Alternative titles; symbols
HGNC Approved Gene Symbol: ORC2
Cytogenetic location: 2q33.1 Genomic coordinates (GRCh38): 2:200,908,977-200,963,660 (from NCBI)
The origin recognition complex (ORC) is a multiprotein assemblage identified in S. cerevisiae that binds to the ARS (autonomously replicating sequence) consensus, a DNA motif that is an essential part of yeast origins of replication. ORC binding has also been implicated in transcriptional silencing at certain yeast loci. One component of the ORC is a 72-kD protein designated ORC2, mutations of which disrupt silencing at the HMR-E silencer and cause cell cycle arrest between late G1 and the initiation of DNA replication. Takahara et al. (1996) isolated a cDNA from a mouse embryonic stem cell library and a human placenta library whose predicted 577-amino acid protein resembles the yeast sequence. The mouse and human sequences of the ORC2L (ORC2-like) proteins are 47.9% and 46.3% similar to yeast ORC2, respectively. Northern blots showed highest levels of ORC2L expression in testes.
Gavin et al. (1995) used degenerate PCR to clone a human homolog of the yeast ORC2 gene, which they named HsORC2. Gavin et al. (1995) showed that the HsORC1 (ORC1L; 601902) and HsORC2 gene products can be coimmunoprecipitated, and concluded that they form a complex in vivo.
Ohtani et al. (1996) noted that expression of HsORC2 is not regulated during different phases of cell proliferation.
Takahara et al. (1996) mapped the ORC2L gene by fluorescence in situ hybridization to 2q33.
Initiation of DNA replication requires the assembly of a prereplication complex (pre-RC) in late mitosis and G1, with sequential loading of ORC, CDC6 (602627), CDT1 (605525), and the MCM2-7 complex (see MCM2; 116945) onto replication origins. Upon initiation of DNA replication, the pre-RC is disassembled, and CDT1 and CDC6 are released from the origins to prevent rereplication. Using human cell lines, Shen et al. (2012) showed that ORCA (LRWD1; 615167) was required for pre-RC assembly and replication initiation. Knockdown of ORCA via small interfering RNA reduced association of ORC and MCM2-7 with chromatin and caused failure of cell cycle progression through S phase. ORCA associated dynamically with different pre-RC components during the cell cycle: it associated with ORC and CDT1 at G1, with ORC(2-5) and the CDT1 inhibitor geminin (GMNN; 602842) in S phase, and with ORC(2-5), phosphorylated CDT1, and phosphorylated geminin during mitosis. ORCA interacted directly with ORC2, CDT1, and geminin, and ORC2 was required for ORCA stability. Overexpression of geminin reduced the affinity of CDT1 for ORCA, and loss of association between ORCA and CDT1 appeared to be a key step in disassembling the pre-RC at the end of G1 phase.
Dhar et al. (2001) used homologous recombination to replace the third exon encoding the initiator ATG of the ORC2 gene with a neomycin phosphotransferase gene in a colon carcinoma cell line. This hypomorphic mutation decreased ORC2 protein levels by 90%. The G1 phase of the cell cycle was prolonged, but there was no effect on the utilization of either the MYC (190080) or beta-globin (141900) cellular origins of replication. Cells carrying the ORC2 mutation failed to support the replication of a plasmid bearing the oriP replicator of Epstein-Barr virus (EBV), and this defect could be rescued by reintroduction of ORC2. ORC2 specifically associated with oriP in cells, most likely through its interaction with EBV nuclear antigen-1. Geminin (602842), an inhibitor of the mammalian replication initiation complex, inhibited replication from oriP. Therefore, ORC and the human replication initiation apparatus is required for replication from a viral origin of replication.
Dhar, S. K., Yoshida, K., Machida, Y., Khaira, P., Chaudhuri, B., Wohlschlegel, J. A., Leffak, M., Yates, J., Dutta, A. Replication from oriP of Epstein-Barr virus requires human ORC and is inhibited by geminin. Cell 106: 287-296, 2001. [PubMed: 11509178] [Full Text: https://doi.org/10.1016/s0092-8674(01)00458-5]
Gavin, K. A., Hidaka, M., Stillman, B. Conserved initiator proteins in eukaryotes. Science 270: 1667-1671, 1995. [PubMed: 7502077] [Full Text: https://doi.org/10.1126/science.270.5242.1667]
Ohtani, K., DeGregori, J., Leone, G., Herendeen, D. R., Kelly, T. J., Nevins, J. R. Expression of the HsOrc1 gene, a human ORC1 homolog, is regulated by cell proliferation via the E2F transcription factor. Molec. Cell. Biol. 16: 6977-6984, 1996. [PubMed: 8943353] [Full Text: https://doi.org/10.1128/MCB.16.12.6977]
Shen, Z., Chakraborty, A., Jain, A., Giri, S., Ha, T., Prasanth, K. V., Prasanth, S. G. Dynamic association of ORCA with prereplicative complex components regulates DNA replication initiation. Molec. Cell. Biol. 32: 3107-3120, 2012. [PubMed: 22645314] [Full Text: https://doi.org/10.1128/MCB.00362-12]
Takahara, K., Bong, M., Brevard, R., Eddy, R. L., Haley, L. L., Sait, S. J., Shows, T. B., Hoffman, G. G., Greenspan, D. S. Mouse and human homologues of the yeast origin of replication recognition complex subunit ORC2 and chromosomal localization of the cognate human gene ORC2L. Genomics 31: 119-122, 1996. [PubMed: 8808289] [Full Text: https://doi.org/10.1006/geno.1996.0018]