Petropoulos L, Hiscott J
Terry Fox Molecular Oncology Group, Lady Davis Institute for Medical Research, Sir Mortimer B. Davis-Jewish General Hospital, Montréal, Québec, Canada.
Virology. 1998 Dec 5;252(1):189-99. doi: 10.1006/viro.1998.9430.
Biological, molecular, and epidemiological data have demonstrated that human T cell leukemia virus type 1 (HTLV-1) encoded Tax protein plays a central role in the initiation of T cell malignancy. The 40-kDa Tax oncoprotein serves as a potent transcriptional activator that induces viral gene expression driven by the HTLV-1 long terminal repeats and also stimulates multiple cellular genes involved in T cell activation, cell cycle regulation, and gene activation. Since Tax has been shown to interact directly and indirectly with the NF-kappa B/I kappa B regulatory proteins, we examined the significance of an in vivo association between Tax and the I kappa B alpha inhibitor. Using GST affinity chromatography, Tax was shown to interact with the I kappa B alpha ankyrin repeats which are essential for interaction with the NF-kappa B/Rel proteins. In vivo, using I kappa B alpha mutants and co-immunoprecipitation, a preferential interaction between HTLV-1 Tax and N-terminally hypophosphorylated I kappa B alpha was detected. Tax also enhanced binding of I kappa B alpha to the proteasome subunit HsN3, resulting in a Tax-enhanced, constitutive degradation of wild-type and mutated forms of I kappa B alpha in the absence of phosphorylation and ubiquitination. Binding of I kappa B alpha to proteasome subunit HC9 was also observed, but this interaction occurred independently of Tax. Taken together, these results suggest a role for Tax as a viral chaperone resulting in the enhanced constitutive turnover of I kappa B alpha. The association of Tax with hypophosphorylated I kappa B alpha may prevent I kappa B alpha from binding to NF-kappa B and also target I kappa B alpha to the proteasome for degradation via a phosphorylation-independent pathway.
生物学、分子学和流行病学数据表明，人类1型T细胞白血病病毒（HTLV-1）编码的Tax蛋白在T细胞恶性肿瘤的起始过程中发挥着核心作用。40 kDa的Tax癌蛋白是一种强效转录激活因子，可诱导由HTLV-1长末端重复序列驱动的病毒基因表达，还能刺激多个参与T细胞活化、细胞周期调控和基因激活的细胞基因。由于Tax已被证明可直接和间接与NF-κB/IκB调节蛋白相互作用，我们研究了Tax与IκBα抑制剂在体内结合的意义。利用谷胱甘肽S-转移酶（GST）亲和层析法，发现Tax与IκBα锚蛋白重复序列相互作用，而该序列对于与NF-κB/Rel蛋白的相互作用至关重要。在体内，利用IκBα突变体和免疫共沉淀法，检测到HTLV-1 Tax与N端低磷酸化的IκBα之间存在优先相互作用。Tax还增强了IκBα与蛋白酶体亚基HsN3的结合，导致在无磷酸化和泛素化的情况下，Tax增强了野生型和突变型IκBα的组成型降解。还观察到IκBα与蛋白酶体亚基HC9的结合，但这种相互作用独立于Tax发生。综上所述，这些结果表明Tax作为一种病毒伴侣蛋白，可导致IκBα的组成型周转增强。Tax与低磷酸化IκBα的结合可能会阻止IκBα与NF-κB结合，还会通过非磷酸化途径将IκBα靶向蛋白酶体进行降解。
