The amino-terminal TPR domain of Dia2 tethers SCF(Dia2) to the replisome progression complex

A Decade of Discovery-Eukaryotic Replisome Disassembly at Replication Termination

The eukaryotic replicative helicase (CMG complex) is assembled during DNA replication initiation in a highly regulated manner, which is described in depth by other manuscripts in this Issue. During DNA replication, the replicative helicase moves through the chromatin, unwinding DNA and facilitating nascent DNA synthesis by polymerases. Once the duplication of a replicon is complete, the CMG helicase and the remaining components of the replisome need to be removed from the chromatin. Research carried out over the last ten years has produced a breakthrough in our understanding, revealing that replication termination, and more specifically replisome disassembly, is indeed a highly regulated process. This review brings together our current understanding of these processes and highlights elements of the mechanism that are conserved or have undergone divergence throughout evolution. Finally, we discuss events beyond the classic termination of DNA replication in S-phase and go over the known mechanisms of replicative helicase removal from chromatin in these particular situations.

DciA Helicase Operators Exhibit Diversity across Bacterial Phyla

A fundamental requirement for life is the replication of an organism's DNA. Studies in Escherichia coli and Bacillus subtilis have set the paradigm for DNA replication in bacteria. During replication initiation in E. coli and B. subtilis, the replicative helicase is loaded onto the DNA at the origin of replication by an ATPase helicase loader. However, most bacteria do not encode homologs to the helicase loaders in E. coli and B. subtilis. Recent work has identified the DciA protein as a predicted helicase operator that may perform a function analogous to the helicase loaders in E. coli and B. subtilis. DciA proteins, which are defined by the presence of a DUF721 domain (termed the DciA domain herein), are conserved in most bacteria but have only been studied in mycobacteria and gammaproteobacteria (Pseudomonas aeruginosa and Vibrio cholerae). Sequences outside the DciA domain in Mycobacterium tuberculosis DciA are essential for protein function but are not conserved in the P. aeruginosa and V. cholerae homologs, raising questions regarding the conservation and evolution of DciA proteins across bacterial phyla. To comprehensively define the DciA protein family, we took a computational evolutionary approach and analyzed the domain architectures and sequence properties of DciA domain-containing proteins across the tree of life. These analyses identified lineage-specific domain architectures among DciA homologs, as well as broadly conserved sequence-structural motifs. The diversity of DciA proteins represents the evolution of helicase operation in bacterial DNA replication and highlights the need for phylum-specific analyses of this fundamental biological process. IMPORTANCE Despite the fundamental importance of DNA replication for life, this process remains understudied in bacteria outside Escherichia coli and Bacillus subtilis. In particular, most bacteria do not encode the helicase-loading proteins that are essential in E. coli and B. subtilis for DNA replication. Instead, most bacteria encode a DciA homolog that likely constitutes the predominant mechanism of helicase operation in bacteria. However, it is still unknown how DciA structure and function compare across diverse phyla that encode DciA proteins. In this study, we performed computational evolutionary analyses to uncover tremendous diversity among DciA homologs. These studies provide a significant advance in our understanding of an essential component of the bacterial DNA replication machinery.

The p97 segregase cofactor Ubxn7 facilitates replisome disassembly during S-phase

Complex cellular processes are driven by the regulated assembly and disassembly of large multiprotein complexes. While we are beginning to understand the molecular mechanism for assembly of the eukaryotic DNA replication machinery (replisome), we still know relatively little about the regulation of its disassembly at replication termination. Recently, the first elements of this process have emerged, revealing that the replicative helicase, at the heart of the replisome, is polyubiquitylated prior to unloading and that this unloading requires p97 segregase activity. Two different E3 ubiquitin ligases have now been shown to ubiquitylate the helicase under different conditions: Cul2^Lrr1 and TRAIP. Here, using Xenopus laevis egg extract cell-free system and biochemical approaches, we have found two p97 cofactors, Ubxn7 and Faf1, which can interact with p97 during replisome disassembly during S-phase. We show only Ubxn7, however, facilitates efficient replisome disassembly. Ubxn7 delivers this role through its interaction via independent domains with both Cul2^Lrr1 and p97 to allow coupling between Mcm7 ubiquitylation and its removal from chromatin. Our data therefore characterize Ubxn7 as the first substrate-specific p97 cofactor regulating replisome disassembly in vertebrates and a rationale for the efficacy of the Cul2^Lrr1 replisome unloading pathway in unperturbed S-phase.

The Effect of Dia2 Protein Deficiency on the Cell Cycle, Cell Size, and Recruitment of Ctf4 Protein in Saccharomyces cerevisiae

Cells have evolved elaborate mechanisms to regulate DNA replication machinery and cell cycles in response to DNA damage and replication stress in order to prevent genomic instability and cancer. The E3 ubiquitin ligase SCFDia2 in S. cerevisiae is involved in the DNA replication and DNA damage stress response, but its effect on cell growth is still unclear. Here, we demonstrate that the absence of Dia2 prolongs the cell cycle by extending both S- and G2/M-phases while, at the same time, activating the S-phase checkpoint. In these conditions, Ctf4-an essential DNA replication protein and substrate of Dia2-prolongs its binding to the chromatin during the extended S- and G2/M-phases. Notably, the prolonged cell cycle when Dia2 is absent is accompanied by a marked increase in cell size. We found that while both DNA replication inhibition and an absence of Dia2 exerts effects on cell cycle duration and cell size, Dia2 deficiency leads to a much more profound increase in cell size and a substantially lesser effect on cell cycle duration compared to DNA replication inhibition. Our results suggest that the increased cell size in dia2∆ involves a complex mechanism in which the prolonged cell cycle is one of the driving forces.

A conserved mechanism for regulating replisome disassembly in eukaryotes

Replisome disassembly is the final step of eukaryotic DNA replication and is triggered by ubiquitylation of the CDC45-MCM-GINS (CMG) replicative helicase1-3. Despite being driven by evolutionarily diverse E3 ubiquitin ligases in different eukaryotes (SCFDia2 in budding yeast1, CUL2LRR1 in metazoa4-7), replisome disassembly is governed by a common regulatory principle, in which ubiquitylation of CMG is suppressed before replication termination, to prevent replication fork collapse. Recent evidence suggests that this suppression is mediated by replication fork DNA8-10. However, it is unknown how SCFDia2 and CUL2LRR1 discriminate terminated from elongating replisomes, to selectively ubiquitylate CMG only after termination. Here we used cryo-electron microscopy to solve high-resolution structures of budding yeast and human replisome-E3 ligase assemblies. Our structures show that the leucine-rich repeat domains of Dia2 and LRR1 are structurally distinct, but bind to a common site on CMG, including the MCM3 and MCM5 zinc-finger domains. The LRR-MCM interaction is essential for replisome disassembly and, crucially, is occluded by the excluded DNA strand at replication forks, establishing the structural basis for the suppression of CMG ubiquitylation before termination. Our results elucidate a conserved mechanism for the regulation of replisome disassembly in eukaryotes, and reveal a previously unanticipated role for DNA in preserving replisome integrity.

