title: The ORC1 cycle in human cells: II. Dynamic changes in the human ORC complex during the cell cycle.
abstract: The origin recognition complex (ORC) plays a central role in regulating the initiation of DNA replication in eukaryotes. The level of the ORC1 subunit oscillates throughout the cell cycle, defining an ORC1 cycle. ORC1 accumulates in G1 and is degraded in S phase, although other ORC subunits (ORCs 2-5) remain at almost constant levels. The behavior of ORC components in human cell nuclei with respect to the ORC1 cycle demonstrates that ORCs 2-5 form a complex that is present throughout the cell cycle and that associates with ORC1 when it accumulates in G1 nuclei. ORCs 2-5 are found in both nuclease-insoluble and -soluble fractions. The appearance of nuclease-insoluble ORCs 2-5 parallels the increase in the level of ORC1 associating with nuclease-insoluble, non-chromatin nuclear structures. Thus, ORCs 2-5 are temporally recruited to nuclease-insoluble structures by formation of the ORC1-5 complex. An artificial reduction in the level of ORC1 in human cells by RNA interference results in a shift of ORC2 to the nuclease-soluble fraction, and the association of MCM proteins with chromatin fractions is also blocked by this treatment. These results indicate that ORC1 regulates the status of the ORC complex in human nuclei by tethering ORCs 2-5 to nuclear structures. This dynamic shift is further required for the loading of MCM proteins onto chromatin. Thus, the pre-replication complex in human cells may be regulated by the temporal accumulation of ORC1 in G1 nuclei.
authors.affiliation: Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0101, Japan.
authors.forename: Satoshi
authors.initials: S
authors.lastname: Ohta
authors.forename: Yasutoshi
authors.initials: Y
authors.lastname: Tatsumi
authors.forename: Masatoshi
authors.initials: M
authors.lastname: Fujita
authors.forename: Toshiki
authors.initials: T
authors.lastname: Tsurimoto
authors.forename: Chikashi
authors.initials: C
authors.lastname: Obuse
mesh_terms: D000595:Amino Acid Sequence; D002453:Cell Cycle; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism; D002843:Chromatin / Q000378:metabolism; D004247:DNA / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000737:chemistry / Q000502:physiology*; D016193:G1 Phase; D006367:HeLa Cells; D006801:Humans; D015151:Immunoblotting; D013058:Mass Spectrometry; D008954:Models, Biological; D008969:Molecular Sequence Data; D051738:Origin Recognition Complex; D011233:Precipitin Tests; D034622:RNA Interference; D016196:S Phase; D013997:Time Factors
chemical_list: D002843:Chromatin; D004268:DNA-Binding Proteins; C497223:ORC1 protein, human; C497224:ORC2 protein, human; D051738:Origin Recognition Complex; D004247:DNA