Interaction between PCNA and diubiquitinated Mcm10 is essential for cell growth in budding yeast

The minichromosome maintenance protein 10 (Mcm10) is an evolutionarily conserved factor that is essential for replication initiation and elongation. Mcm10 is part of the eukaryotic replication fork and interacts with a variety of proteins, including the Mcm2-7 helicase and DNA polymerase alpha/primase complexes. A motif search revealed a match to the proliferating cell nuclear antigen (PCNA)-interacting protein (PIP) box in Mcm10. Here, we demonstrate a direct interaction between Mcm10 and PCNA that is alleviated by mutations in conserved residues of the PIP box. Interestingly, only the diubiquitinated form of Mcm10 binds to PCNA. Diubiquitination of Mcm10 is cell cycle regulated; it first appears in late G(1) and persists throughout S phase. During this time, diubiquitinated Mcm10 is associated with chromatin, suggesting a direct role in DNA replication. Surprisingly, a Y245A substitution in the PIP box of Mcm10 that inhibits the interaction with PCNA abolishes cell proliferation. This severe-growth phenotype, which has not been observed for analogous mutations in other PCNA-interacting proteins, is rescued by a compensatory mutation in PCNA that restores interaction with Mcm10-Y245A. Taken together, our results suggest that diubiquitinated Mcm10 interacts with PCNA to facilitate an essential step in DNA elongation.     

Unligated Okazaki Fragments Induce PCNA Ubiquitination and a Requirement for Rad59-Dependent Replication Fork Progression

Deficiency in DNA ligase I, encoded by CDC9 in budding yeast, leads to the accumulation of unligated Okazaki fragments and triggers PCNA ubiquitination at a non-canonical lysine residue. This signal is crucial to activate the S phase checkpoint, which promotes cell cycle delay. We report here that a pol30-K107 mutation alleviated cell cycle delay in cdc9 mutants, consistent with the idea that the modification of PCNA at K107 affects the rate of DNA synthesis at replication forks. To determine whether PCNA ubiquitination occurred in response to nicks or was triggered by the lack of PCNA-DNA ligase interaction, we complemented cdc9 cells with either wild-type DNA ligase I or a mutant form, which fails to interact with PCNA. Both enzymes reversed PCNA ubiquitination, arguing that the modification is likely an integral part of a novel nick-sensory mechanism and not due to non-specific secondary mutations that could have occurred spontaneously in cdc9 mutants. To further understand how cells cope with the accumulation of nicks during DNA replication, we utilized cdc9-1 in a genome-wide synthetic lethality screen, which identified RAD59 as a strong negative interactor. In comparison to cdc9 single mutants, cdc9 rad59Δ double mutants did not alter PCNA ubiquitination but enhanced phosphorylation of the mediator of the replication checkpoint, Mrc1. Since Mrc1 resides at the replication fork and is phosphorylated in response to fork stalling, these results indicate that Rad59 alleviates nick-induced replication fork slowdown. Thus, we propose that Rad59 promotes fork progression when Okazaki fragment processing is compromised and counteracts PCNA-K107 mediated cell cycle arrest.     

Solution NMR Structure of the C-terminal DNA Binding Domain of Mcm10 Reveals a Conserved MCM Motif

The eukaryotic DNA replication protein Mcm10 associates with chromatin in early S-phase and is required for assembly and function of the replication fork protein machinery. Xenopus laevis (X) Mcm10 binds DNA via a highly conserved internal domain (ID) and a C-terminal domain (CTD) that is unique to higher eukaryotes. Although the structural basis of the interactions of the ID with DNA and polymerase α is known, little information is available for the CTD. We have identified the minimal DNA binding region of the XMcm10-CTD and determined its three-dimensional structure by solution NMR. The CTD contains a globular domain composed of two zinc binding motifs. NMR chemical shift perturbation and mutational analysis show that ssDNA binds only to the N-terminal (CCCH-type) zinc motif, whose structure is unique to Mcm10. The second (CCCC-type) zinc motif is not involved in DNA binding. However, it is structurally similar to the CCCC zinc ribbon in the N-terminal oligomerization domain of eukaryotic and archaeal MCM helicases. NMR analysis of a construct spanning both the ID and CTD reveals that the two DNA binding domains are structurally independent in solution, supporting a modular architecture for vertebrate Mcm10. Our results provide insight in the action of Mcm10 in the replisome and support a model in which it serves as a central scaffold through coupling of interactions with partner proteins and the DNA.     