Coordinating DNA Replication and Mitosis through Ubiquitin/SUMO and CDK1

Post-translational modification of the DNA replication machinery by ubiquitin and SUMO plays key roles in the faithful duplication of the genetic information. Among other functions, ubiquitination and SUMOylation serve as signals for the extraction of factors from chromatin by the AAA ATPase VCP. In addition to the regulation of DNA replication initiation and elongation, we now know that ubiquitination mediates the disassembly of the replisome after DNA replication termination, a process that is essential to preserve genomic stability. Here, we review the recent evidence showing how active DNA replication restricts replisome ubiquitination to prevent the premature disassembly of the DNA replication machinery. Ubiquitination also mediates the removal of the replisome to allow DNA repair. Further, we discuss the interplay between ubiquitin-mediated replisome disassembly and the activation of CDK1 that is required to set up the transition from the S phase to mitosis. We propose the existence of a ubiquitin–CDK1 relay, where the disassembly of terminated replisomes increases CDK1 activity that, in turn, favors the ubiquitination and disassembly of more replisomes. This model has important implications for the mechanism of action of cancer therapies that induce the untimely activation of CDK1, thereby triggering premature replisome disassembly and DNA damage.

The Fate of Two Unstoppable Trains After Arriving Destination: Replisome Disassembly During DNA Replication Termination

In eukaryotes, the perfect duplication of the chromosomes is executed by a dynamic molecular machine called the replisome. As a key step to finishing DNA replication, replisome disassembly is triggered by ubiquitylation of the MCM7 subunit of the helicase complex CMG. Afterwards, the CDC48/p97 "unfoldase" is recruited to the ubiquitylated helicase to unfold MCM7 and disassemble the replisome. Here we summarise recently discovered mechanisms of replisome disassembly that are likely to be broadly conserved in eukaryotes. We also discuss two crucial questions that remain to be explored further in the future. Firstly, how is CMG ubiquitylation repressed by the replication fork throughout elongation? Secondly, what is the biological significance of replisome disassembly and what are the consequences of failing to ubiquitylate and disassemble the CMG helicase?

Mechanisms of eukaryotic replisome disassembly

DNA replication is a complex process that needs to be executed accurately before cell division in order to maintain genome integrity. DNA replication is divided into three main stages: initiation, elongation and termination. One of the key events during initiation is the assembly of the replicative helicase at origins of replication, and this mechanism has been very well described over the last decades. In the last six years however, researchers have also focused on deciphering the molecular mechanisms underlying the disassembly of the replicative helicase during termination. Similar to replisome assembly, the mechanism of replisome disassembly is strictly regulated and well conserved throughout evolution, although its complexity increases in higher eukaryotes. While budding yeast rely on just one pathway for replisome disassembly in S phase, higher eukaryotes evolved an additional mitotic pathway over and above the default S phase specific pathway. Moreover, replisome disassembly has been recently found to be a key event prior to the repair of certain DNA lesions, such as under-replicated DNA in mitosis and inter-strand cross-links (ICLs) in S phase. Although replisome disassembly in human cells has not been characterised yet, they possess all of the factors involved in these pathways in model organisms, and de-regulation of many of them are known to contribute to tumorigenesis and other pathological conditions.

The DNA replication fork suppresses CMG unloading from chromatin before termination

When converging replication forks meet during replication termination, the CMG (Cdc45-MCM2-7-GINS) helicase is polyubiquitylated by CRL2Lrr1 and unloaded from chromatin by the p97 ATPase. Here, we investigate the signal that triggers CMG unloading in Xenopus egg extracts using single-molecule and ensemble approaches. We show that converging CMGs pass each other and keep translocating at the same speed as before convergence, whereafter they are rapidly and independently unloaded. When CMG unloading is blocked, diverging CMGs do not support DNA synthesis, indicating that after bypass CMGs encounter the nascent lagging strands of the converging fork and then translocate along double-stranded DNA (dsDNA). However, translocation on dsDNA is not required for CMG's removal from chromatin because in the absence of nascent strand synthesis, converging CMGs are still unloaded. Moreover, recombinant CMG added to nuclear extract undergoes ubiquitylation and disassembly in the absence of any DNA, and DNA digestion triggers CMG ubiquitylation at stalled replication forks. Our findings suggest that DNA suppresses CMG ubiquitylation during elongation and that this suppression is relieved when CMGs converge, leading to CMG unloading.

In Vitro Reconstitution Defines the Minimal Requirements for Cdc48-Dependent Disassembly of the CMG Helicase in Budding Yeast

Disassembly of the replisome is the final step of chromosome duplication in eukaryotes. In budding yeast and metazoa, cullin ubiquitin ligases are required to ubiquitylate the Cdc45-MCM-GINS (CMG) helicase that lies at the heart of the replisome, leading to a disassembly reaction that is dependent upon the ATPase known as Cdc48 or p97. Here, we describe the reconstitution of replisome disassembly, using a purified complex of the budding yeast replisome in association with the cullin ligase SCF^Dia2. Upon addition of E1 and E2 enzymes, together with ubiquitin and ATP, the CMG helicase is ubiquitylated on its Mcm7 subunit. Subsequent addition of Cdc48, together with its cofactors Ufd1-Npl4, drives efficient disassembly of ubiquitylated CMG, thereby recapitulating the steps of replisome disassembly that are observed in vivo. Our findings define the minimal requirements for disassembly of the eukaryotic replisome and provide a model system for studying the disassembly of protein complexes by Cdc48-Ufd1-Npl4.

CMG helicase disassembly is controlled by replication fork DNA, replisome components and a ubiquitin threshold

The eukaryotic replisome assembles around the CMG helicase, which stably associates with DNA replication forks throughout elongation. When replication terminates, CMG is ubiquitylated on its Mcm7 subunit and disassembled by the Cdc48/p97 ATPase. Until now, the regulation that restricts CMG ubiquitylation to termination was unknown, as was the mechanism of disassembly. By reconstituting these processes with purified budding yeast proteins, we show that ubiquitylation is tightly repressed throughout elongation by the Y-shaped DNA structure of replication forks. Termination removes the repressive DNA structure, whereupon long K48-linked ubiquitin chains are conjugated to CMG-Mcm7, dependent on multiple replisome components that bind to the ubiquitin ligase SCFDia2. This mechanism pushes CMG beyond a '5-ubiquitin threshold' that is inherent to Cdc48, which specifically unfolds ubiquitylated Mcm7 and thereby disassembles CMG. These findings explain the exquisite regulation of CMG disassembly and provide a general model for the disassembly of ubiquitylated protein complexes by Cdc48.

Cryo-EM Structure of the Fork Protection Complex Bound to CMG at a Replication Fork

The eukaryotic replisome, organized around the Cdc45-MCM-GINS (CMG) helicase, orchestrates chromosome replication. Multiple factors associate directly with CMG, including Ctf4 and the heterotrimeric fork protection complex (Csm3/Tof1 and Mrc1), which has important roles including aiding normal replication rates and stabilizing stalled forks. How these proteins interface with CMG to execute these functions is poorly understood. Here we present 3 to 3.5 Å resolution electron cryomicroscopy (cryo-EM) structures comprising CMG, Ctf4, and the fork protection complex at a replication fork. The structures provide high-resolution views of CMG-DNA interactions, revealing a mechanism for strand separation, and show Csm3/Tof1 “grip” duplex DNA ahead of CMG via a network of interactions important for efficient replication fork pausing. Although Mrc1 was not resolved in our structures, we determine its topology in the replisome by cross-linking mass spectrometry. Collectively, our work reveals how four highly conserved replisome components collaborate with CMG to facilitate replisome progression and maintain genome stability.

