Egelman E H, Yu X, Wild R, Hingorani M M, Patel S S
Department of Cell Biology and Neuroanatomy, University of Minnesota Medical School, Minneapolis 55455, USA.
Proc Natl Acad Sci U S A. 1995 Apr 25;92(9):3869-73. doi: 10.1073/pnas.92.9.3869.
Most helicases studied to date have been characterized as oligomeric, but the relation between their structure and function has not been understood. The bacteriophage T7 gene 4 helicase/primase proteins act in T7 DNA replication. We have used electron microscopy, three-dimensional reconstruction, and protein crosslinking to demonstrate that both proteins form hexameric rings around single-stranded DNA. Each subunit has two lobes, so the hexamer appears to be two-tiered, with a small ring stacked on a large ring. The single-stranded DNA passes through the central hole of the hexamer, and the data exclude substantial wrapping of the DNA about or within the protein ring. Further, the hexamer binds DNA with a defined polarity as the smaller ring of the hexamer points toward the 5' end of the DNA. The similarity in three-dimensional structure of the T7 gene 4 proteins to that of the Escherichia coli RuvB helicase suggests that polar rings assembled around DNA may be a general feature of numerous hexameric helicases involved in DNA replication, transcription, recombination, and repair.
迄今为止研究的大多数解旋酶都被表征为寡聚体，但它们的结构与功能之间的关系尚未被理解。噬菌体T7基因4解旋酶/引发酶蛋白在T7 DNA复制中起作用。我们利用电子显微镜、三维重建和蛋白质交联来证明这两种蛋白围绕单链DNA形成六聚体环。每个亚基有两个叶，因此六聚体似乎是两层的，一个小环堆叠在一个大环上。单链DNA穿过六聚体的中心孔，并且数据排除了DNA在蛋白环周围或内部的大量缠绕。此外，六聚体以确定的极性结合DNA，因为六聚体的较小环指向DNA的5'端。T7基因4蛋白与大肠杆菌RuvB解旋酶在三维结构上的相似性表明，围绕DNA组装的极性环可能是参与DNA复制、转录、重组和修复的众多六聚体解旋酶的一个普遍特征。