Fission Yeast Cdc24 Is a Replication Factor C- and Proliferating Cell Nuclear Antigen-Interacting Factor Essential for S-Phase Completion

At the nonpermissive temperature the fission yeast cdc24-M38 mutant arrests in the cell cycle with incomplete DNA replication as indicated by pulsed-field gel electrophoresis. The cdc24⁺ gene encodes a 501-amino-acid protein with no significant homology to any known proteins. The temperature-sensitive cdc24 mutant is effectively rescued by pcn1⁺, rfc1⁺ (a fission yeast homologue of RFC1), and hhp1⁺, which encode the proliferating cell nuclear antigen (PCNA), the large subunit of replication factor C (RFC), and a casein kinase I involved in DNA damage repair, respectively. The Cdc24 protein binds PCNA and RFC1 in vivo, and the domains essential for Cdc24 function and for RFC1 and PCNA binding colocalize in the N-terminal two-thirds of the molecule. In addition, cdc24⁺ genetically interacts with the gene encoding the catalytic subunit of DNA polymerase , which is stimulated by PCNA and RFC, and with those encoding the fission yeast counterparts of Mcm2, Mcm4, and Mcm10. These results indicate that Cdc24 is an RFC- and PCNA-interacting factor required for DNA replication and might serve as a target for regulation.     

Mcm10 deficiency causes defective-replisome-induced mutagenesis and a dependency on error-free postreplicative repair

Mcm10 is a multifunctional replication factor with reported roles in origin activation, polymerase loading, and replication fork progression. The literature supporting these variable roles is controversial, and it has been debated whether Mcm10 has an active role in elongation. Here, we provide evidence that the mcm10–1 allele confers alterations in DNA synthesis that lead to defective-replisome-induced mutagenesis (DRIM). Specifically, we observed that mcm10–1 cells exhibited elevated levels of PCNA ubiquitination and activation of the translesion polymerase, pol-ζ. Whereas translesion synthesis had no measurable impact on viability, mcm10–1 mutants also engaged in error-free postreplicative repair (PRR), and this pathway promoted survival at semi-permissive conditions. Replication gaps in mcm10–1 were likely caused by elongation defects, as dbf4–1 mutants, which are compromised for origin activation did not display any hallmarks of replication stress. Furthermore, we demonstrate that deficiencies in priming, induced by a pol1–1 mutation, also resulted in DRIM, but not in error-free PRR. Similar to mcm10–1 mutants, DRIM did not rescue the replication defect in pol1–1 cells. Thus, it appears that DRIM is not proficient to fill replication gaps in pol1–1 and mcm10–1 mutants. Moreover, the ability to correctly prime nascent DNA may be a crucial prerequisite to initiate error-free PRR.     

The N-terminus of Mcm10 is important for interaction with the 9-1-1 clamp and in resistance to DNA damage

Accurate replication of the genome requires the evolutionarily conserved minichromosome maintenance protein, Mcm10. Although the details of the precise role of Mcm10 in DNA replication are still debated, it interacts with the Mcm2-7 core helicase, the lagging strand polymerase, DNA polymerase-α and the replication clamp, proliferating cell nuclear antigen. Loss of these interactions caused by the depletion of Mcm10 leads to chromosome breakage and cell cycle checkpoint activation. However, whether Mcm10 has an active role in DNA damage prevention is unknown. Here, we present data that establish a novel role of the N-terminus of Mcm10 in resisting DNA damage. We show that Mcm10 interacts with the Mec3 subunit of the 9-1-1 clamp in response to replication stress evoked by UV irradiation or nucleotide shortage. We map the interaction domain with Mec3 within the N-terminal region of Mcm10 and demonstrate that its truncation causes UV light sensitivity. This sensitivity is not further enhanced by a deletion of MEC3, arguing that MCM10 and MEC3 operate in the same pathway. Since Rad53 phosphorylation in response to UV light appears to be normal in N-terminally truncated mcm10 mutants, we propose that Mcm10 may have a role in replication fork restart or DNA repair.     