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Cryo-EM structure of Csm3/Tof1 and Ctf4 bound to the eukaryotic CMG helicase

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Csm3/Tof1 are positioned at the front of the replisome where they grip duplex DNA

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High-resolution views of CMG-DNA contacts suggest a mechanism for strand separation

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Mrc1 binds across one side of CMG contacting the front and back of the replisome

DNA replication machinery prevents Rad52-dependent single-strand annealing that leads to gross chromosomal rearrangements at centromeres

Homologous recombination between repetitive sequences can lead to gross chromosomal rearrangements (GCRs). At fission yeast centromeres, Rad51-dependent conservative recombination predominantly occurs between inverted repeats, thereby suppressing formation of isochromosomes whose arms are mirror images. However, it is unclear how GCRs occur in the absence of Rad51 and how GCRs are prevented at centromeres. Here, we show that homology-mediated GCRs occur through Rad52-dependent single-strand annealing (SSA). The rad52-R45K mutation, which impairs SSA activity of Rad52 protein, dramatically reduces isochromosome formation in rad51 deletion cells. A ring-like complex Msh2-Msh3 and a structure-specific endonuclease Mus81 function in the Rad52-dependent GCR pathway. Remarkably, mutations in replication fork components, including DNA polymerase α and Swi1/Tof1/Timeless, change the balance between Rad51-dependent recombination and Rad52-dependent SSA at centromeres, increasing Rad52-dependent SSA that forms isochromosomes. Our results uncover a role of DNA replication machinery in the recombination pathway choice that prevents Rad52-dependent GCRs at centromeres.

Yeast Genome Maintenance by the Multifunctional PIF1 DNA Helicase Family

The two PIF1 family helicases in Saccharomyces cerevisiae, Rrm3, and ScPif1, associate with thousands of sites throughout the genome where they perform overlapping and distinct roles in telomere length maintenance, replication through non-histone proteins and G4 structures, lagging strand replication, replication fork convergence, the repair of DNA double-strand break ends, and transposable element mobility. ScPif1 and its fission yeast homolog Pfh1 also localize to mitochondria where they protect mitochondrial genome integrity. In addition to yeast serving as a model system for the rapid functional evaluation of human Pif1 variants, yeast cells lacking Rrm3 have proven useful for elucidating the cellular response to replication fork pausing at endogenous sites. Here, we review the increasingly important cellular functions of the yeast PIF1 helicases in maintaining genome integrity, and highlight recent advances in our understanding of their roles in facilitating fork progression through replisome barriers, their functional interactions with DNA repair, and replication stress response pathways.

Regulation of Mammalian DNA Replication via the Ubiquitin-Proteasome System

Proper regulation of DNA replication ensures the faithful transmission of genetic material essential for optimal cellular and organismal physiology. Central to this regulation is the activity of a set of enzymes that induce or reverse posttranslational modifications of various proteins critical for the initiation, progression, and termination of DNA replication. This is particularly important when DNA replication proceeds in cancer cells with elevated rates of genomic instability and increased proliferative capacities. Here, we describe how DNA replication in mammalian cells is regulated via the activity of the ubiquitin-proteasome system as well as the consequence of derailed ubiquitylation signaling involved in this important cellular activity.

New insights into donor directionality of mating-type switching in Schizosaccharomyces pombe

Mating-type switching in Schizosaccharomyces pombe entails programmed gene conversion events regulated by DNA replication, heterochromatin, and the HP1-like chromodomain protein Swi6. The whole mechanism remains to be fully understood. Using a gene deletion library, we screened ~ 3400 mutants for defects in the donor selection step where a heterochromatic locus, mat2-P or mat3-M, is chosen to convert the expressed mat1 locus. By measuring the biases in mat1 content that result from faulty directionality, we identified in total 20 factors required for donor selection. Unexpectedly, these included the histone H3 lysine 4 (H3K4) methyltransferase complex subunits Set1, Swd1, Swd2, Swd3, Spf1 and Ash2, the BRE1-like ubiquitin ligase Brl2 and the Elongator complex subunit Elp6. The mutant defects were investigated in strains with reversed donor loci (mat2-M mat3-P) or when the SRE2 and SRE3 recombination enhancers, adjacent to the donors, were deleted or transposed. Mutants in Set1C, Brl2 or Elp6 altered balanced donor usage away from mat2 and the SRE2 enhancer, towards mat3 and the SRE3 enhancer. The defects in these mutants were qualitatively similar to heterochromatin mutants lacking Swi6, the NAD⁺-dependent histone deacetylase Sir2, or the Clr4, Raf1 or Rik1 subunits of the histone H3 lysine 9 (H3K9) methyltransferase complex, albeit not as extreme. Other mutants showed clonal biases in switching. This was the case for mutants in the NAD⁺-independent deacetylase complex subunits Clr1, Clr2 and Clr3, the casein kinase CK2 subunit Ckb1, the ubiquitin ligase component Pof3, and the CENP-B homologue Cbp1, as well as for double mutants lacking Swi6 and Brl2, Pof3, or Cbp1. Thus, we propose that Set1C cooperates with Swi6 and heterochromatin to direct donor choice to mat2-P in M cells, perhaps by inhibiting the SRE3 recombination enhancer, and that in the absence of Swi6 other factors are still capable of imposing biases to donor choice.

Effects of chromatin structure on recombination can be studied in the fission yeast S. pombe where two heterochromatic loci, mat2 and mat3, are chosen in a cell-type specific manner to convert the expressed mat1 locus and switch the yeast mating-type. The system has previously revealed the determining role of heterochromatin, histone H3K9 methylation and HP1 family protein Swi6, in donor selection. Here, we find that other chromatin modifiers and protein complexes, including components of the histone H3K4 methyltransferase complex Set1C, the histone H2B ubiquitin ligase HULC and Elongator, also participate in donor selection. Our findings open up new research paths to study mating-type switching in fission yeast and the roles of these complexes in recombination.

Dependency of Heterochromatin Domains on Replication Factors

Chromatin structure regulates both genome expression and dynamics in eukaryotes, where large heterochromatic regions are epigenetically silenced through the methylation of histone H3K9, histone deacetylation, and the assembly of repressive complexes. Previous genetic screens with the fission yeast Schizosaccharomyces pombe have led to the identification of key enzymatic activities and structural constituents of heterochromatin. We report here on additional factors discovered by screening a library of deletion mutants for silencing defects at the edge of a heterochromatic domain bound by its natural boundary-the IR-R+ element-or by ectopic boundaries. We found that several components of the DNA replication progression complex (RPC), including Mrc1/Claspin, Mcl1/Ctf4, Swi1/Timeless, Swi3/Tipin, and the FACT subunit Pob3, are essential for robust heterochromatic silencing, as are the ubiquitin ligase components Pof3 and Def1, which have been implicated in the removal of stalled DNA and RNA polymerases from chromatin. Moreover, the search identified the cohesin release factor Wpl1 and the forkhead protein Fkh2, both likely to function through genome organization, the Ssz1 chaperone, the Fkbp39 proline cis-trans isomerase, which acts on histone H3P30 and P38 in Saccharomyces cerevisiae, and the chromatin remodeler Fft3. In addition to their effects in the mating-type region, to varying extents, these factors take part in heterochromatic silencing in pericentromeric regions and telomeres, revealing for many a general effect in heterochromatin. This list of factors provides precious new clues with which to study the spatiotemporal organization and dynamics of heterochromatic regions in connection with DNA replication.

