The human origin recognition complex protein 1 dissociates from chromatin during S phase in HeLa cells

We investigated the association of human origin recognition complex (ORC) proteins hOrc1p and hOrc2p with chromatin in HeLa cells. Independent procedures including limited nuclease digestion and differential salt extraction of isolated nuclei showed that a complex containing hOrc1p and hOrc2p occurs in a nuclease-resistant compartment of chromatin and can be eluted with moderate high salt concentrations. A second fraction of hOrc2p that dissociates in vitro at low salt conditions was found to occur in a chromatin compartment characterized by its high accessibility to micrococcal nuclease. Functional differences between these two sites become apparent in HeLa cells that synchronously enter the S phase after a release from a double-thymidine block. The hOrc1p/hOrc2p-containing complexes dissociate from their chromatin sites during S phase and reassociate at the end of mitosis. In contrast, the fraction of hOrc2p in nuclease-accessible, more open chromatin remains bound during all phases of the cell cycle. We propose that the hOrc1p/hOrc2p-containing complexes are components of the human origin recognition complex. Thus, the observed cell cycle-dependent release of the hOrc1p/hOrc2p-containing complexes is in line with previous studies with Xenopus and Drosophila systems, which indicated that a change in ORC stability occurs after prereplication complex formation. This could be a powerful mechanism that prevents the rereplication of already replicated chromatin in the metazoan cell cycle.