Topuzlu Ece, Lawrence C Martin
a Department of Chemistry and Biochemistry , Montana State University , Bozeman , MT 59717 , USA.
RNA Biol. 2016;13(2):254-7. doi: 10.1080/15476286.2015.1130209.
The CRISPR/Cas adaptive immune system shows extreme diversity in the number of CRISPR/Cas types and subtypes, and in the multitude of CRISPR associated protein families of which they are composed. Despite this diversity, the roles of many Cas protein families are now defined with regard to spacer acquisition, crRNA biogenesis, and DNA or RNA surveillance and targeting. However, a number of unclassified CRISPR-Cas proteins remain. Such proteins have traditionally been designated as CRISPR subtype x (Csx). Here we revisit the structural analysis of one such protein, Csx3, and show that this homodimeric protein utilizes a Rossmann fold for the recognition of an RNA tetranucleotide. Tertiary and quaternary structural similarities of Csx3 to CRISPR/Cas proteins Csx1 and Csa3 are identified and suggest Csx3 is a new member of the CRISPR Associated Rossmann Fold (CARF) superfamily. The structure of the Csx3/RNA complex illustrates one way CARF domain proteins may recognize pseudo-symmetric polynucleotides.
CRISPR/Cas适应性免疫系统在CRISPR/Cas类型和亚型的数量以及它们所组成的众多CRISPR相关蛋白家族方面表现出极大的多样性。尽管存在这种多样性，但现在许多Cas蛋白家族在间隔序列获取、crRNA生物合成以及DNA或RNA监测与靶向方面的作用已得到明确。然而，仍有一些未分类的CRISPR-Cas蛋白。这类蛋白传统上被指定为CRISPR亚型x（Csx）。在此，我们重新审视其中一种蛋白Csx3的结构分析，并表明这种同二聚体蛋白利用罗斯曼折叠来识别一个RNA四核苷酸。确定了Csx3与CRISPR/Cas蛋白Csx1和Csa3的三级和四级结构相似性，这表明Csx3是CRISPR相关罗斯曼折叠（CARF）超家族的一个新成员。Csx3/RNA复合物的结构阐明了CARF结构域蛋白识别假对称多核苷酸的一种方式。