Degradation of Mrc1 promotes recombination-mediated restart of stalled replication forks

The DNA replication or S-phase checkpoint monitors the integrity of DNA synthesis. Replication stress or DNA damage triggers fork stalling and checkpoint signaling to activate repair pathways. Recovery from checkpoint activation is critical for cell survival following DNA damage. Recovery from the S-phase checkpoint includes inactivation of checkpoint signaling and restart of stalled replication forks. Previous studies demonstrated that degradation of Mrc1, the Saccharomyces cerevisiae ortholog of human Claspin, is facilitated by the SCF^Dia2 ubiquitin ligase and is important for cell cycle re-entry after DNA damage-induced S-phase checkpoint activation. Here, we show that degradation of Mrc1 facilitated by the SCF^Dia2 complex is critical to restart stalled replication forks during checkpoint recovery. Using DNA fiber analysis, we showed that Dia2 functions with the Sgs1 and Mph1 helicases (orthologs of human BLM and FANCM, respectively) in the recombination-mediated fork restart pathway. In addition, Dia2 physically interacts with Sgs1 upon checkpoint activation. Importantly, failure to target Mrc1 for degradation during recovery inhibits Sgs1 chromatin association, but this can be alleviated by induced proteolysis of Mrc1 after checkpoint activation. Together, these studies provide new mechanistic insights into how cells recover from activation of the S-phase checkpoint.

Dynamic ubiquitin signaling in cell cycle regulation

Gilberto and Peter discuss the role of ubiquitylation in the regulation of DNA replication and mitosis.

The cell division cycle is driven by a collection of enzymes that coordinate DNA duplication and separation, ensuring that genomic information is faithfully and perpetually maintained. The activity of the effector proteins that perform and coordinate these biological processes oscillates by regulated expression and/or posttranslational modifications. Ubiquitylation is a cardinal cellular modification and is long known for driving cell cycle transitions. In this review, we emphasize emerging concepts of how ubiquitylation brings the necessary dynamicity and plasticity that underlie the processes of DNA replication and mitosis. New studies, often focusing on the regulation of chromosomal proteins like DNA polymerases or kinetochore kinases, are demonstrating that ubiquitylation is a versatile modification that can be used to fine-tune these cell cycle events, frequently through processes that do not involve proteasomal degradation. Understanding how the increasing variety of identified ubiquitin signals are transduced will allow us to develop a deeper mechanistic perception of how the multiple factors come together to faithfully propagate genomic information. Here, we discuss these and additional conceptual challenges that are currently under study toward understanding how ubiquitin governs cell cycle regulation.

Chromosome Duplication in Saccharomyces cerevisiae

The accurate and complete replication of genomic DNA is essential for all life. In eukaryotic cells, the assembly of the multi-enzyme replisomes that perform replication is divided into stages that occur at distinct phases of the cell cycle. Replicative DNA helicases are loaded around origins of DNA replication exclusively during G₁ phase. The loaded helicases are then activated during S phase and associate with the replicative DNA polymerases and other accessory proteins. The function of the resulting replisomes is monitored by checkpoint proteins that protect arrested replisomes and inhibit new initiation when replication is inhibited. The replisome also coordinates nucleosome disassembly, assembly, and the establishment of sister chromatid cohesion. Finally, when two replisomes converge they are disassembled. Studies in Saccharomyces cerevisiae have led the way in our understanding of these processes. Here, we review our increasingly molecular understanding of these events and their regulation.

Regulating telomere length from the inside out: the replication fork model

In this Hypothesis, Greider describes a new model for telomere length regulation, which links DNA replication and telomere elongation.

Telomere length is regulated around an equilibrium set point. Telomeres shorten during replication and are lengthened by telomerase. Disruption of the length equilibrium leads to disease; thus, it is important to understand the mechanisms that regulate length at the molecular level. The prevailing protein-counting model for regulating telomerase access to elongate the telomere does not explain accumulating evidence of a role of DNA replication in telomere length regulation. Here I present an alternative model: the replication fork model that can explain how passage of a replication fork and regulation of origin firing affect telomere length.

CUL-2LRR-1 and UBXN-3 drive replisome disassembly during DNA replication termination and mitosis

Replisome disassembly is the final step of DNA replication in eukaryotes, involving the ubiquitylation and CDC48-dependent dissolution of the CMG helicase (CDC45-MCM-GINS). Using Caenorhabditis elegans early embryos and Xenopus laevis egg extracts, we show that the E3 ligase CUL-2^LRR-1 associates with the replisome and drives ubiquitylation and disassembly of CMG, together with the CDC-48 cofactors UFD-1 and NPL-4. Removal of CMG from chromatin in frog egg extracts requires CUL2 neddylation, and our data identify chromatin recruitment of CUL2^LRR1 as a key regulated step during DNA replication termination. Interestingly, however, CMG persists on chromatin until prophase in worms that lack CUL-2^LRR-1, but is then removed by a mitotic pathway that requires the CDC-48 cofactor UBXN-3, orthologous to the human tumour suppressor FAF1. Partial inactivation of lrr-1 and ubxn-3 leads to synthetic lethality, suggesting future approaches by which a deeper understanding of CMG disassembly in metazoa could be exploited therapeutically.

Ufd1-Npl4 Recruit Cdc48 for Disassembly of Ubiquitylated CMG Helicase at the End of Chromosome Replication

Disassembly of the Cdc45-MCM-GINS (CMG) DNA helicase is the key regulated step during DNA replication termination in eukaryotes, involving ubiquitylation of the Mcm7 helicase subunit, leading to a disassembly process that requires the Cdc48 "segregase". Here, we employ a screen to identify partners of budding yeast Cdc48 that are important for disassembly of ubiquitylated CMG helicase at the end of chromosome replication. We demonstrate that the ubiquitin-binding Ufd1-Npl4 complex recruits Cdc48 to ubiquitylated CMG. Ubiquitylation of CMG in yeast cell extracts is dependent upon lysine 29 of Mcm7, which is the only detectable site of ubiquitylation both in vitro and in vivo (though in vivo other sites can be modified when K29 is mutated). Mutation of K29 abrogates in vitro recruitment of Ufd1-Npl4-Cdc48 to the CMG helicase, supporting a model whereby Ufd1-Npl4 recruits Cdc48 to ubiquitylated CMG at the end of chromosome replication, thereby driving the disassembly reaction.

Roles of SUMO in Replication Initiation, Progression, and Termination

Accurate genome duplication during cell division is essential for life. This process is accomplished by the close collaboration between replication factors and many additional proteins that provide assistant roles. Replication factors establish the replication machineries capable of copying billions of nucleotides, while regulatory proteins help to achieve accuracy and efficiency of replication. Among regulatory proteins, protein modification enzymes can bestow fast and reversible changes to many targets, leading to coordinated effects on replication. Recent studies have begun to elucidate how one type of protein modification, sumoylation, can modify replication proteins and regulate genome duplication through multiple mechanisms. This chapter summarizes these new findings, and how they can integrate with the known regulatory circuitries of replication. As this area of research is still at its infancy, many outstanding questions remain to be explored, and we discuss these issues in light of the new advances.

