The role of MCM/P1 proteins in the licensing of DNA replication

Cell cycle control of replication initiation in eukaryotes

Studies on the initiation of DNA replication in eukaryotes have progressed recently through different approaches that promise to converge. Proteins interacting with the origin recognition complex form a prereplicative complex early in the cell cycle. The regulation of the binding of MCM/P1 proteins to chromatin plays a key role in the replication licensing system which prevents re-replication in a single cell cycle. Cyclin-dependent kinases provide an overall control of the cell cycle by stimulating S-phase entry and possibly by preventing re-establishment of prereplicative complexes in G2 phase.

The Xenopus origin recognition complex is essential for DNA replication and MCM binding to chromatin

BACKGROUND

The origin recognition complex (ORC) and the minichromosome maintenance (MCM) protein complex were initially discovered in yeast and shown to be essential for DNA replication. Homologues of ORC and MCM proteins exist in higher eukaryotes, including Xenopus. The Xenopus MCM proteins and the Xenopus homologues of Saccharomyces cerevisiae Orc 1p and Orc2p (XOrc1 and XOrc2) have recently been shown to be essential for DNA replication. Here, we describe the different but interdependent functions of the ORC and MCM complexes in DNA replication in Xenopus egg extracts.

RESULTS

The XOrc1 and XOrc2 proteins are present in the same multiprotein complex in Xenopus egg extracts. Immunodepletion of ORC inhibits DNA replication of Xenopus sperm nuclei. Mixing MCM-depleted and ORC-depleted extracts restores replication capacity. ORC does not co-localize with sites of DNA replication during elongation. However, at initiation the two staining patterns overlap. In contrast to MCMs, which are displaced from chromatin during S phase, XOrc1 and XOrc2 are nuclear chromatin-bound proteins throughout interphase and move to the cytoplasm in mitosis. Permeable HeLa G1- and G2-phase nuclei can replicate in ORC-depleted extract, consistent with the presence of chromatin-bound ORC in both pre-replicative and post-replicative nuclei. Interestingly, the binding of ORC to chromatin does not require the presence of MCMs; however, the binding of MCM proteins to chromatin is dependent on the presence of ORC.

CONCLUSIONS

The Xenopus ORC and the MCM protein complex perform essential, non-redundant functions in DNA replication. Xenopus ORC is bound to chromatin throughout interphase but, in contrast to S. cerevisiae ORC, it appears to be, at least partly, displaced from chromatin during mitosis. The binding of MCM proteins requires the presence of ORC. Thus, the assembly of replication-competent chromatin involves the sequential binding of ORC and MCMs to DNA.

Interaction between the origin recognition complex and the replication licensing system in Xenopus

The origin recognition complex (ORC) binds to origins of replication in budding yeast. We have cloned a Xenopus homolog of the largest ORC polypeptide (XORC1). Immunodepletion of XOrc1 from Xenopus egg extracts blocks the initiation of DNA replication. We have purified Xenopus ORC, consisting of a protein complex similar to yeast ORC. In Xenopus egg extracts, ORC associates with chromatin throughout G1 and S phases. RLF-M, a component of the replication licensing system, also associates with chromatin early in the cell cycle but dissociates during S phase. We show that the assembly of RLF-M onto chromatin is dependent on the presence of chromatin-bound ORC, leading to sequential assembly of initiation proteins onto replication origins during the cell cycle.

The Xenopus Cdc6 protein is essential for the initiation of a single round of DNA replication in cell-free extracts

We have cloned a Xenopus Cdc6 homolog (Xcdc6) and characterized its role in DNA replication with Xenopus egg extracts. Immunodepletion of Xcdc6 abolishes chromosomal replication but not elongation on single-stranded DNA templates. Xcdc6 binds to chromatin at the beginning of interphase but disappears from chromatin upon initiation of replication. Immunodepletion studies indicate that binding of Xcdc6 to chromatin requires Xorc2, a component of the origin recognition complex. Moreover, Xmcm3 cannot bind to chromatin lacking Xcdc6, suggesting that Xorc2, Xcdc6, and Xmcm3 associate with the DNA sequentially. In postreplicative nuclei, Xcdc6 is associated with the nuclear envelope. These studies indicate that Xcdc6, is essential for initiation of replication in vertebrates and that interaction with the nuclear envelope may regulate its function.

