Connor L M, Marriott S J
Department of Molecular Virology and Microbiology, Baylor College of Medicine, Houston, Texas 77030, USA.
Virology. 2000 May 10;270(2):328-36. doi: 10.1006/viro.2000.0262.
The human T cell leukemia virus type I (HTLV-I) Tax protein activates transcription from the viral long terminal repeat and select cellular promoters by interacting with cellular DNA-binding proteins. The HTLV-I promoter contains three copies of a Tax-responsive element (TRE-1), each of which possesses a core cAMP response element (CRE). The cAMP response element-binding protein, CREB, binds TRE-1 and mediates Tax association with, and transactivation of, the viral promoter. These activities depend on DNA sequences that flank the core CRE. Although CREs are found in a variety of cellular promoters, cellular CREs vary in sequence from TRE-1, especially in the flanking regions, and are generally not Tax responsive. The molecular basis for differential Tax responsiveness of viral and cellular CREs has not been determined. Here we demonstrate that the conformation of CREB is influenced by the nucleotide sequence of its DNA-binding element. CREB showed altered sensitivity to V8, chymotrypsin, and trypsin proteases when bound to the HTLV-I TRE-1 element as compared to the rat somatostatin CRE element. The phosphorylation state of CREB did not influence its protease sensitivity on either element. Sequences flanking the core CRE-binding site in each element were found to specify protease sensitivity. Since the TRE-1-flanking sequences also modulate Tax association with CREB, and Tax transactivation of CREB-dependent LTR transcription, these results suggest that CREB conformation may determine the ability of Tax to bind CREB.
人类I型T细胞白血病病毒（HTLV-I）的Tax蛋白通过与细胞DNA结合蛋白相互作用，激活病毒长末端重复序列和特定细胞启动子的转录。HTLV-I启动子包含三个Tax反应元件（TRE-1）拷贝，每个拷贝都有一个核心环磷酸腺苷反应元件（CRE）。环磷酸腺苷反应元件结合蛋白（CREB）与TRE-1结合，并介导Tax与病毒启动子的结合及反式激活。这些活性依赖于核心CRE侧翼的DNA序列。尽管CRE存在于多种细胞启动子中，但细胞CRE在序列上与TRE-1不同，尤其是在侧翼区域，并且通常对Tax无反应。病毒和细胞CRE对Tax反应性差异的分子基础尚未确定。在此我们证明，CREB的构象受其DNA结合元件核苷酸序列的影响。与大鼠生长抑素CRE元件相比，当CREB与HTLV-I TRE-1元件结合时，其对V8蛋白酶、胰凝乳蛋白酶和胰蛋白酶的敏感性发生改变。CREB的磷酸化状态对其在任何一个元件上的蛋白酶敏感性均无影响。发现每个元件中核心CRE结合位点侧翼的序列决定蛋白酶敏感性。由于TRE-1侧翼序列也调节Tax与CREB的结合以及Tax对CREB依赖性LTR转录的反式激活，这些结果表明CREB构象可能决定Tax结合CREB的能力。
