Xu Y L, Adya N, Siores E, Gao Q S, Giam C Z
Department of Biochemistry, University of Nebraska Medical Center, College of Medicine, Omaha 68198-4525.
J Biol Chem. 1990 Nov 25;265(33):20285-92.
Human T-cell leukemia virus type I (HTLV-I) encodes a 40-kDa nuclear protein, Tax, which stimulates transcription from three 21-base pair (bp) repeats in its U3 region. Tax trans-activation is mediated via cellular factors that interact with the TGACGT motifs in the 21-bp repeats. Gel mobility shift assay and UV cross-linking analysis show that two proteins of 52 and 46 kDa in size bind the 21-bp repeat specifically. Base substitutions in the TGACGT motif which abolished Tax trans-activation abrogated factor binding whereas the repeats containing mutations that did not affect Tax trans-activation supported factor binding as the wild-type repeat. The 52- and 46-kDa factors are present in human T-cell lines Jurkat and MT4 (HTLV-I transformed) and in HeLa cells but are undetectable in a human placental cell line JEG-3, which gave a reduced level of trans-activation. JEG-3 extracts contain a distinct DNA binding activity that shows analogous sequence requirements as the 52- and 46-kDa proteins in interacting with the various 21-bp repeats. c-Jun and CREB (cAMP-responsive element binding factor) can stimulate transcription from HTLV-I long terminal repeats in JEG-3 cells. At least two copies of the 21-bp repeats are required for optimal trans-activation by c-Jun and CREB. Most single point mutations in the TGACGT motif that abolished Tax trans-activation, however, did not affect c-Jun- or CREB-directed transcriptional enhancement. These data indicate that many transcription factors including c-Jun and CREB exert stimulatory effects on HTLV-I transcription although they do not directly respond to Tax. The 52- and 46-kDa cellular proteins most likely are involved directly in Tax-mediated trans-activation, and they are tentatively named Tax activation factors I and II, respectively.
人类I型T细胞白血病病毒（HTLV-I）编码一种40 kDa的核蛋白Tax，它可刺激其U3区域中三个21碱基对（bp）重复序列的转录。Tax的反式激活是通过与21 bp重复序列中的TGACGT基序相互作用的细胞因子介导的。凝胶迁移率变动分析和紫外线交联分析表明，两种大小分别为52 kDa和46 kDa的蛋白质特异性结合21 bp重复序列。TGACGT基序中的碱基替换消除了Tax的反式激活，同时也消除了因子结合，而含有不影响Tax反式激活的突变的重复序列则像野生型重复序列一样支持因子结合。52 kDa和46 kDa的因子存在于人类T细胞系Jurkat和MT4（HTLV-I转化的）以及HeLa细胞中，但在人类胎盘细胞系JEG-3中未检测到，后者的反式激活水平较低。JEG-3提取物含有一种独特的DNA结合活性，在与各种21 bp重复序列相互作用时，其显示出与52 kDa和46 kDa蛋白质类似的序列要求。c-Jun和CREB（cAMP反应元件结合因子）可刺激JEG-3细胞中HTLV-I长末端重复序列的转录。c-Jun和CREB实现最佳反式激活至少需要两个拷贝的21 bp重复序列。然而，TGACGT基序中大多数消除Tax反式激活的单点突变并不影响c-Jun或CREB介导的转录增强。这些数据表明，包括c-Jun和CREB在内的许多转录因子对HTLV-I转录发挥刺激作用，尽管它们不直接响应Tax。52 kDa和46 kDa的细胞蛋白很可能直接参与Tax介导的反式激活，它们暂分别命名为Tax激活因子I和Tax激活因子II。