Functions of Ubiquitin and SUMO in DNA Replication and Replication Stress

Complete and faithful duplication of its entire genetic material is one of the essential prerequisites for a proliferating cell to maintain genome stability. Yet, during replication DNA is particularly vulnerable to insults. On the one hand, lesions in replicating DNA frequently cause a stalling of the replication machinery, as most DNA polymerases cannot cope with defective templates. This situation is aggravated by the fact that strand separation in preparation for DNA synthesis prevents common repair mechanisms relying on strand complementarity, such as base and nucleotide excision repair, from working properly. On the other hand, the replication process itself subjects the DNA to a series of hazardous transformations, ranging from the exposure of single-stranded DNA to topological contortions and the generation of nicks and fragments, which all bear the risk of inducing genomic instability. Dealing with these problems requires rapid and flexible responses, for which posttranslational protein modifications that act independently of protein synthesis are particularly well suited. Hence, it is not surprising that members of the ubiquitin family, particularly ubiquitin itself and SUMO, feature prominently in controlling many of the defensive and restorative measures involved in the protection of DNA during replication. In this review we will discuss the contributions of ubiquitin and SUMO to genome maintenance specifically as they relate to DNA replication. We will consider cases where the modifiers act during regular, i.e., unperturbed stages of replication, such as initiation, fork progression, and termination, but also give an account of their functions in dealing with lesions, replication stalling and fork collapse.

MCM: one ring to rule them all

Precise replication of the eukaryotic genome is achieved primarily through strict regulation of the enzyme responsible for DNA unwinding, the replicative helicase. The motor of this helicase is a hexameric AAA+ ATPase called MCM. The loading of MCM onto DNA and its subsequent activation and disassembly are each restricted to separate cell cycle phases; this ensures that a functional replisome is only built once at any replication origin. In recent years, biochemical and structural studies have shown that distinct conformational changes in MCM, each requiring post-translational modifications and/or the activity of other replication proteins, define the various stages of the chromosome replication cycle. Here, we review recent progress in this area.

A new MCM modification cycle regulates DNA replication initiation

The MCM DNA helicase is a central regulatory target during genome replication. MCM is kept inactive during G1, and it initiates replication after being activated in S phase. During this transition, the only known chemical change to MCM is the gain of multisite phosphorylation that promotes cofactor recruitment. Because replication initiation is intimately linked to multiple biological cues, additional changes to MCM can provide further regulatory points. Here, we describe a yeast MCM SUMOylation cycle that regulates replication. MCM subunits undergo SUMOylation upon loading at origins in G1 before MCM phosphorylation. MCM SUMOylation levels then decline as MCM phosphorylation levels rise, thus suggesting an inhibitory role of MCM SUMOylation during replication. Indeed, increasing MCM SUMOylation impairs replication initiation, partly through promoting the recruitment of a phosphatase that decreases MCM phosphorylation and activation. We propose that MCM SUMOylation counterbalances kinase-based regulation, thus ensuring accurate control of replication initiation.

The Replisome-Coupled E3 Ubiquitin Ligase Rtt101Mms22 Counteracts Mrc1 Function to Tolerate Genotoxic Stress

Faithful DNA replication and repair requires the activity of cullin 4-based E3 ubiquitin ligases (CRL4), but the underlying mechanisms remain poorly understood. The budding yeast Cul4 homologue, Rtt101, in complex with the linker Mms1 and the putative substrate adaptor Mms22 promotes progression of replication forks through damaged DNA. Here we characterized the interactome of Mms22 and found that the Rtt101^Mms22 ligase associates with the replisome progression complex during S-phase via the amino-terminal WD40 domain of Ctf4. Moreover, genetic screening for suppressors of the genotoxic sensitivity of rtt101Δ cells identified a cluster of replication proteins, among them a component of the fork protection complex, Mrc1. In contrast to rtt101Δ and mms22Δ cells, mrc1Δ rtt101Δ and mrc1Δmms22Δ double mutants complete DNA replication upon replication stress by facilitating the repair/restart of stalled replication forks using a Rad52-dependent mechanism. Our results suggest that the Rtt101^Mms22 E3 ligase does not induce Mrc1 degradation, but specifically counteracts Mrc1’s replicative function, possibly by modulating its interaction with the CMG (Cdc45-MCM-GINS) complex at stalled forks.

Post-translational protein modifications, such as ubiquitylation, are essential for cells to respond to environmental cues. In order to understand how eukaryotes cope with DNA damage, we have investigated a conserved E3 ubiquitin ligase complex required for the resistance to carcinogenic chemicals. This complex, composed of Rtt101, Mms1 and Mms22 in budding yeast, plays a critical role in regulating the fate of stalled DNA replication. Here, we found that the Rtt101^Mms22 E3 ubiquitin ligase complex interacts with the replisome during S-phase, and orchestrates the repair/restart of DNA synthesis after stalling by activating a Rad52-dependent homologous recombination pathway. Our findings indicate that Rtt101^Mms22 specifically counteracts the replicative activity of Mrc1, a subunit of the fork protection complex, possibly by modulating its interaction with the CMG (Cdc45-MCM-GINS) helicase complex upon fork stalling. Altogether, our study unravels a functional protein cluster that is essential to understand how eukaryotic cells cope with DNA damage during replication and, thus deepens our knowledge of the biology that underlies carcinogenesis.

Analysis of the Histone H3.1 Interactome: A Suitable Chaperone for the Right Event

Despite minimal disparity at the sequence level, mammalian H3 variants bind to distinct sets of polypeptides. Although histone H3.1 predominates in cycling cells, our knowledge of the soluble complexes that it forms en route to deposition or following eviction from chromatin remains limited. Here, we provide a comprehensive analysis of the H3.1-binding proteome, with emphasis on its interactions with histone chaperones and components of the replication fork. Quantitative mass spectrometry revealed 170 protein interactions, whereas a large-scale biochemical fractionation of H3.1 and associated enzymatic activities uncovered over twenty stable protein complexes in dividing human cells. The sNASP and ASF1 chaperones play pivotal roles in the processing of soluble histones but do not associate with the active CDC45/MCM2-7/GINS (CMG) replicative helicase. We also find TONSL-MMS22L to function as a H3-H4 histone chaperone. It associates with the regulatory MCM5 subunit of the replicative helicase.

Proteolytic regulation of metabolic enzymes by E3 ubiquitin ligase complexes: lessons from yeast

Eukaryotic organisms use diverse mechanisms to control metabolic rates in response to changes in the internal and/or external environment. Fine metabolic control is a highly responsive, energy-saving process that is mediated by allosteric inhibition/activation and/or reversible modification of preexisting metabolic enzymes. In contrast, coarse metabolic control is a relatively long-term and expensive process that involves modulating the level of metabolic enzymes. Coarse metabolic control can be achieved through the degradation of metabolic enzymes by the ubiquitin-proteasome system (UPS), in which substrates are specifically ubiquitinated by an E3 ubiquitin ligase and targeted for proteasomal degradation. Here, we review select multi-protein E3 ligase complexes that directly regulate metabolic enzymes in Saccharomyces cerevisiae. The first part of the review focuses on the endoplasmic reticulum (ER) membrane-associated Hrd1 and Doa10 E3 ligase complexes. In addition to their primary roles in the ER-associated degradation pathway that eliminates misfolded proteins, recent quantitative proteomic analyses identified native substrates of Hrd1 and Doa10 in the sterol synthesis pathway. The second part focuses on the SCF (Skp1-Cul1-F-box protein) complex, an abundant prototypical multi-protein E3 ligase complex. While the best-known roles of the SCF complex are in the regulation of the cell cycle and transcription, accumulating evidence indicates that the SCF complex also modulates carbon metabolism pathways. The increasing number of metabolic enzymes whose stability is directly regulated by the UPS underscores the importance of the proteolytic regulation of metabolic processes for the acclimation of cells to environmental changes.

