Mechanism of Foreign DNA Selection in a Bacterial Adaptive Immune System
Mechanism of Foreign DNA Selection in a Bacterial Adaptive Immune System
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DOI：
10.1016/j.molcel.2012.03.020
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发表时间：
2012-06-08
期刊：
影响因子：
16
通讯作者：
Doudna, Jennifer A.
中科院分区：
文献类型：
--
作者：
Sashital, Dipali G.;Wiedenheft, Blake;Doudna, Jennifer A.
在细菌和古细菌CRISPR免疫途径中，来自入侵噬菌体或质粒的DNA序列被整合到宿主基因组内的CRISPR基因座中，从而赋予针对后续感染的免疫力。核糖核蛋白复合物Cascade利用从这些基因座产生的RNA靶向互补的“非自身”DNA序列进行破坏，同时避免与CRISPR基因座内的“自身”序列结合。在这里，我们表明CasA，Cascade的最大蛋白质亚基，是非自身靶点识别和结合所必需的。将CasA的2.3埃晶体结构与Cascade的cryo-EM结构相结合，我们已经确定了病毒防御所需的环。该环接触非自身靶选择所需的保守的三个碱基对基序。我们的数据表明，CasA环在靶不稳定和结合之前扫描DNA中的这个短基序，从而最大限度地提高Cascade的DNA监视效率。
In bacterial and archaeal CRISPR immune pathways, DNA sequences from invading bacteriophage or plasmids are integrated into CRISPR loci within the host genome, conferring immunity against subsequent infections. The ribonucleoprotein complex Cascade utilizes RNAs generated from these loci to target complementary "nonself" DNA sequences for destruction, while avoiding binding to "self" sequences within the CRISPR locus. Here we show that CasA, the largest protein subunit of Cascade, is required for nonself target recognition and binding. Combining a 2.3 angstrom crystal structure of CasA with cryo-EM structures of Cascade, we have identified a loop that is required for viral defense. This loop contacts a conserved three base pair motif that is required for nonself target selection. Our data suggest a model in which the CasA loop scans DNA for this short motif prior to target destabilization and binding, maximizing the efficiency of DNA surveillance by Cascade.