The sophisticated tail structures of DNA bacteriophages play essential roles in life cycles. Podoviruses P22 and Sf6 have short tails consisting of multiple proteins, among which is a tail adaptor protein that connects the portal protein to the other tail proteins. Assembly of the tail has been shown to occur in a sequential manner to ensure proper molecular interactions, but the underlying mechanism remains to be understood. Here, we report the high-resolution structure of the tail adaptor protein gp7 from phage Sf6. The structure exhibits distinct distribution of opposite charges on two sides of the molecule. A gp7 dodecameric ring model shows an entirely negatively charged surface, suggesting that the assembly of the dodecamer occurs through head-to-tail interactions of the bipolar monomers. The N-terminal helix-loop structure undergoes rearrangement compared with that of the P22 homolog complexed with the portal, which is achieved by repositioning of two consecutive repeats of a conserved octad sequence motif. We propose that the conformation of the N-terminal helix-loop observed in the Sf6-gp7 and P22 portal:gp4 complex represents the pre- and postassembly state, respectively. Such motif repositioning may serve as a conformational switch that creates the docking site for the tail nozzle only after the assembly of adaptor protein to the portal. In addition, the C-terminal portion of gp7 shows conformational flexibility, indicating an induced fit on binding to the portal. These results provide insight into the mechanistic role of the adaptor protein in mediating the sequential assembly of the phage tail.

中文翻译：

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足病毒尾部衔接子的高分辨率结构建议重新定位介导顺序尾部组装的八单元体基序

DNA噬菌体的复杂尾巴结构在生命周期中起着至关重要的作用。足病毒P22和Sf6具有由多种蛋白质组成的短尾巴，其中一种是将门户蛋白与其他尾巴蛋白连接的尾巴衔接子蛋白。已显示尾部的组装以顺序的方式进行以确保适当的分子相互作用，但是其潜在机理尚待了解。在这里，我们报道了噬菌体Sf6的尾巴衔接蛋白gp7的高分辨率结构。该结构在分子的两侧表现出不同的相反电荷分布。gp7十二聚体环模型显示出完全带负电荷的表面，表明十二聚体的组装是通过双极性单体的头尾相互作用而发生的。N末端螺旋环结构与与门形成复合物的P22同源物相比，发生了重排，这是通过重新排列保守的octad序列基序的两个连续重复来实现的。我们建议在Sf6-gp7和P22 portal：gp4复合物中观察到的N末端螺旋环的构象分别代表组装前和组装后的状态。这样的基序重新定位可以用作构象开关，该构象开关仅在衔接子蛋白组装到门上之后才为尾喷嘴建立对接位点。此外，gp7的C端部分显示构象柔韧性，表明与门户结合时诱导适合。这些结果提供了对衔接子蛋白在介导噬菌体尾部的顺序组装中的机械作用的见解。