title: The ORC1 cycle in human cells: I. cell cycle-regulated oscillation of human ORC1.
abstract: Components of ORC (the origin recognition complex) are highly conserved among eukaryotes and are thought to play an essential role in the initiation of DNA replication. The level of the largest subunit of human ORC (ORC1) during the cell cycle was studied in several human cell lines with a specific antibody. In all cell lines, ORC1 levels oscillate: ORC1 starts to accumulate in mid-G1 phase, reaches a peak at the G1/S boundary, and decreases to a basal level in S phase. In contrast, the levels of other ORC subunits (ORCs 2-5) remain constant throughout the cell cycle. The oscillation of ORC1, or the ORC1 cycle, also occurs in cells expressing ORC1 ectopically from a constitutive promoter. Furthermore, the 26 S proteasome inhibitor MG132 blocks the decrease in ORC1, suggesting that the ORC1 cycle is mainly due to 26 S proteasome-dependent degradation. Arrest of the cell cycle in early S phase by hydroxyurea, aphidicolin, or thymidine treatment is associated with basal levels of ORC1, indicating that ORC1 proteolysis starts in early S phase and is independent of S phase progression. These observations indicate that the ORC1 cycle in human cells is highly linked with cell cycle progression, allowing the initiation of replication to be coordinated with the cell cycle and preventing origins from refiring.
authors.affiliation: Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0101, Japan.
authors.forename: Yasutoshi
authors.initials: Y
authors.lastname: Tatsumi
authors.forename: Satoshi
authors.initials: S
authors.lastname: Ohta
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Kimura
authors.forename: Toshiki
authors.initials: T
authors.lastname: Tsurimoto
authors.forename: Chikashi
authors.initials: C
authors.lastname: Obuse
mesh_terms: D016590:Aphidicolin / Q000494:pharmacology; D001973:Bromodeoxyuridine / Q000494:pharmacology; D002453:Cell Cycle; D002460:Cell Line; D004247:DNA / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D016193:G1 Phase; D006367:HeLa Cells; D006801:Humans; D006918:Hydroxyurea / Q000494:pharmacology; D018613:Microscopy, Confocal; D019384:Nucleic Acid Synthesis Inhibitors / Q000494:pharmacology; D051738:Origin Recognition Complex; D010447:Peptide Hydrolases / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex; D016196:S Phase; D013936:Thymidine / Q000494:pharmacology; D013997:Time Factors
chemical_list: D004268:DNA-Binding Proteins; D004791:Enzyme Inhibitors; D019384:Nucleic Acid Synthesis Inhibitors; C497223:ORC1 protein, human; D051738:Origin Recognition Complex; D016590:Aphidicolin; D004247:DNA; D010447:Peptide Hydrolases; D046988:Proteasome Endopeptidase Complex; C061553:ATP dependent 26S protease; D001973:Bromodeoxyuridine; D013936:Thymidine; D006918:Hydroxyurea