Identification of novel human Cdt1-binding proteins by a proteomics approach: proteolytic regulation by APC/CCdh1

HBO1 histone acetylase activity is essential for DNA replication licensing and inhibited by Geminin

HBO1, an H4-specific histone acetylase, is a coactivator of the DNA replication licensing factor Cdt1. HBO1 acetylase activity is required for licensing, because a histone acetylase (HAT)-defective mutant of HBO1 bound at origins is unable to load the MCM complex. H4 acetylation at origins is cell-cycle regulated, with maximal activity at the G1/S transition, and coexpression of HBO1 and Jade-1 increases histone acetylation and MCM complex loading. Overexpression of the Set8 histone H4 tail-binding domain specifically inhibits MCM loading, suggesting that histones are a physiologically relevant target for licensing. Lastly, Geminin inhibits HBO1 acetylase activity in the context of a Cdt1-HBO1 complex, and it associates with origins and inhibits H4 acetylation and licensing in vivo. Thus, H4 acetylation at origins by HBO1 is critical for replication licensing by Cdt1, and negative regulation of licensing by Geminin is likely to involve inhibition of HBO1 histone acetylase activity.

CDC6 interaction with ATR regulates activation of a replication checkpoint in higher eukaryotic cells

CDC6, a replication licensing protein, is partially exported to the cytoplasm in human cells through phosphorylation by Cdk during S phase, but a significant proportion remains in the nucleus. We report here that human CDC6 physically interacts with ATR, a crucial checkpoint kinase, in a manner that is stimulated by phosphorylation by Cdk. CDC6 silencing by siRNAs affected ATR-dependent inhibition of mitotic entry elicited by modest replication stress. Whereas a Cdk-phosphorylation-mimicking CDC6 mutant could rescue the checkpoint defect by CDC6 silencing, a phosphorylation-deficient mutant could not. Furthermore, we found that the CDC6-ATR interaction is conserved in Xenopus. We show that the presence of Xenopus CDC6 during S phase is essential for Xenopus ATR to bind to chromatin in response to replication inhibition. In addition, when human CDC6 amino acid fragment 180-220, which can bind to both human and Xenopus ATR, was added to Xenopus egg extracts after assembly of the pre-replication complex, Xenopus Chk1 phosphorylation was significantly reduced without lowering replication, probably through a sequestration of CDC6-mediated ATR-chromatin interaction. Thus, CDC6 might regulate replication-checkpoint activation through the interaction with ATR in higher eukaryotic cells.

Geminin stabilizes Cdt1 during meiosis in Xenopus oocytes

During the mitotic cell cycle, Geminin can act both as a promoter and inhibitor of initiation of DNA replication. As a promoter, Geminin stabilizes Cdt1 and facilitates its accumulation leading to the assembly of the pre-replication complex on DNA. As an inhibitor, Geminin prevents Cdt1 from loading the mini-chromosome maintenance complex onto pre-replication complexes in late S, G(2), and M phases. Here we show that during meiosis Geminin functions as a stabilizer of Cdt1 promoting its accumulation for the early division cycles of the embryo. Depletion of Geminin in Xenopus immature oocytes leads to a decrease of Cdt1 protein levels during maturation and after activation of these oocytes. Injection of exogenous recombinant Geminin into the depleted oocytes rescues Cdt1 levels demonstrating that Geminin stabilizes Cdt1 during meiosis and after fertilization. Furthermore, Geminin-depleted oocytes did not replicate their DNA after meiosis I indicating that Geminin does not act as an inhibitor of initiation of DNA replication between meiosis I and meiosis II.

