Crystal structure of the eukaryotic origin recognition complex

摘要

真核生物利用一个有六个亚单元的DNA结合复合物(起始识别复合体，即ORC)来启动DNA复制。James Berger及同事以3.5A的分辨率获得了这一270kDa复合物的结构。ORC有缺口的环结构将使其能够包围双螺旋DNA，同时与"mini-chromosome maintenance 2-7(MCM2-7)"解旋酶复合物相接触的"域面"(domain face)也被识别了出来。Orc1亚单元的位置显示了该亚单元会怎样调控该复合物的一个自抑制状态、以使复制启动能够对细胞周期或发育提示做出反应。%Initiation of cellular DNA replication is tightly controlled to sustain genomic integrity. In eukaryotes, the heterohexameric origin recognition complex (ORC) is essential for coordinating replication onset. Here we describe the crystal structure of Drosophila ORC at 3.5 A resolution, showing that the 270 kilodalton initiator core complex comprises a two-layered notched ring in which a collar of winged-helix domains from the Orc1-5 subunits sits atop a layer of AAA+ (ATPases associated with a variety of cellular activities) folds. Although canonical inter-AAA+ domain interactions exist between four of the six ORC subunits, unanticipated features are also evident. These include highly interdigitated domain - swapping interactions between the winged-helix folds and AAA+ modules of neighbouring protomers, and a quasi-spiral arrangement of DNA binding elements that circumnavigate an approximately 20 A wide channel in the centre of the complex. Comparative analyses indicate that ORC encircles DNA, using its winged-helix domain face to engage the mini-chromosome maintenance 2-7 (MCM2-7) complex during replicative helicase loading; however, an observed out-of-plane rotation of more than 90° for the Orcl AAA+ domain disrupts interactions with catalytic amino acids in Orc4, narrowing and sealing off entry into the central channel. Prima facie, our data indicate that Drosophila ORC can switch between active and autoinhibited conformations, suggesting a novel means for cell cycle and/or developmental control of ORC functions.