Prokaryotic adaptive immune systems use Clustered Regularly Interspaced Short Palindromic Repeats (CRISPRs) and CRISPR-associated (Cas) proteins for RNA-guided cleavage of foreign genetic elements. The focus of this review, Type VI CRISPR-Cas systems, contain a single protein, Cas13 (formerly C2c2) that when assembled with a CRISPR RNA (crRNA) forms a crRNA-guided RNA-targeting effector complex. Type VI CRISPR-Cas systems can be divided into four subtypes (A-D) based on Cas13 phylogeny. All Cas13 proteins studied to date possess two enzymatically distinct ribonuclease activities that are required for optimal interference. One RNase is responsible for pre-crRNA processing to form mature Type VI interference complexes, while the other RNase activity provided by the two Higher Eukaryotes and Prokaryotes Nucleotide-binding (HEPN) domains, is required for degradation of target-RNA during viral interference. In this review, I will compare and contrast what is known about the molecular architecture and behavior of Type VI (A-D) CRISPR-Cas13 interference complexes, how this allows them to carry out their RNA-targeting function, how Type VI accessory proteins are able to modulate Cas13 activity, and how together all of these features have led to the rapid development of a range of RNA-targeting applications. Throughout I will also discuss some of the outstanding questions regarding Cas13's molecular behavior, and its role in bacterial adaptive immunity and RNA-targeting applications.

中文翻译：

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含 Cas13 的 VI 型 CRISPR-Cas 系统的 RNA 靶向分子机制。


原核适应性免疫系统使用成簇规则间隔短回文重复序列 (CRISPR) 和 CRISPR 相关 (Cas) 蛋白进行 RNA 引导的外源遗传元件切割。本次综述的重点是 VI 型 CRISPR-Cas 系统，包含单一蛋白质 Cas13（以前称为 C2c2），当与 CRISPR RNA (crRNA) 组装时，形成 crRNA 引导的 RNA 靶向效应复合物。 VI型CRISPR-Cas系统根据Cas13系统发育可分为四个亚型（AD）。迄今为止研究的所有 Cas13 蛋白都具有两种不同的核糖核酸酶活性，这是最佳干扰所必需的。一种 RNase 负责前 crRNA 加工以形成成熟的 VI 型干扰复合物，而另一种 RNase 活性由两个高等真核生物和原核生物核苷酸结合 (HEPN) 结构域提供，是病毒干扰期间靶标 RNA 降解所必需的。在这篇综述中，我将比较和对比 VI 型 (AD) CRISPR-Cas13 干扰复合物的分子结构和行为的已知信息，这如何使它们能够执行其 RNA 靶向功能，VI 型辅助蛋白如何能够调节 Cas13 活性，以及​​所有这些功能如何共同导致一系列 RNA 靶向应用的快速发展。在整个过程中，我还将讨论有关 Cas13 分子行为及其在细菌适应性免疫和 RNA 靶向应用中的作用的一些悬而未决的问题。