Tethering of SCF(Dia2) to the Replisome Promotes Efficient Ubiquitylation and Disassembly of the CMG Helicase

Disassembly of the Cdc45-MCM-GINS (CMG) DNA helicase, which unwinds the parental DNA duplex at eukaryotic replication forks, is the key regulated step during replication termination but is poorly understood [1, 2]. In budding yeast, the F-box protein Dia2 drives ubiquitylation of the CMG helicase at the end of replication, leading to a disassembly pathway that requires the Cdc48 segregase [3]. The substrate-binding domain of Dia2 comprises leucine-rich repeats, but Dia2 also has a TPR domain at its amino terminus that interacts with the Ctf4 and Mrc1 subunits of the replisome progression complex [4, 5], which assembles around the CMG helicase at replication forks [6]. Previous studies suggested two disparate roles for the TPR domain of Dia2, either mediating replisome-specific degradation of Mrc1 and Ctf4 [4] or else tethering SCF^Dia2 (SCF [Skp1/cullin/F-box protein]) to the replisome to increase its local concentration at replication forks [5]. Here, we show that SCF^Dia2 does not mediate replisome-specific degradation of Mrc1 and Ctf4, either during normal S phase or in response to replication stress. Instead, the tethering of SCF^Dia2 to the replisome progression complex increases the efficiency of ubiquitylation of the Mcm7 subunit of CMG, both in vitro and in vivo. Correspondingly, loss of tethering reduces the efficiency of CMG disassembly in vivo and is synthetic lethal in combination with a disassembly-defective allele of CDC48. Residual ubiquitylation of Mcm7 in dia2-ΔTPR cells is still CMG specific, highlighting the complex regulation of the final stages of chromosome replication, about which much still remains to be learned.

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Replisome tethering of SCF^Dia2 promotes efficient ubiquitylation of the CMG helicase

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Loss of tethering and mutation of Cdc48 cause synthetic CMG disassembly defects

The DNA-Binding Domain of S. pombe Mrc1 (Claspin) Acts to Enhance Stalling at Replication Barriers

During S-phase replication forks can stall at specific genetic loci. At some loci, the stalling events depend on the replisome components Schizosaccharomyces pombe Swi1 (Saccharomyces cerevisiae Tof1) and Swi3 (S. cerevisiae Csm3) as well as factors that bind DNA in a site-specific manner. Using a new genetic screen we identified Mrc1 (S. cerevisiae Mrc1/metazoan Claspin) as a replisome component involved in replication stalling. Mrc1 is known to form a sub-complex with Swi1 and Swi3 within the replisome and is required for the intra-S phase checkpoint activation. This discovery is surprising as several studies show that S. cerevisiae Mrc1 is not required for replication barrier activity. In contrast, we show that deletion of S. pombe mrc1 leads to an approximately three-fold reduction in barrier activity at several barriers and that Mrc1’s role in replication fork stalling is independent of its role in checkpoint activation. Instead, S. pombe Mrc1 mediated fork stalling requires the presence of a functional copy of its phylogenetically conserved DNA binding domain. Interestingly, this domain is on the sequence level absent from S. cerevisiae Mrc1. Our study indicates that direct interactions between the eukaryotic replisome and the DNA are important for site-specific replication stalling.

Regulation of Unperturbed DNA Replication by Ubiquitylation

Posttranslational modification of proteins by means of attachment of a small globular protein ubiquitin (i.e., ubiquitylation) represents one of the most abundant and versatile mechanisms of protein regulation employed by eukaryotic cells. Ubiquitylation influences almost every cellular process and its key role in coordination of the DNA damage response is well established. In this review we focus, however, on the ways ubiquitylation controls the process of unperturbed DNA replication. We summarise the accumulated knowledge showing the leading role of ubiquitin driven protein degradation in setting up conditions favourable for replication origin licensing and S-phase entry. Importantly, we also present the emerging major role of ubiquitylation in coordination of the active DNA replication process: preventing re-replication, regulating the progression of DNA replication forks, chromatin re-establishment and disassembly of the replisome at the termination of replication forks.

Termination of DNA replication forks: "Breaking up is hard to do"

To ensure duplication of the entire genome, eukaryotic DNA replication initiates from thousands of replication origins. The replication forks move through the chromatin until they encounter forks from neighboring origins. During replication fork termination forks converge, the replisomes disassemble and topoisomerase II resolves the daughter DNA molecules. If not resolved efficiently, terminating forks result in genomic instability through the formation of pathogenic structures. Our recent findings shed light onto the mechanism of replisome disassembly upon replication fork termination. We have shown that termination-specific polyubiquitylation of the replicative helicase component – Mcm7, leads to dissolution of the active helicase in a process dependent on the p97/VCP/Cdc48 segregase. The inhibition of terminating helicase disassembly resulted in a replication termination defect. In this extended view we present hypothetical models of replication fork termination and discuss remaining and emerging questions in the DNA replication termination field.

Replisome function during replicative stress is modulated by histone h3 lysine 56 acetylation through Ctf4

Histone H3 lysine 56 acetylation in Saccharomyces cerevisiae is required for the maintenance of genome stability under normal conditions and upon DNA replication stress. Here we show that in the absence of H3 lysine 56 acetylation replisome components become deleterious when replication forks collapse at natural replication block sites. This lethality is not a direct consequence of chromatin assembly defects during replication fork progression. Rather, our genetic analyses suggest that in the presence of replicative stress H3 lysine 56 acetylation uncouples the Cdc45-Mcm2-7-GINS DNA helicase complex and DNA polymerases through the replisome component Ctf4. In addition, we discovered that the N-terminal domain of Ctf4, necessary for the interaction of Ctf4 with Mms22, an adaptor protein of the Rtt101-Mms1 E3 ubiquitin ligase, is required for the function of the H3 lysine 56 acetylation pathway, suggesting that replicative stress promotes the interaction between Ctf4 and Mms22. Taken together, our results indicate that Ctf4 is an essential member of the H3 lysine 56 acetylation pathway and provide novel mechanistic insights into understanding the role of H3 lysine 56 acetylation in maintaining genome stability upon replication stress.

Closing the MCM cycle at replication termination sites

The initiation of eukaryotic DNA replication is a highly regulated process conserved from yeast to human. The past decade has seen significant advances in understanding how the CMG (Cdc45‐MCM‐GINS) replicative helicase is loaded onto DNA. However, very little was known on how this complex is removed from chromatin at the end of S phase. Two papers in a recent issue of Science [1], [2] show that in yeast and in Xenopus, the CMG complex is unloaded at replication termination sites by an active mechanism involving the polyubiquitylation of Mcm7.

The subunits of the S-phase checkpoint complex Mrc1/Tof1/Csm3: dynamics and interdependence

Background

The S-phase checkpoint aims to prevent cells from generation of extensive single-stranded DNA that predisposes to genome instability. The S. cerevisiae complex Tof1/Csm3/Mrc1 acts to restrain the replicative MCM helicase when DNA synthesis is prohibited. Keeping the replication machinery intact allows restart of the replication fork when the block is relieved. Although the subunits of the Tof1/Csm3/Mrc1 complex are well studied, the impact of every single subunit on the triple complex formation and function needs to be established.

