High impact information on ORC1

• ATP binding and hydrolysis by Orc1p are both regulated by origin DNA in a sequence-specific manner [1].
• Here we report the cloning of the genes encoding the 120 kDa (ORC1), 62 kDa (ORC3), and 56 kDa (ORC4) subunits of ORC and the reconstitution of the complete complex after expression of all six subunits in insect cells [2].
• Orc1p is related to Cdc6p and Cdc18p, which regulate DNA replication and mitosis, and to Sir3p, a regulator of transcriptional silencing [2].
• The multidomain structure of Orc1p reveals similarity to regulators of DNA replication and transcriptional silencing [2].
• Genetic analysis of orc1+ from S. pombe reveals that it is essential for cell viability [3].

Biological context of ORC1

• Conservation of structures among members of the ORC1-related proteins suggests that these proteins play a key role in the intiation of DNA replication in all eukaryotes [4].
• This HLH motif is also highly conserved in all eukaryotic ORC1 proteins [5].
• The eight missense mutations all map within a 17-amino-acid segment of Sir1p, and this segment was also required for Sir1p's interaction with Orc1p [6].
• Unlike cdc18+, orp1+ is expressed constitutively during the cell cycle. cdc18+, CDC6, and orp1+ belong to a family of related genes that also includes the gene ORC1, which encodes a subunit of the origin recognition complex (ORC) of S. cerevisiae [7].
• This was blocked completely in the cells that had progressed to the S phase under conditions deficient in ORC1 protein followed by cell death [8].

Anatomical context of ORC1

• In the ovary, the sub-nuclear distribution of ORC1 shifts during a developmentally regulated switch from endoreplication of the entire genome to amplification of the chorion gene clusters [9].
• We find ORC1, 2 and ORC4 to be metabolically stable proteins that co-fractionate with chromatin throughout the cell-cycle in Chinese hamster fibroblasts [10].
• Orc1 is degraded by the 26S proteasome only when released into the cytosol, and peptide additions to Orc1 make it hypersensitive to polyubiquitination [11].
• Assignment of the homologue of the yeast origin recognition complex subunit ORC4 (ORC4L) to human chromosome band 2q22-->q23 by in situ hybridization and somatic cell hybrid analysis [12].

Associations of ORC1 with chemical compounds

• Two-dimensional gel analysis of the replication intermediates after the galactose removal revealed that the depletion of ORC1 protein caused a decrease in the frequency of initiation of chromosomal replication, eventually resulting in the inhibition of replication as a whole [8].
• The N-terminal alanine residues of the silencing protein Sir3 and of Orc1 are acetylated by the NatA Nalpha-acetyltransferase [13].

Physical interactions of ORC1

• The Sir1 protein plays a key role in establishing a silent chromatin structure at the cryptic mating-type loci HMR and HML in Saccharomyces cerevisiae by interacting with the bromo-adjacent homology (BAH) domain of the Orc1p subunit of the origin recognition complex (ORC) [14].

Regulatory relationships of ORC1

• Sir3p is a target of mitogen-activated protein (MAP) kinase cascade regulation and has significant similarity to the Orc1p subunit of the DNA replication origin recognition complex [15].

Other interactions of ORC1

• Thus the amino acids identified are required for Sir1 protein's recognition of the HMR-E silencer and interaction with Orc1p, but not for its ability to function in silencing per se [6].
• In plants, however, the only cloned putative homologs of ORC subunits are the Arabidopsis ORC2 and the rice ORC1 [16].
• Genetic evidence further supported a functional link between NatA and ORC: (i) nat1Delta was synthetically lethal with orc2-1 and (ii) the synthetic lethality between nat1Delta and SUM1-1 required the Orc1 N terminus [17].
• When it is visualized by the chromosome-spreading method followed by immunostaining, Mcm10p forms punctate foci on chromatin throughout the cell cycle and these foci mostly overlap with those of Orc1p, a component of ORC [18].
• These findings indicated that DNA lesions caused by a defect in Orc1p function trigger the RAD9-dependent checkpoint control, which ensures genomic integrity either by stopping the cell cycle progress until lesion repair, or by inducing cell death when the lesion is not properly repaired [19].

Analytical, diagnostic and therapeutic context of ORC1

• Using the yeast two-hybrid system and immunoprecipitation, we report here that murine HP1alpha interacts with the preRC proteins ORC1, ORC2 and CDC6 [20].
• Sequence analysis showed that eight of the nine sir3-eso alleles have mutations within the N-terminal region that is highly similar to the DNA replication initiation protein Orc1p [21].

References