Spartan/C1orf124 is important to prevent UV-induced mutagenesis

Uninterrupted replication across damaged DNA is critical to prevent replication fork collapse and resulting double-strand DNA breaks. Rad18-mediated PCNA ubiquitination is a crucial event that triggers a number of downstream pathways important for lesion bypass. Here, we report characterization of Spartan, an evolutionarily conserved protein containing a PCNA-interacting peptide motif, called a PIP box, and a UBZ4 ubiquitin-binding domain. Spartan is a nuclear protein and forms DNA damage-induced foci that colocalize with markers for stalled DNA replication. Focus formation of Spartan requires its PIP-box and the UBZ4 domain and is dependent on Rad18 and the PCNA ubiquitination site, indicating that Spartan is recruited to ubiquitinated PCNA. Spartan depletion results in increased mutagenesis during replication of UV-damaged DNA. Taken together, our data suggest that Spartan is recruited to sites of stalled replication via ubiquitinated PCNA and plays an important role to prevent mutations associated with replication of damaged DNA.     

Spartan/C1orf124 is important to prevent UV-induced mutagenesis

Uninterrupted replication across damaged DNA is critical to prevent replication fork collapse and resulting double-strand DNA breaks. Rad18-mediated PCNA ubiquitination is a crucial event that triggers a number of downstream pathways important for lesion bypass. Here, we report characterization of Spartan, an evolutionarily conserved protein containing a PCNA-interacting peptide motif, called a PIP box, and a UBZ4 ubiquitin-binding domain. Spartan is a nuclear protein and forms DNA damage-induced foci that colocalize with markers for stalled DNA replication. Focus formation of Spartan requires its PIP-box and the UBZ4 domain and is dependent on Rad18 and the PCNA ubiquitination site, indicating that Spartan is recruited to ubiquitinated PCNA. Spartan depletion results in increased mutagenesis during replication of UV-damaged DNA. Taken together, our data suggest that Spartan is recruited to sites of stalled replication via ubiquitinated PCNA and plays an important role to prevent mutations associated with replication of damaged DNA.     

Ubiquitination of PCNA and the Polymerase Switch in Human Cells

Replicative DNA polymerases are blocked by damage in the template DNA. To get past this damage, the cell employs specialised translesion synthesis (TLS) polymerases, which have reduced stringency and are able to bypass different lesions. For example, DNA polymerase ? (pol?) is able to carry out TLS past UV-induced cyclobutane pyrimidine dimers. How does the cell bring about the switch from replicative to TLS polymerase? We have shown that, in human cells, when the replication machinery is blocked at DNA damage, PCNA, the sliding clamp required for DNA replication, is mono-ubiquitinated and that this modified form of PCNA has increased affinity for pol?. This provides a mechanism for the polymerase switch. In this Extra-View, we discuss the possible signals that might trigger ubiquitination of PCNA, whether PCNA becomes de-ubiquitinated after TLS has been accomplished and the role of the hREV1 protein in TLS. We point out some apparent differences between mechanisms in Saccharomyces cerevisiae and human cells.     

The human homolog of Saccharomyces cerevisiae Mcm10 interacts with replication factors and dissociates from nuclease-resistant nuclear structures in G(2) phase