Binding of human minichromosome maintenance proteins with histone H3

Minichromosome maintenance (MCM) proteins play essential roles in eukaryotic DNA replication, but their biochemical properties remain to be determined. We detected in HeLa cell extracts six proteins, CDC47, CDC46/MCM5, Cdc21, P1/MCM3, Mis5, and BM28/MCM2, by their binding to a specific antibody and by partial sequencing. The human homologs of the MCM2 (BM28), Mis5, Cdc21, and CDC47 proteins were tightly bound to a histone-Sepharose column and purified to near homogeneity, whereas the P1/MCM3 and CDC46/MCM5 proteins passed through. Among the four core histones, the human BM28/MCM2, Mis5, Cdc21, and CDC47 proteins had high affinity for histone H3. Immunoprecipitation with anti-Cdc21 antibody revealed that these four MCM proteins form complexes. These results are consistent with the findings that MCM proteins bind with chromatin in vivo.

Both cyclin A and cyclin E have S-phase promoting (SPF) activity in Xenopus egg extracts

Extracts of activated Xenopus eggs in which protein synthesis has been inhibited support a single round of chromosomal DNA replication. Affinity-depletion of cyclin dependent kinases (Cdks) from these extracts blocks the initiation of DNA replication. We define ‘S-phase promoting factor’ (SPF) as the Cdk activity required for DNA replication in these Cdk-depleted extracts. Recombinant cyclins A and E, but not cyclin B, showed significant SPF activity. High concentrations of cyclin A promoted entry into mitosis, which inhibited DNA replication. In contrast, high concentrations of cyclin E1 promoted neither nuclear envelope disassembly nor full chromosome condensation. In the early embryo cyclin E1 complexes exclusively with Cdk2 and cyclin A is complexed predominantly with Cdc2; only later in development does cyclin A associate with Cdk2. We show that baculovirus-produced complexes of cyclin A-Cd2, cyclin A-Cdk2 and cyclin E-Cdk2 could each provide SPF activity. These results suggest that although in the early Xenopus embryo cyclin E1-Cdk2 is sufficient to support entry into S-phase, cyclin A-Cdc2 provides a significant additional quantity of SPF as its levels rise during S phase.

Mechanisms restricting DNA replication to once per cell cycle: MCMS, pre-replicative complexes and kinases

An important aspect of cell behaviour is that DNA replication happens only once per cell cycle. Replicated DNA is unable to re-replicate until cell division has occurred. Unreplicated DNA is in a replication-competent or 'licensed' state. The ability to replicate is lost in S phase and regained following passage through mitosis. Recent evidence has implicated an MCM (minichromosome maintenance) protein complex and the Cdc6 protein in determining replication competence. Regeneration of replication competence upon passage through mitosis entails changes in protein kinase activity, of which the MCMs are a likely target. Features of the mechanism that restricts DNA replication to once per cell cycle appear to be conserved throughout eukaryotes.

Cell cycle control of eukaryotic DNA replication

A clearer picture of replication control is emerging through the characterization of proteins, such as cdc18/Cdc6 and members of the mini-chromosome maintenance (MCM) protein family, that are involved in the initiation step. Cyclin B dependent kinases have conserved roles in both Saccharomyces cerevisiae and Schizosaccharomyces pombe, switching on DNA replication in G1 and preventing re-replication in G2. A model is suggested where MCMs and CDKs play complementary roles to ensure 'once-per-cell-cycle' replication, with CDKs maintaining a G1 or G2 state, whereas MCMs provide a cis-acting control on chromatin to prevent reinitiation during a single S phase.