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Origin recognition complex (ORC), a six-protein complex, is the most likely initiator of chromosomal DNA replication in eukaryotes. Throughout the cell cycle, ORC binds to chromatin at origins of DNA replication and functions as a ‘landing pad’ for the binding of other proteins, including Cdt1p, to form a prereplicative complex. In this study, we used yeast two-hybrid analysis to examine the interaction between Cdt1p and every ORC subunit. We observed potent interaction with Orc6p, and weaker interaction with Orc2p and Orc5p. Coimmunoprecipitation assay confirmed that Cdt1p interacted with Orc6p, as well as with Orc1p and Orc2p. We mapped the C-terminal region, and a middle region of Orc6p (amino acids residues 394–435, and 121–175, respectively), as important for interaction with Cdt1p. Cdt1p was purified to examine its direct interaction with ORC, and its effect on the activity of ORC. Glutathione- S-transferase pull-down analysis revealed that Cdt1p binds directly to ORC. Cdt1p neither bound to origin DNA and ATP nor affected ORC-binding to origin DNA and ATP. These results suggest that interaction of Cdt1p with ORC is involved in the formation of the prereplicative complex, rather than in regulation of the activity of ORC. [ABSTRACT FROM AUTHOR]

Origin recognition complex (ORC), a six-protein complex (Orc1p–6p), is the most likely initiator of chromosomal DNA replication in eukaryotes. Although ORC of Saccharomyces cerevisiae has been studied extensively from biochemical and genetic perspectives, its quaternary structure remains unknown. Previous studies suggested that ORC has functions other than DNA replication, such as gene silencing, but the molecular mechanisms of these functions have not been determined. In this study, we used yeast two-hybrid analysis to examine the interaction between ORC subunits and to search for ORC-binding proteins. As well as the known Orc4p–Orc5p interaction, we revealed strong interactions between Orc2p and Ord3p (2p–3p), Orc2p and Ord5p (2p–5p), Orc2p and Ord6p (2p–6p) and Orc3p and Ord6p (3p–6p) and weaker interactions between Orc1p and Ord4p (1p–4p), Orc3p and Ord4p (3p–4p), Orc2p and Ord3p (3p–5p) and Orc5p and Ord3p (5p–6p). These results suggest that 2p–3p–6p may form a core complex. Orc2p and Orc6p are phosphorylated in vivo, regulating initiation of DNA replication. However, replacing the phosphorylated amino acid residues with others that cannot be phosphorylated, or that mimic phosphorylation, did not affect subunit interactions. We also identified several proteins that interact with ORC subunits; Sir4p and Mad1p interact with Orc2p; Cac1p and Ykr077wp with Orc3p; Rrm3p and Swi6p with Orc5p; and Mih1p with Orc6p. We discuss roles of these interactions in functions of ORC. [ABSTRACT FROM AUTHOR]