Department of Agricultural Biotechnology, Seoul National University, Seoul 08826, Korea.
Research Institute of Agriculture and Life Sciences, Seoul National University, Seoul 08826, Korea.
Nucleic Acids Res. 2018 Jan 9;46(1):485-492. doi: 10.1093/nar/gkx1181.
Clustered regularly interspaced short palindromic repeats (CRISPRs) and CRISPR-associated (Cas) proteins provide bacteria with RNA-based adaptive immunity against phage infection. To counteract this defense mechanism, phages evolved anti-CRISPR (Acr) proteins that inactivate the CRISPR-Cas systems. AcrIIA1, encoded by Listeria monocytogenes prophages, is the most prevalent among the Acr proteins targeting type II-A CRISPR-Cas systems and has been used as a marker to identify other Acr proteins. Here, we report the crystal structure of AcrIIA1 and its RNA-binding affinity. AcrIIA1 forms a dimer with a novel two helical-domain architecture. The N-terminal domain of AcrIIA1 exhibits a helix-turn-helix motif similar to transcriptional factors. When overexpressed in Escherichia coli, AcrIIA1 associates with RNAs, suggesting that AcrIIA1 functions via nucleic acid recognition. Taken together, the unique structural and functional features of AcrIIA1 suggest its distinct mode of Acr activity, expanding the diversity of the inhibitory mechanisms employed by Acr proteins.
成簇规律间隔短回文重复序列 (CRISPRs) 和 CRISPR 相关 (Cas) 蛋白为细菌提供了基于 RNA 的适应性免疫，以抵御噬菌体感染。为了对抗这种防御机制，噬菌体进化出了抗 CRISPR (Acr) 蛋白，使 CRISPR-Cas 系统失活。AcrIIA1 由李斯特菌噬菌体编码，是针对 II-A 型 CRISPR-Cas 系统的 Acr 蛋白中最常见的一种，并且一直被用作识别其他 Acr 蛋白的标志物。在这里，我们报告了 AcrIIA1 的晶体结构及其 RNA 结合亲和力。AcrIIA1 形成一个具有新颖双螺旋结构域架构的二聚体。AcrIIA1 的 N 端结构域具有类似于转录因子的螺旋-转角-螺旋基序。当在大肠杆菌中过表达时，AcrIIA1 与 RNA 结合，表明 AcrIIA1 通过核酸识别发挥作用。综上所述，AcrIIA1 的独特结构和功能特征表明其 Acr 活性具有独特的模式，扩展了 Acr 蛋白所采用的抑制机制的多样性。
