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The ORC1 cycle in human cells: II. Dynamic changes in the human ORC complex during the cell cycle
Satoshi Ohta1, Yasutoshi Tatsumi, Masatoshi Fujita
1Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0101, Japan.
The Journal of Biological Chemistry
|August 12, 2003
Summary
The origin recognition complex (ORC) subunit ORC1 regulates DNA replication initiation in eukaryotes. ORC1 accumulation in G1 phase recruits other ORC subunits to nuclear structures, enabling MCM protein loading onto chromatin.
Area of Science:
- Cell Biology
- Molecular Biology
- Genetics
Background:
- The origin recognition complex (ORC) is crucial for eukaryotic DNA replication initiation.
- ORC1 subunit levels oscillate during the cell cycle, unlike stable ORCs 2-5.
Purpose of the Study:
- To investigate the role of ORC1 in regulating the ORC complex and DNA replication in human cells.
- To elucidate the mechanism by which ORC1 influences ORC subunit localization and MCM protein loading.
Main Methods:
- Analysis of ORC subunit behavior in human cell nuclei throughout the cell cycle.
- Fractionation of nuclear components into nuclease-insoluble and -soluble fractions.
- RNA interference to reduce ORC1 levels and assess downstream effects on ORC2 and MCM proteins.
Main Results:
- ORCs 2-5 form a stable complex, associating with ORC1 upon its accumulation in G1 phase.
- ORC1 recruits ORCs 2-5 to nuclease-insoluble nuclear structures, forming the ORC1-5 complex.
- Reduced ORC1 levels cause ORC2 to shift to the soluble fraction and block MCM protein chromatin loading.
Conclusions:
- ORC1 acts as a key regulator, tethering the ORC complex to nuclear structures.
- This ORC1-dependent dynamic is essential for MCM protein recruitment to chromatin.
- Temporal ORC1 accumulation in G1 may control pre-replication complex formation in human cells.