Fig. 10.

The mammalian ‘ORC cycle’. All six ORC subunits (shaded circles) are bound tightly to chromatin during G1 phase to provide sites for initiation of pre-replication complex (pre-RC) assembly. DNA synthesis does not begin until pre-RCs are activated by CcnE-Cdk2, a protein kinase whose activity is inhibited during G1 phase by the CDK-specific inhibitor p27/Kip1. With the onset of S phase, the affinity of Orc1 for chromatin is selectively reduced, and Orc1 is targeted for ubiquitylation by SCF^Skp2. Since SCF^Skp2-dependent degradation of p27 is essential for cell-cycle progression (Kossatz et al., 2004), CcnE-Cdk2 activation is accompanied by Orc1 inactivation, resulting in initiation of S phase with concomitant suppression of pre-RC assembly. Orc1 is selectively degraded during S phase in human cells, but not in hamster cells. However, even a single ubiquitin adduct can inactivate Orc1 by transporting it to the cytoplasm. In M phase, Orc1 is hyperphosphorylated and bound tightly to CcnA-Cdk1, but weakly to chromatin.

Hyperphosphorylation favors cytoplasmic localization. When CcnA and CcnB are degraded during the transition from M to G1 phase, Cdk1 is inactivated, and Orc1 is dephosphorylated and rebound tightly to chromatin. If Orc1 is not associated with ORC, and if it is not ubiquitylated or hyperphosphorylated, then Orc1 can induce apoptosis.