Mcm10 (Dna43), first identified in Saccharomyces cerevisiae, is an essential protein which functions in the initiation of DNA synthesis. Mcm10 is a nuclear protein that is localized to replication origins and mediates the interaction of the Mcm2–7 complex with replication origins. We identified and cloned a human cDNA whose product was structurally homologous to the yeast Mcm10 protein. Human Mcm10 (HsMcm10) is a 98-kDa protein of 874 amino acids which shows 23 and 21% overall similarity to Schizosaccharomyces pombe Cdc23 and S.cerevisiae Mcm10, respectively. The messenger RNA level of HsMcm10 increased at the G₁/S-boundary when quiescent human NB1–RGB cells were induced to proliferate as is the case of many replication factors. HsMcm10 associated with nuclease-resistant nuclear structures throughout S phase and dissociated from it in G₂ phase. HsMcm10 associated with human Orc2 protein when overexpressed in COS-1 cells. HsMcm10 also interacted with Orc2, Mcm2 and Mcm6 proteins in the yeast two-hybrid system. These results suggest that HsMcm10 may function in DNA replication through the interaction with Orc and Mcm2–7 complexes.     

Mismatch Repair proteins are recruited to replicating DNA through interaction with Proliferating Cell Nuclear Antigen (PCNA)

Mismatch Repair (MMR) is closely linked to DNA replication; however, other than the role of the replicative sliding clamp (PCNA) in various MMR functions, the linkage between DNA replication and MMR has been difficult to investigate. Here we use an in vitro DNA replication system based on simian virus 40, to investigate MMR recruitment to replicating DNA. Both DNA replication and MMR proteins are recruited to replicating DNA in an origin-dependent fashion. Primer synthesis is required for recruitment of both PCNA and MMR proteins, but not for recruitment of the single-stranded DNA-binding protein (RPA). Blocking PCNA recruitment to replicating DNA with a p21-based polypeptide blocks PCNA and MMR, but not RPA recruitment. Once PCNA and subsequent proteins required for replication are loaded onto DNA, addition of p21 leaves PCNA on the replicating DNA, but actively displaces MMR proteins. These findings indicate that the MMR machinery is recruited to replicating DNA through its interaction with PCNA, and suggests that this occurs via binding of the MMR proteins to the multi-protein interaction sites on PCNA. These studies demonstrate the utility of this system for further investigation of the role of DNA replication in MMR.     

A novel function of DNA polymerase zeta regulated by PCNA

DNA polymerase zeta (Polzeta) participates in translesion DNA synthesis and is involved in the generation of the majority of mutations induced by DNA damage. The mechanisms that license access of Polzeta to the primer terminus and regulate the extent of its participation in genome replication are poorly understood. The Polzeta-dependent damage-induced mutagenesis requires monoubiquitination of proliferating cell nuclear antigen (PCNA) that is triggered by exposure to mutagens. We show that Polzeta contributes to DNA replication and causes mutagenesis not only in response to DNA damage but also in response to malfunction of normal replicative machinery due to mutations in replication genes. These replication defects lead to ubiquitination of PCNA even in the absence of DNA damage. Unlike damage-induced mutagenesis, the Polzeta-dependent spontaneous mutagenesis in replication mutants is reduced in strains defective in both ubiquitination and sumoylation of Lys164 of PCNA. Additionally, studies of a PCNA mutant defective for functional interactions with Polzeta, but not for monoubiquitination by the Rad6/Rad18 complex demonstrate a role for PCNA in regulating the mutagenic activity of Polzeta separate from its modification at Lys164.     

Monoubiquitination of proliferating cell nuclear antigen induced by stalled replication requires uncoupling of DNA polymerase and mini-chromosome maintenance helicase activities

Proliferating cell nuclear antigen (PCNA) is a homotrimeric, ring-shaped protein complex that functions as a processivity factor for DNA polymerases. Following genotoxic stress, PCNA is modified at a conserved site by either a single ubiquitin moiety or a polyubiquitin chain. These modifications are required to coordinate DNA damage tolerance processes with ongoing replication. The molecular mechanisms responsible for inducing PCNA ubiquitination are not well understood. Using Xenopus egg extracts, we show that ultraviolet radiation and aphidicolin treatment induce the mono- and diubiquitination of PCNA. PCNA ubiquitination is replication-dependent and coincides with activation of the ataxia telangiectasia mutated and Rad3-related (ATR)-dependent DNA damage checkpoint pathway. However, loss of ATR signaling by depletion of the ATR-interacting protein (ATRIP) or Rad1, a component of the 911 checkpoint clamp, does not impair PCNA ubiquitination. Primed single-stranded DNA generated by uncoupling of mini-chromosome maintenance helicase and DNA polymerase activities has been shown previously to be necessary for ATR activation. Here we show that PCNA ubiquitination also requires uncoupling of helicase and polymerase activities. We further demonstrate that replicating single-stranded DNA, which mimics the structure produced upon uncoupling, is sufficient to induce PCNA monoubiquitination. Our results suggest that PCNA ubiquitination and ATR activation are two independent events that occur in response to a common single-stranded DNA intermediate generated by functional uncoupling of mini-chromosome maintenance (MCM) helicase and DNA polymerase activities.     