Redundant and differential regulation of multiple licensing factors ensures prevention of re-replication in normal human cells

When human cells enter S-phase, overlapping differential inhibitory mechanisms downregulate the replication licensing factors ORC1, CDC6 and Cdt1. Such regulation prevents re-replication so that deregulation of any individual factor alone would not be expected to induce overt re-replication. However, this has been challenged by the fact that overexpression of Cdt1 or Cdt1+CDC6 causes re-replication in some cancer cell lines. We thought it important to analyze licensing regulations in human non-cancerous cells that are resistant to Cdt1-induced re-replication and examined whether simultaneous deregulation of these licensing factors induces re-replication in two such cell lines, including human fibroblasts immortalized by telomerase. Individual overexpression of either Cdt1, ORC1 or CDC6 induced no detectable re-replication. However, with Cdt1+ORC1 or Cdt1+CDC6, some re-replication was detectable and coexpression of Cdt1+ORC1+CDC6 synergistically acted to give strong re-replication with increased mini-chromosome maintenance (MCM) loading. Coexpression of ORC1+CDC6 was without effect. These results suggest that, although Cdt1 regulation is the key step, differential regulation of multiple licensing factors ensures prevention of re-replication in normal human cells. Our findings also show for the first time the importance of ORC1 regulation for prevention of re-replication.

Replication licensing and cancer--a fatal entanglement?

Correct regulation of the replication licensing system ensures that chromosomal DNA is precisely duplicated in each cell division cycle. Licensing proteins are inappropriately expressed at an early stage of tumorigenesis in a wide variety of cancers. Here we discuss evidence that misregulation of replication licensing is a consequence of oncogene-induced cell proliferation. This misregulation can cause either under- or over-replication of chromosomal DNA, and could explain the genetic instability commonly seen in cancer cells.

The origin recognition complex is dispensable for endoreplication in Drosophila

The origin recognition complex (ORC) is an essential component of the prereplication complex (pre-RC) in mitotic cell cycles. The role of ORC as a foundation to assemble the pre-RC is conserved from yeast to human. Furthermore, in metazoans ORC plays a key role in determining the timing of replication initiation and origin usage. In this report we have produced and analyzed a Drosophila orc1 allele to investigate the roles of ORC1 in three different modes of DNA replication during development. As expected, ORC1 is essential for mitotic replication and proliferation in brains and imaginal discs, as well as for gene amplification in ovarian follicle cells. Surprisingly, however, ORC1 is not required for endoreplication. Decreased cell number in orc1 mutant salivary glands is consistent with the idea that undetectable levels of maternal ORC1 during embryogenesis fail to support further proliferation. Nevertheless, these cells begin endoreplicating normally and reach a final ploidy of >1000C in the absence of zygotic synthesis of ORC1. The dispensability of ORC is further supported by an examination of other ORC members, whereas Double-parked protein/Cdt1 and minichromosome maintenance proteins are apparently essential for endoreplication, implying that some aspects of initiation are shared among the three modes of DNA replication. This study provides insight into the physiologic roles of ORC during metazoan development and proposes that DNA replication initiation is governed differently in mitotic and endocycles.

The Arabidopsis COP9 signalosome is essential for G2 phase progression and genomic stability

The COP9 signalosome (CSN) is required for the full activity of cullin-RING E3 ubiquitin ligases (CRLs) in eukaryotes. CSN exerts its function on CRLs by removing the ubiquitin-related NEDD8 conjugate from the cullin subunit of CRLs. CSN seems, thereby, to control CRL disassembly or CRL subunit stability. In Arabidopsis thaliana, loss of CSN function leads to constitutive photomorphogenic (cop) seedling development and a post-germination growth arrest. The underlying molecular cause of this growth arrest is currently unknown. Here, we show that Arabidopsis csn mutants are delayed in G2 phase progression. This cell cycle arrest correlates with the induction of the DNA damage response pathway and is suggestive of the activation of a DNA damage checkpoint. In support of this hypothesis, we detected gene conversion events in csn mutants that are indicative of DNA double-strand breaks. DNA damage is also apparent in mutants of the NEDD8 conjugation pathway and in mutants of the E3 ligase subunits CULLIN4, COP1 and DET1, which share phenotypes with csn mutants. In summary, our data suggest that Arabidopsis csn mutants undergo DNA damage, which might be the cause of the delay in G2 cell cycle progression.