Findings

This work studies the cellular localization and the chromatin binding of GFP-tagged subunits when the complex is intact and when a subunit is missing. We demonstrate that the complex is formed in cell nucleus, not the cytoplasm, as Tof1, Csm3 and Mrc1 enter the nucleus independently from one another. Via in situ chromatin binding assay we show that a Tof1-Csm3 dimer formation and chromatin binding is required to ensure the attachment of Mrc1 to chromatin. Our study indicates that the translocation into the nucleus is not the process to regulate the timing of chromatin association of Mrc1. We also studied the nuclear behavior of Mrc1 subunit in the process of adaptation to the presence hydroxyurea. Our results indicate that after prolonged HU incubation, cells bypass the S-phase checkpoint and proceed throughout the cell cycle. This process is accompanied by Mrc1 chromatin detachment and Rad53 dephosphorylation.

Conclusions

In S. cerevisiae the subunits of the S-phase checkpoint complex Mrc1/Tof1/Csm3 independently enter the cell nucleus, where a Tof1-Csm3 dimer is formed to ensure the chromatin binding of Mrc1 and favor DNA replication and S-phase checkpoint fork arrest. In the process of adaptation to the presence of hydroxyurea Mrc1 is detached from chromatin and Rad53 checkpoint activity is diminished in order to allow S-phase checkpoint escape and completion of the cell cycle.

Cdc48 and a ubiquitin ligase drive disassembly of the CMG helicase at the end of DNA replication

Chromosome replication is initiated by a universal mechanism in eukaryotic cells, involving the assembly and activation at replication origins of the CMG (Cdc45-MCM-GINS) DNA helicase, which is essential for the progression of replication forks. Disassembly of CMG is likely to be a key regulated step at the end of chromosome replication, but the mechanism was unknown until now. Here we show that the ubiquitin ligase known as SCF(Dia2) promotes ubiquitylation of CMG during the final stages of chromosome replication in Saccharomyces cerevisiae. The Cdc48/p97 segregase then associates with ubiquitylated CMG, leading rapidly to helicase disassembly. These findings indicate that the end of chromosome replication in eukaryotes is controlled in a similarly complex fashion to the much-better-characterized initiation step.

At short telomeres Tel1 directs early replication and phosphorylates Rif1

The replication time of Saccharomyces cerevisiae telomeres responds to TG1-3 repeat length, with telomeres of normal length replicating late during S phase and short telomeres replicating early. Here we show that Tel1 kinase, which is recruited to short telomeres, specifies their early replication, because we find a tel1Δ mutant has short telomeres that nonetheless replicate late. Consistent with a role for Tel1 in driving early telomere replication, initiation at a replication origin close to an induced short telomere was reduced in tel1Δ cells, in an S phase blocked by hydroxyurea. The telomeric chromatin component Rif1 mediates late replication of normal telomeres and is a potential substrate of Tel1 phosphorylation, so we tested whether Tel1 directs early replication of short telomeres by inactivating Rif1. A strain lacking both Rif1 and Tel1 behaves like a rif1Δ mutant by replicating its telomeres early, implying that Tel1 can counteract the delaying effect of Rif1 to control telomere replication time. Proteomic analyses reveals that in yku70Δ cells that have short telomeres, Rif1 is phosphorylated at Tel1 consensus sequences (S/TQ sites), with phosphorylation of Serine-1308 being completely dependent on Tel1. Replication timing analysis of a strain mutated at these phosphorylation sites, however, suggested that Tel1-mediated phosphorylation of Rif1 is not the sole mechanism of replication timing control at telomeres. Overall, our results reveal two new functions of Tel1 at shortened telomeres: phosphorylation of Rif1, and specification of early replication by counteracting the Rif1-mediated delay in initiation at nearby replication origins.

Two-way communications between ubiquitin-like modifiers and DNA

Many aspects of nucleic acid metabolism, such as DNA replication, repair and transcription, are regulated by the post-translational modifiers ubiquitin and SUMO. Not surprisingly, DNA itself plays an integral part in determining the modification of most chromatin-associated targets. Conversely, ubiquitination or SUMOylation of a protein can impinge on its DNA-binding properties. This review describes mechanistic principles governing the mutual interactions between DNA and ubiquitin or SUMO.

Replisome components--post-translational modifications and their effects

The process of DNA replication is highly regulated, but at the same time very dynamic. Once S-phase is initiated and replication elongation is occurring, the cells are committed to complete replication in order to ensure genome stability and survival. Many pathways exist to resolve situations where normal replisome progression is not possible. It is becoming more and more evident that post-translational modifications of replisome components play a key role in regulating these pathways which ensure fork progression. Here we review the known modifications of the progressing replisome and how these modifications are thought to affect DNA replication in unperturbed and perturbed S-phases.

Proteasome-dependent degradation of replisome components regulates faithful DNA replication

The replication machinery, or the replisome, collides with a variety of obstacles during the normal process of DNA replication. In addition to damaged template DNA, numerous chromosome regions are considered to be difficult to replicate owing to the presence of DNA secondary structures and DNA-binding proteins. Under these conditions, the replication fork stalls, generating replication stress. Stalled forks are prone to collapse, posing serious threats to genomic integrity. It is generally thought that the replication checkpoint functions to stabilize the replisome and replication fork structure upon replication stress. This is important in order to allow DNA replication to resume once the problem is solved. However, our recent studies demonstrated that some replisome components undergo proteasome-dependent degradation during DNA replication in the fission yeast Schizosaccharomyces pombe. Our investigation has revealed the involvement of the SCF^Pof3 (Skp1-Cullin/Cdc53-F-box) ubiquitin ligase in replisome regulation. We also demonstrated that forced accumulation of the replisome components leads to abnormal DNA replication upon replication stress. Here we review these findings and present additional data indicating the importance of replisome degradation for DNA replication. Our studies suggest that cells activate an alternative pathway to degrade replisome components in order to preserve genomic integrity.

Coordinated degradation of replisome components ensures genome stability upon replication stress in the absence of the replication fork protection complex

The stabilization of the replisome complex is essential in order to achieve highly processive DNA replication and preserve genomic integrity. Conversely, it would also be advantageous for the cell to abrogate replisome functions to prevent inappropriate replication when fork progression is adversely perturbed. However, such mechanisms remain elusive. Here we report that replicative DNA polymerases and helicases, the major components of the replisome, are degraded in concert in the absence of Swi1, a subunit of the replication fork protection complex. In sharp contrast, ORC and PCNA, which are also required for DNA replication, were stably maintained. We demonstrate that this degradation of DNA polymerases and helicases is dependent on the ubiquitin-proteasome system, in which the SCF(Pof3) ubiquitin ligase is involved. Consistently, we show that Pof3 interacts with DNA polymerase ε. Remarkably, forced accumulation of replisome components leads to abnormal DNA replication and mitotic catastrophes in the absence of Swi1. Swi1 is known to prevent fork collapse at natural replication block sites throughout the genome. Therefore, our results suggest that the cell elicits a program to degrade replisomes upon replication stress in the absence of Swi1. We also suggest that this program prevents inappropriate duplication of the genome, which in turn contributes to the preservation of genomic integrity.