Mcm3 is polyubiquitinated during mitosis before establishment of the pre-replication complex

To ensure fidelity in genome duplication, eukaryotes restrict DNA synthesis to once every cell division by a cascade of regulated steps. Central to this cascade is the periodic assembly of the hexameric MCM2-7 complex at replication origins. However, in Saccharomyces cerevisiae, only a fraction of each MCM protein is able to assemble into hexamers and associate with replication origins during M phase, suggesting that MCM complex assembly and recruitment may be regulated post-translationally. Here we show that a small fraction of Mcm3p is polyubiquitinated at the onset of MCM complex assembly. Reducing the rate of ubiquitination by uba1-165, a suppressor of mcm3-10, restored the interaction of Mcm3-10p with subunits of the MCM complex and its recruitment to the replication origin. Possible roles for ubiquitinated Mcm3p in the assembly of the MCM complex at replication origins are discussed.     

The Mini-Chromosome Maintenance (Mcm) Complexes Interact with DNA Polymerase α-Primase and Stimulate Its Ability to Synthesize RNA Primers

The Mini-chromosome maintenance (Mcm) proteins are essential as central components for the DNA unwinding machinery during eukaryotic DNA replication. DNA primase activity is required at the DNA replication fork to synthesize short RNA primers for DNA chain elongation on the lagging strand. Although direct physical and functional interactions between helicase and primase have been known in many prokaryotic and viral systems, potential interactions between helicase and primase have not been explored in eukaryotes. Using purified Mcm and DNA primase complexes, a direct physical interaction is detected in pull-down assays between the Mcm2∼7 complex and the hetero-dimeric DNA primase composed of the p48 and p58 subunits. The Mcm4/6/7 complex co-sediments with the primase and the DNA polymerase α-primase complex in glycerol gradient centrifugation and forms a Mcm4/6/7-primase-DNA ternary complex in gel-shift assays. Both the Mcm4/6/7 and Mcm2∼7 complexes stimulate RNA primer synthesis by DNA primase in vitro. However, primase inhibits the Mcm4/6/7 helicase activity and this inhibition is abolished by the addition of competitor DNA. In contrast, the ATP hydrolysis activity of Mcm4/6/7 complex is not affected by primase. Mcm and primase proteins mutually stimulate their DNA-binding activities. Our findings indicate that a direct physical interaction between primase and Mcm proteins may facilitate priming reaction by the former protein, suggesting that efficient DNA synthesis through helicase-primase interactions may be conserved in eukaryotic chromosomes.     

Developmental Regulation of the Tetrahymena thermophila Origin Recognition Complex

The Tetrahymena thermophila DNA replication machinery faces unique demands due to the compartmentalization of two functionally distinct nuclei within a single cytoplasm, and complex developmental program. Here we present evidence for programmed changes in ORC and MCM abundance that are not consistent with conventional models for DNA replication. As a starting point, we show that ORC dosage is critical during the vegetative cell cycle and development. A moderate reduction in Orc1p induces genome instability in the diploid micronucleus, aberrant division of the polyploid macronucleus, and failure to generate a robust intra-S phase checkpoint response. In contrast to yeast ORC2 mutants, replication initiation is unaffected; instead, replication forks elongation is perturbed, as Mcm6p levels decline in parallel with Orc1p. Experimentally induced down-regulation of ORC and MCMs also impairs endoreplication and gene amplification, consistent with essential roles during development. Unexpectedly Orc1p and Mcm6p levels fluctuate dramatically in developing wild type conjugants, increasing for early cycles of conventional micronuclear DNA replication and macronuclear anlagen replication (endoreplication phase I, rDNA gene amplification). This increase does not reflect the DNA replication load, as much less DNA is synthesized during this developmental window compared to vegetative S phase. Furthermore, although Orc1p levels transiently increase prior to endoreplication phase II, Orc1p and Mcm6p levels decline when the replication load increases and unconventional DNA replication intermediates are produced. We propose that replication initiation is re-programmed to meet different requirements or challenges during the successive stages of Tetrahymena development.     

