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Assembly of the ORC complex at the origin of replication

Summary

Organism

Homo sapiens (human)

Reactome

R-HSA-68616

PubChem

R-HSA-68616

Description

Human ORC1 can associate with DNA origin of replication sites independently of other origin of replication complex (ORC) subunits (Hoshina et al. 2013; Eladl et al. 2021). ORC1 localizes to condensed chromosomes during early mitosis (M phase) and serves as a nucleating center for the assembly of the ORC and, subsequently, the pre-replication complex. ORC1 remains associated with late replication origins throughout late G1. Upon S phase entry, ORC1 undergoes ubiquitin-mediated degradation, leading to dissociation of the ORC from chromatin (Kara et al. 2015).

Most human replication origins contain guanine (G)-rich sequences which may form G-quadruplex (G4) structures (Besnard et al. 2012) and these G4 structures may mediate the recognition of replication origins by ORC1 (Hoshina et al. 2013; Eladl et al. 2021). Besides binding to nucleosome-free replication origin DNA, ORC1 interacts with neighboring nucleosomes (Hizume et al. 2013), in particular with nucleosomes containing histone H4 dimethylated at lysine 21 (H4K20me2 mark), which is enriched at replication origins. Binding of ORC1 to H4K20me2 facilitates ORC1 binding to replication origins and ORC chromatin loading (Kuo et al. 2012, Zhang et al. 2015).

ORC1 binding sites are universally associated with transcription start sites (TSSs) of coding and non-coding RNAs. Replication origins associated with moderate to high transcription level TSSs (belonging to coding RNAs) fire in early S phase, while those associated with low transcription level TSSs (belonging to non-coding RNAs) fire throughout the S phase (Dellino et al. 2013).

ORC2 forms a heterodimer with ORC3, which is a prerequisite for the association of ORC5 and, subsequently, ORC4 (Ranjan and Gossen 2006; Siddiqui and Stillman 2007). ORC1 binds to the ORC(2-5) complex in the nucleus to form a stable ORC(1-5) complex (Radichev et al. 2006; Ghosh et al. 2011). ORC1 is necessary for the association of the ORC(2-5) complex to chromatin (Radichev et al. 2006). The ORC(2-5) complex exhibits a tightly autoinhibited conformation, with the winged-helix domain (WHD) of ORC2 completely blocking the central DNA-binding channel. Binding of ORC1 remodels the WHD of ORC2, moving it away from the central channel and partially relieving the autoinhibition (Cheng et al. 2020, Jaremko et al. 2020). ORC6 associates with the ORC(1-5) complex to form the ORC(1-6) complex (Ghosh et al. 2011). The association of ORC6 with the ORC(1-5) complex is weak and it frequently does not co-immunoprecipitate with the other ORC(1-5) subunits. ORC4 is the only ORC(1-5) subunit that was shown to directly bind to ORC6 (Radichev et al. 2006). Some ORC6 mutations reported in Meier-Gorlin syndrome were shown to interfere with ORC6 incorporation into the ORC (Balasov et al. 2015).

Click on a node on the pathway to see its details. Glycoproteins are marked with a glycoprotein icon in their name.

SPV: a JavaScript Signaling Pathway Visualizer, Bioinformatics, 2018

Displaying entries **1 - 10** of 25 in total

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UniProt ID	Protein Name	Gene Symbol	Pathway Viewer
O43913	Origin recognition complex subunit 5	ORC5 ORC5L	view
O60814	Histone H2B type 1-K	H2BC12 H2BFT HIRIP1 HIST1H2BK	view
P04908	Histone H2A type 1-B/E	H2AC4; H2AC8 H2AFA H2AFM HIST1H2AB HIST1H2AE	view
P06899	Histone H2B type 1-J	H2BC11 H2BFR HIST1H2BJ	view
P16104	Histone H2AX	H2AFX H2AX	view
P23527	Histone H2B type 1-O	H2BC17 H2BFH H2BFN HIST1H2BO	view
P33778	Histone H2B type 1-B	H2BC3 H2BFF HIST1H2BB	view
P57053	Histone H2B type F-S	H2BC12L H2BFS H2BS1	view
P58876	Histone H2B type 1-D	H2BC4; H2BC6; H2BC7; H2BC8; H2BC10 H2BC5 H2BFA H2BFB H2BFG H2BFH H2BFK H2BFL HIRIP2 HIST1H2BC HIST1H2BD HIST1H2BE HIST1H2BF HIST1H2BG HIST1H2BI	view
P62805	Histone H4	H4-16 H4/A H4/B H4/C H4/D H4/E H4/G H4/H H4/I H4/J H4/K H4/M H4/N H4/O H4C1; H4C2; H4C3; H4C4; H4C5; H4C6; H4C8; H4C9; H4C11; H4C12; H4C13; H4C14; H4C15; H4C16 H4F2 H4FA H4FB H4FC H4FD H4FE H4FG H4FH H4FI H4FJ H4FK H4FM H4FN H4FO HIST1H4A HIST1H4B HIST1H4C HIST1H4D HIST1H4E HIST1H4F HIST1H4H HIST1H4I HIST1H4J HIST1H4K HIST1H4L HIST2H4 HIST2H4A HIST2H4B HIST4H4	view

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