Moarefi I, Jeruzalmi D, Turner J, O'Donnell M, Kuriyan J
Howard Hughes Medical Institute, The Rockefeller University, 1230 York Avenue, New York, NY 10021, USA.
J Mol Biol. 2000 Mar 10;296(5):1215-23. doi: 10.1006/jmbi.1999.3511.
The protein encoded by gene 45 of T4 bacteriophage (gene 45 protein or gp45), is responsible for tethering the catalytic subunit of T4 DNA Polymerase to DNA during high-speed replication. Also referred to as a sliding DNA clamp, gp45 is similar in its function to the processivity factors of bacterial and eukaryotic DNA polymerases, the beta-clamp and PCNA, respectively. Crystallographic analysis has shown that the beta-clamp and PCNA form highly symmetrical ring-shaped structures through which duplex DNA can be threaded. Gp45 shares no sequence similarity with beta-clamp or PCNA, and sequence comparisons have not been able to establish whether it adopts a similar structure. We have determined the crystal structure of gp45 from T4 bacteriophage at 2.4 A resolution, using multiple isomorphous replacement. The protein forms a trimeric ring-shaped assembly with overall dimensions that are similar to those of the bacterial and eukaryotic processivity factors. Each monomer of gp45 contains two domains that are very similar in chain fold to those of beta-clamp and PCNA. Despite an overall negative charge, the inner surface of the ring is in a region of positive electrostatic potential, consistent with a mechanism in which DNA is threaded through the ring.
T4噬菌体基因45编码的蛋白质（基因45蛋白或gp45），负责在高速复制过程中将T4 DNA聚合酶的催化亚基与DNA相连。gp45也被称为滑动DNA夹，其功能分别与细菌和真核生物DNA聚合酶的持续性因子β夹和增殖细胞核抗原（PCNA）相似。晶体学分析表明，β夹和PCNA形成高度对称的环形结构，双链DNA可从中穿过。Gp45与β夹或PCNA没有序列相似性，序列比较也无法确定它是否采用类似结构。我们利用多重同晶置换法，以2.4埃的分辨率确定了T4噬菌体gp45的晶体结构。该蛋白形成三聚体环形组装体，其整体尺寸与细菌和真核生物的持续性因子相似。gp45的每个单体包含两个结构域，其链折叠与β夹和PCNA的结构域非常相似。尽管整体带负电荷，但环的内表面处于正静电势区域，这与DNA穿过环中心的机制一致。
