host cell transcription factors, including nuclear factor-κB (NF-κB), cyclic AMP response element-binding protein (CREB) and serum responsive factor (SRF), thereby perturbing transcriptional networks in host cells [6]. Among these factors, accumulating evidence indicates that persistent activation of NF-κB by Tax is crucial for T cell transformation and ATL development [7,8,9]. The canonical pathway is activated by cytokines, such as tumor necrosis factor (TNF)-α and interleukin (IL), whose stimulation leads to activation of the IκB kinase (IKK) complex, which is composed of the catalytic subunits IKKα and IKKβ and the regulatory subunit NEMO [11]. Tax is able to activate both canonical and noncanonical pathways, which are thought to be coordinately involved in leukemogenesis [13]

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Conclusion