Regulation of translesion DNA synthesis: Posttranslational modification of lysine residues in key proteins

Posttranslational modification of proteins often controls various aspects of their cellular function. Indeed, over the past decade or so, it has been discovered that posttranslational modification of lysine residues plays a major role in regulating translesion DNA synthesis (TLS) and perhaps the most appreciated lysine modification is that of ubiquitination. Much of the recent interest in ubiquitination stems from the fact that proliferating cell nuclear antigen (PCNA) was previously shown to be specifically ubiquitinated at K164 and that such ubiquitination plays a key role in regulating TLS. In addition, TLS polymerases themselves are now known to be ubiquitinated. In the case of human polymerase η, ubiquitination at four lysine residues in its C-terminus appears to regulate its ability to interact with PCNA and modulate TLS. Within the past few years, advances in global proteomic research have revealed that many proteins involved in TLS are, in fact, subject to a previously underappreciated number of lysine modifications. In this review, we will summarize the known lysine modifications of several key proteins involved in TLS; PCNA and Y-family polymerases η, ι, κ and Rev1 and we will discuss the potential regulatory effects of such modification in controlling TLS in vivo.     

Ciprofloxacin is an inhibitor of the Mcm2-7 replicative helicase

Most currently available small molecule inhibitors of DNA replication lack enzymatic specificity, resulting in deleterious side effects during use in cancer chemotherapy and limited experimental usefulness as mechanistic tools to study DNA replication. Towards development of targeted replication inhibitors, we have focused on Mcm2-7 (minichromosome maintenance protein 2–7), a highly conserved helicase and key regulatory component of eukaryotic DNA replication. Unexpectedly we found that the fluoroquinolone antibiotic ciprofloxacin preferentially inhibits Mcm2-7. Ciprofloxacin blocks the DNA helicase activity of Mcm2-7 at concentrations that have little effect on other tested helicases and prevents the proliferation of both yeast and human cells at concentrations similar to those that inhibit DNA unwinding. Moreover, a previously characterized mcm mutant (mcm4chaos3) exhibits increased ciprofloxacin resistance. To identify more potent Mcm2-7 inhibitors, we screened molecules that are structurally related to ciprofloxacin and identified several that compromise the Mcm2-7 helicase activity at lower concentrations. Our results indicate that ciprofloxacin targets Mcm2-7 in vitro, and support the feasibility of developing specific quinolone-based inhibitors of Mcm2-7 for therapeutic and experimental applications.     

A role for PCNA ubiquitination in immunoglobulin hypermutation

Proliferating cell nuclear antigen (PCNA) is a DNA polymerase cofactor and regulator of replication-linked functions. Upon DNA damage, yeast and vertebrate PCNA is modified at the conserved lysine K164 by ubiquitin, which mediates error-prone replication across lesions via translesion polymerases. We investigated the role of PCNA ubiquitination in variants of the DT40 B cell line that are mutant in K164 of PCNA or in Rad18, which is involved in PCNA ubiquitination. Remarkably, the PCNA^K164R mutation not only renders cells sensitive to DNA-damaging agents, but also strongly reduces activation induced deaminase-dependent single-nucleotide substitutions in the immunoglobulin light-chain locus. This is the first evidence, to our knowledge, that vertebrates exploit the PCNA-ubiquitin pathway for immunoglobulin hypermutation, most likely through the recruitment of error-prone DNA polymerases.