Cancer Research UK Nucleic Acid Structure Research Group,MSI/WTB Complex,The University of Dundee,Dow Street,Dundee DD1 5EH,UK.
Q Rev Biophys. 2018 Jan;51:e5. doi: 10.1017/S0033583518000033.
The kink-turn (k-turn) is a widespread structural motif found in functional RNA species. It typically comprises a three-nucleotide bulge followed by tandem trans sugar edge-Hoogsteen G:A base pairs. It introduces a sharp kink into the axis of duplex RNA, juxtaposing the minor grooves. Cross-strand H-bonds form at the interface, accepted by the conserved adenine nucleobases of the G:A basepairs. Alternative acceptors for one of these divides the k-turns into two conformational classes N3 and N1. The base pair that follows the G:A pairs (3b:3n) determines which conformation is adopted by a given k-turn. k-turns often mediate tertiary contacts in folded RNA species and frequently bind proteins. Common k-turn binding proteins include members of the L7Ae family, such as the human 15·5k protein. A recognition helix within these proteins binds in the widened major groove on the outside of the k-turn, that makes specific H-bonds with the conserved guanine nucleobases of the G:A pairs. L7Ae binds with extremely high affinity, and single-molecule data are consistent with folding by conformational selection. The standard, simple k-turn can be elaborated in a variety of ways, that include the complex k-turns and the k-junctions. In free solution in the absence of added metal ions or protein k-turns do not adopt the tightly-kinked conformation. They undergo folding by the binding of proteins, by the formation of tertiary contacts, and some (but not all) will fold on the addition of metal ions. Whether or not folding occurs in the presence of metal ions depends on local sequence, including the 3b:3n position, and the -1b:-1n position (5' to the bulge). In most cases -1b:-1n = C:G, so that the 3b:3n position is critical since it determines both folding properties and conformation. In general, the selection of these sequence matches a given k-turn to its biological requirements. The k-turn structure is now very well understood, to the point at which they can be used as a building block for the formation of RNA nano-objects, including triangles and squares.
发夹环（k-turn）是在功能性 RNA 物种中发现的一种广泛存在的结构基序。它通常由三个核苷酸的凸起组成，然后是串联的转糖边缘- Hoogsteen G:A 碱基对。它在双螺旋 RNA 的轴线上引入了一个急剧的弯曲，使小沟并列。在界面处形成跨链氢键，由 G:A 碱基对的保守腺嘌呤核苷接受。这些接受体之一的替代物将 k 环分为两个构象类 N3 和 N1。紧随 G:A 对的碱基对（3b:3n）决定了给定 k 环采用哪种构象。k 环通常介导折叠 RNA 物种中的三级接触，并且经常与蛋白质结合。常见的 k 环结合蛋白包括 L7Ae 家族的成员，例如人类 15·5k 蛋白。这些蛋白质中的识别螺旋结合在 k 环外侧的加宽主沟中，与 G:A 对的保守鸟嘌呤核苷形成特定的氢键。L7Ae 以极高的亲和力结合，单分子数据与构象选择折叠一致。标准的简单 k 环可以以多种方式进行修饰，包括复杂的 k 环和 k 结。在没有添加金属离子或蛋白质的游离溶液中，k 环不会采用紧密弯曲的构象。它们通过蛋白质的结合、三级接触的形成以及一些（但不是全部）金属离子的添加来折叠。在金属离子存在与否的情况下是否发生折叠取决于局部序列，包括 3b:3n 位置和-1b:-1n 位置（凸起的 5'端）。在大多数情况下，-1b:-1n=C:G，因此 3b:3n 位置至关重要，因为它决定了折叠特性和构象。通常，这些序列的选择将给定的 k 环与其生物学要求匹配。k 环结构现在已经得到了很好的理解，可以将其用作形成 RNA 纳米物体（包括三角形和正方形）的构建块。
