York Structural Biology Laboratory, Department of Chemistry, University of York, York YO10 5DD, UK.
Structural and Computational Biology Unit, European Molecular Biology Laboratory, 69117Heidelberg, Germany.
Nucleic Acids Res. 2023 Jul 21;51(13):7025-7035. doi: 10.1093/nar/gkad480.
Double-stranded DNA viruses utilise machinery, made of terminase proteins, to package viral DNA into the capsid. For cos bacteriophage, a defined signal, recognised by small terminase, flanks each genome unit. Here we present the first structural data for a cos virus DNA packaging motor, assembled from the bacteriophage HK97 terminase proteins, procapsids encompassing the portal protein, and DNA containing a cos site. The cryo-EM structure is consistent with the packaging termination state adopted after DNA cleavage, with DNA density within the large terminase assembly ending abruptly at the portal protein entrance. Retention of the large terminase complex after cleavage of the short DNA substrate suggests that motor dissociation from the capsid requires headful pressure, in common with pac viruses. Interestingly, the clip domain of the 12-subunit portal protein does not adhere to C12 symmetry, indicating asymmetry induced by binding of the large terminase/DNA. The motor assembly is also highly asymmetric, showing a ring of 5 large terminase monomers, tilted against the portal. Variable degrees of extension between N- and C-terminal domains of individual subunits suggest a mechanism of DNA translocation driven by inter-domain contraction and relaxation.
双链 DNA 病毒利用由末端酶蛋白组成的机器将病毒 DNA 包装到衣壳中。对于 cos 噬菌体，小末端酶识别的特定信号侧翼每个基因组单元。在这里，我们展示了第一个 cos 病毒 DNA 包装马达的结构数据，该马达由噬菌体 HK97 末端酶蛋白、包含门户蛋白的原衣壳和含有 cos 位点的 DNA 组装而成。低温电镜结构与 DNA 切割后采用的包装终止状态一致，在大末端酶组装内的 DNA 密度在门户蛋白入口处突然终止。短 DNA 底物切割后大末端酶复合物的保留表明，与 pac 病毒一样，马达从衣壳解离需要满负荷压力。有趣的是，12 亚基门户蛋白的夹域不遵守 C12 对称，表明结合大末端酶/DNA 诱导了不对称。马达组装也高度不对称，显示出 5 个大末端酶单体的环，倾斜于门户。单个亚基的 N 端和 C 端结构域之间的可变性延伸表明 DNA 转位的机制是由结构域收缩和松弛驱动的。
