Nishimura Mitsuhiro, Wang Junjie, Wakata Aika, Sakamoto Kento, Mori Yasuko
Division of Clinical Virology, Kobe University Graduate School of Medicine, Kobe, Japan.
Division of Clinical Virology, Kobe University Graduate School of Medicine, Kobe, Japan
J Virol. 2017 Oct 13;91(21). doi: 10.1128/JVI.01121-17. Print 2017 Nov 1.
Immediate early proteins of human herpesvirus 6A (HHV-6A) are expressed at the outset of lytic infection and thereby regulate viral gene expression. Immediate early protein 2 (IE2) of HHV-6A is a transactivator that drives a variety of promoters. The C-terminal region of HHV-6A IE2 is shared among IE2 homologs in betaherpesviruses and is involved in dimerization, DNA binding, and transcription factor binding. In this study, the structure of the IE2 C-terminal domain (IE2-CTD) was determined by X-ray crystallography at a resolution of 2.5 Å. IE2-CTD forms a homodimer stabilized by a β-barrel core with two interchanging long loops. Unexpectedly, the core structure resembles those of the gammaherpesvirus factors EBNA1 of Epstein-Barr virus and LANA of Kaposi sarcoma-associated herpesvirus, but the interchanging loops are longer in IE2-CTD and form helix-turn-helix (HTH)-like motifs at their tips. The HTH and surrounding α-helices form a structural feature specific to the IE2 group. The apparent DNA-binding site (based on structural similarity with EBNA1 and LANA) resides on the opposite side of the HTH-like motifs, surrounded by positive electrostatic potential. Mapping analysis of conserved residues on the three-dimensional structure delineated a potential factor-binding site adjacent to the expected DNA-binding site. The predicted bi- or tripartite functional sites indicate a role for IE2-CTD as an adapter connecting the promoter and transcriptional factors that drive gene expression. Human herpesvirus 6A (HHV-6A) and HHV-6B belong to betaherpesvirus subfamily. Both viruses establish lifelong latency after primary infection, and their reactivation poses a significant risk to immunocompromised patients. Immediate early protein 2 (IE2) of HHV-6A and HHV-6B is a transactivator that triggers viral replication and contains a DNA-binding domain shared with other betaherpesviruses such as human herpesvirus 7 and human cytomegalovirus. In this study, an atomic structure of the DNA-binding domain of HHV-6A IE2 was determined and analyzed, enabling a structure-based understanding of the functions of IE2, specifically DNA recognition and interaction with transcription factors. Unexpectedly, the dimeric core resembles the DNA-binding domain of transcription regulators from gammaherpesviruses, showing structural conservation as a DNA-binding domain but with its own unique structural features. These findings facilitate further characterization of this key viral transactivator.
人类疱疹病毒6A（HHV - 6A）的立即早期蛋白在裂解感染开始时表达，从而调节病毒基因表达。HHV - 6A的立即早期蛋白2（IE2）是一种反式激活因子，可驱动多种启动子。HHV - 6A IE2的C末端区域在β疱疹病毒的IE2同源物中是共享的，并且参与二聚化、DNA结合和转录因子结合。在本研究中，通过X射线晶体学以2.5 Å的分辨率确定了IE2 C末端结构域（IE2 - CTD）的结构。IE2 - CTD形成一个同型二聚体，由一个带有两个互换长环的β桶核心稳定。出乎意料的是，核心结构类似于爱泼斯坦 - 巴尔病毒的γ疱疹病毒因子EBNA1和卡波西肉瘤相关疱疹病毒的LANA，但IE2 - CTD中的互换环更长，并且在其末端形成螺旋 - 转角 - 螺旋（HTH）样基序。HTH和周围的α螺旋形成了IE2组特有的结构特征。明显的DNA结合位点（基于与EBNA1和LANA的结构相似性）位于HTH样基序的另一侧，被正静电势包围。对三维结构上保守残基的图谱分析描绘了一个与预期DNA结合位点相邻的潜在因子结合位点。预测的双部分或三部分功能位点表明IE2 - CTD作为连接启动子和驱动基因表达的转录因子的衔接子发挥作用。人类疱疹病毒6A（HHV - 6A）和HHV - 6B属于β疱疹病毒亚科。两种病毒在初次感染后都会建立终身潜伏，它们的重新激活对免疫功能低下的患者构成重大风险。HHV - 6A和HHV - 6B的立即早期蛋白2（IE2）是一种触发病毒复制的反式激活因子，并且包含与其他β疱疹病毒（如人类疱疹病毒7和人类巨细胞病毒）共享的DNA结合结构域。在本研究中，确定并分析了HHV - 6A IE2的DNA结合结构域的原子结构，从而能够基于结构理解IE2的功能，特别是DNA识别以及与转录因子的相互作用。出乎意料的是，二聚体核心类似于γ疱疹病毒转录调节因子的DNA结合结构域，显示出作为DNA结合结构域的结构保守性，但具有其自身独特的结构特征。这些发现有助于进一步表征这种关键的病毒反式激活因子。
