Structural insight into the assembly and conformational activation of human origin recognition complex

Jiaxuan Cheng,Xiaohan Wang,Yuanliang Zhai,Ning Gao,Ningning Li,Jiazhi Hu

doi:10.1038/s41421-020-00232-3

Abstract

The function of the origin recognition complex (ORC) in DNA replication is highly conserved in recognizing and marking the initiation sites. The detailed molecular mechanisms by which human ORC is reconfigured into a state competent for origin association remain largely unknown. Here, we present structural characterizations of human ORC1–5 and ORC2–5 assemblies. ORC2–5 exhibits a tightly autoinhibited conformation with the winged-helix domain of ORC2 completely blocking the central DNA-binding channel. The binding of ORC1 partially relieves the autoinhibitory effect of ORC2–5 through remodeling ORC2-WHD, which makes ORC2-WHD away from the central channel creating a still autoinhibited but more dynamic structure. In particular, the AAA+ domain of ORC1 is highly flexible to sample a variety of conformations from inactive to potentially active states. These results provide insights into the detailed mechanisms regulating the autoinhibition of human ORC and its subsequent activation for DNA binding.

Highlights

In eukaryotes, DNA replication initiation is tightly regulated to ensure that the genome can be replicated once and only once per cell cycle
HsORC in two functional forms, ORC2–5 and ORC1–5. Both complexes adopt autoinhibited conformations, and in particular, HsORC2–5 appears to be more compact with its DNA-binding channel completely occupied by ORC2-winged-helix domain (WHD) when compared with HsORC1–5 and Drosophila melanogaster ORC (DmORC)
The binding of ORC1 rearranges ORC2–5 core complex into a less inhibited state with a narrow central channel. These results provide critical insights for us to understand the mechanisms of origin recognition complex (ORC) regulation in metazoan to restrict pre-replication complex (pre-RC) assembly only within G1 phase

Summary

Introduction

DNA replication initiation is tightly regulated to ensure that the genome can be replicated once and only once per cell cycle. Central to this regulation is the loading and subsequent activation of the mini-chromosome maintenance (Mcm2–7) helicase complex at replication origins[1,2,3]. Among Orc[1,2,3,4,5], Orc[1], Orc[4], and Orc[5] each contains a functional AAA+ domain for nucleotide binding; in contrast, Orc[2] and Orc[3] only contain a RecA-like subdomain with no predicted ATPase activities[4,5,6]. Orc[6] bears little resemblance to other Orc subunits but is essential for helicase loading[5,7,8,9]

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Conclusion