The Hect domain E3 ligase Tom1 and the F-box protein Dia2 control Cdc6 degradation in G1 phase

The accurate replication of genetic information is critical to maintaining chromosomal integrity. Cdc6 functions in the assembly of pre-replicative complexes and is specifically required to load the Mcm2-7 replicative helicase complex at replication origins. Cdc6 is targeted for protein degradation by multiple mechanisms in Saccharomyces cerevisiae, although only a single pathway and E3 ubiquitin ligase for Cdc6 has been identified, the SCF(Cdc4) (Skp1/Cdc53/F-box protein) complex. Notably, Cdc6 is unstable during the G(1) phase of the cell cycle, but the ubiquitination pathway has not been previously identified. Using a genetic approach, we identified two additional E3 ubiquitin ligase components required for Cdc6 degradation, the F-box protein Dia2 and the Hect domain E3 Tom1. Both Dia2 and Tom1 control Cdc6 turnover during G(1) phase of the cell cycle and act separately from SCF(Cdc4). Ubiquitination of Cdc6 is significantly reduced in dia2Δ and tom1Δ cells. Tom1 and Dia2 each independently immunoprecipitate Cdc6, binding to a C-terminal region of the protein. Tom1 and Dia2 cannot compensate for each other in Cdc6 degradation. Cdc6 and Mcm4 chromatin association is aberrant in tom1Δ and dia2Δ cells in G(1) phase. Together, these results present evidence for a novel degradation pathway that controls Cdc6 turnover in G(1) that may regulate pre-replicative complex assembly.

The ubiquitin-proteasome system of Saccharomyces cerevisiae

Protein modifications provide cells with exquisite temporal and spatial control of protein function. Ubiquitin is among the most important modifiers, serving both to target hundreds of proteins for rapid degradation by the proteasome, and as a dynamic signaling agent that regulates the function of covalently bound proteins. The diverse effects of ubiquitylation reflect the assembly of structurally distinct ubiquitin chains on target proteins. The resulting ubiquitin code is interpreted by an extensive family of ubiquitin receptors. Here we review the components of this regulatory network and its effects throughout the cell.

Inn1 and Cyk3 regulate chitin synthase during cytokinesis in budding yeasts

The chitin synthase that makes the primary septum during cell division in budding yeasts is an important therapeutic target with an unknown activation mechanism. We previously found that the C2-domain of the Saccharomyces cerevisiae Inn1 protein plays an essential but uncharacterised role at the cleavage site during cytokinesis. By combining a novel degron allele of INN1 with a point mutation in the C2-domain, we screened for mutations in other genes that suppress the resulting defect in cell division. In this way, we identified 22 dominant mutations of CHS2 (chitin synthase II) that map to two neighbouring sites in the catalytic domain. Chs2 in isolated cell membranes is normally nearly inactive (unless protease treatment is used to bypass inhibition); however, the dominant suppressor allele Chs2-V377I has enhanced activity in vitro. We show that Inn1 associates with Chs2 in yeast cell extracts. It also interacts in a yeast two-hybrid assay with the N-terminal 65% of Chs2, which contains the catalytic domain. In addition to compensating for mutations in the Inn1 C2-domain, the dominant CHS2 alleles suppress cytokinesis defects produced by the lack of the Cyk3 protein. Our data support a model in which the C2-domain of Inn1 acts in conjunction with Cyk3 to regulate the catalytic domain of Chs2 during cytokinesis. These findings suggest novel approaches for developing future drugs against important fungal pathogens.

Hect E3 ubiquitin ligase Tom1 controls Dia2 degradation during the cell cycle

The ubiquitin proteasome system plays a pivotal role in controlling the cell cycle. The budding yeast F-box protein Dia2 is required for genomic stability and is targeted for ubiquitin-dependent degradation in a cell cycle-dependent manner, but the identity of the ubiquitination pathway is unknown. We demonstrate that the Hect domain E3 ubiquitin ligase Tom1 is required for Dia2 protein degradation. Deletion of DIA2 partially suppresses the temperature-sensitive phenotype of tom1 mutants. Tom1 is required for Dia2 ubiquitination and degradation during G1 and G2/M phases of the cell cycle, whereas the Dia2 protein is stabilized during S phase. We find that Tom1 binding to Dia2 is enhanced in G1 and reduced in S phase, suggesting a mechanism for this proteolytic switch. Tom1 recognizes specific, positively charged residues in a Dia2 degradation/NLS domain. Loss of these residues blocks Tom1-mediated turnover of Dia2 and causes a delay in G1-to-S phase progression. Deletion of DIA2 rescues a delay in the G1-to-S phase transition in the tom1Δ mutant. Together our results suggest that Tom1 targets Dia2 for degradation during the cell cycle by recognizing positively charged residues in the Dia2 degradation/NLS domain and that Dia2 protein degradation contributes to G1-to-S phase progression.

The SCFDia2 ubiquitin E3 ligase ubiquitylates Sir4 and functions in transcriptional silencing

In budding yeast, transcriptional silencing, which is important to regulate gene expression and maintain genome integrity, requires silent information regulator (Sir) proteins. In addition, Rtt106, a histone chaperone involved in nucleosome assembly, functions in transcriptional silencing. However, how transcriptional silencing is regulated during mitotic cell division is not well understood. We show that cells lacking Dia2, a component of the SCF^Dia2 E3 ubiquitin ligase involved in DNA replication, display defects in silencing at the telomere and HMR locus and that the F-box and C-terminal regions of Dia2, two regions important for Dia2's ubiquitylation activity, are required for proper transcriptional silencing at these loci. In addition, we show that Sir proteins are mislocalized in dia2Δ mutant cells. Mutations in Dia2 and Rtt106 result in a synergistic loss of silencing at the HMR locus and significant elevation of Sir4 proteins at the HMR locus, suggesting that silencing defects in dia2Δ mutant cells are due, at least in part, to the altered levels of Sir4 at silent chromatin. Supporting this idea, we show that SCF^Dia2 ubiquitylates Sir4 in vitro and in vivo. Furthermore, Sir4 binding to silent chromatin is dynamically regulated during the cell cycle, and this regulation is lost in dia2Δ mutant cells. These results demonstrate that the SCF^Dia2 complex is involved in transcriptional silencing, ubiquitylates Sir4, and regulates transcriptional silencing during the cell cycle.

Heterochromatin is important for the maintenance of genome stability and regulation of gene expression. Heterochromatin protein 1 (HP1), a protein that binds to histone H3 methylated at lysine 9 (H3K9me3) at heterochromatin loci in mammalian cells, is dynamically regulated during the cell cycle by phosphorylation of histone H3 serine 10 (H3S10ph). Compared to mammalian cells, transcriptional silencing at budding yeast silent chromatin requires silent information regulator (Sir) proteins, and H3K9me3 and H3S10ph are not present in budding yeast. Therefore, it is not known whether and how silent chromatin in budding yeast is regulated during the cell cycle. Here, we show that the SCF^Dia2 ubiquitin E3 ligase complex regulates transcriptional silencing. We show that SCF^Dia2 ubiquitylates Sir4, a structural component of yeast silent chromatin, and that Sir4 levels decrease during the cell cycle in a Dia2-dependent manner. Concomitant with the reduction of Sir4 at telomeric silent chromatin during the cell cycle, the expression of a telomere-linked gene increases. Therefore, we propose that transcriptional silencing at budding yeast silent chromatin is regulated during the cell cycle, in part by SCF^Dia2-mediated Sir4 ubiquitylation on chromatin.