Aboul-ela F, Karn J, Varani G
MRC Laboratory of Molecular Biology, Cambridge, UK.
Nucleic Acids Res. 1996 Oct 15;24(20):3974-81. doi: 10.1093/nar/24.20.3974.
Efficient transcription from the human immunodeficiency virus (HIV) promoter depends on binding of the viral regulatory protein Tat to a cis-acting RNA regulatory element, TAR. Tat binds at a trinucleotide bulge located near the apex of the TAR stem-loop structure. An essential feature of Tat-TAR interaction is that the protein induces a conformational change in TAR that repositions the functional groups on the bases and the phosphate backbone that are critical for specific intermolecular recognition of TAR RNA. We have previously determined a high resolution structure for the bound form of TAR RNA using heteronuclear NMR. Here, we describe a high resolution structure of the free TAR RNA based on 871 experimentally determined restraints. In the free TAR RNA, bulged residues U23 and C24 are stacked within the helix, while U25 is looped out. This creates a major distortion of the phosphate backbone between C24 and G26. In contrast, in the bound TAR RNA, each of the three residues from the bulge are looped out of the helix and U23 is drawn into proximity with G26 through contacts with an arginine residue that is inserted between the two bases. Thus, TAR RNA undergoes a transition from a structure with an open and accessible major groove to a much more tightly packed structure that is folded around basic side chains emanating from the Tat protein.
人类免疫缺陷病毒（HIV）启动子的高效转录取决于病毒调节蛋白Tat与顺式作用RNA调节元件TAR的结合。Tat结合在TAR茎环结构顶端附近的一个三核苷酸凸起处。Tat-TAR相互作用的一个基本特征是，该蛋白会诱导TAR发生构象变化，从而重新定位碱基上的官能团以及对TAR RNA特异性分子间识别至关重要的磷酸主链。我们之前利用异核核磁共振确定了结合形式的TAR RNA的高分辨率结构。在此，我们基于871个实验确定的约束条件描述了游离TAR RNA的高分辨率结构。在游离TAR RNA中，凸起的残基U23和C24堆积在螺旋内部，而U25则呈环状突出。这在C24和G26之间造成了磷酸主链的重大扭曲。相比之下，在结合的TAR RNA中，来自凸起的三个残基中的每一个都从螺旋中呈环状突出，并且U23通过与插入两个碱基之间的精氨酸残基接触而与G26靠近。因此，TAR RNA经历了从具有开放且可及的大沟的结构到围绕源自Tat蛋白的碱性侧链折叠的更为紧密堆积的结构的转变。
