Nuclear localization of Schizosaccharomyces pombe Mcm2/Cdc19p requires MCM complex assembly

Different phenotypes in vivo are associated with ATPase motif mutations in Schizosaccharomyces pombe minichromosome maintenance proteins

The six conserved MCM proteins are essential for normal DNA replication. They share a central core of homology that contains sequences related to DNA-dependent and AAA(+) ATPases. It has been suggested that the MCMs form a replicative helicase because a hexameric subcomplex formed by MCM4, -6, and -7 proteins has in vitro DNA helicase activity. To test whether ATPase and helicase activities are required for MCM protein function in vivo, we mutated conserved residues in the Walker A and Walker B motifs of MCM4, -6, and -7 and determined that equivalent mutations in these three proteins have different in vivo effects in fission yeast. Some mutations reported to abolish the in vitro helicase activity of the mouse MCM4/6/7 subcomplex do not affect the in vivo function of fission yeast MCM complex. Mutations of consensus CDK sites in Mcm4p and Mcm7p also have no phenotypic consequences. Co-immunoprecipitation analyses and in situ chromatin-binding experiments were used to study the ability of the mutant Mcm4ps to associate with the other MCMs, localize to the nucleus, and bind to chromatin. We conclude that the role of ATP binding and hydrolysis is different for different MCM subunits.

Interdependent nuclear accumulation of budding yeast Cdt1 and Mcm2-7 during G1 phase

Cdt1 is essential for loading Mcm2-7 proteins into prereplicative complexes (pre-RCs) during replication licensing and has been found in organisms as diverse as fission yeast and humans. We have identified a homologue of Cdt1 in Saccharomyces cerevisiae, which is required for pre-RC assembly. We show that, like Mcm2-7p, Cdt1p accumulates in the nucleus during G1 phase and is excluded from the nucleus later in the cell cycle by cyclin dependent kinases (cdks). Cdt1p interacts with the Mcm2--7p complex, and the nuclear accumulation of these proteins during G1 is interdependent. This coregulation of Cdt1p and Mcm2-7p represents a novel level of pre-RC control.

Essential role of MCM proteins in premeiotic DNA replication

A critical event in eukaryotic DNA replication involves association of minichromosome maintenance (MCM2-7) proteins with origins, to form prereplicative complexes (pre-RCs) that are competent for initiation. The ability of mutants defective in MCM2-7 function to complete meiosis had suggested that pre-RC components could be irrelevant to premeiotic S phase. We show here that MCM2-7 proteins bind to chromatin in fission yeast cells preparing for meiosis and during premeiotic S phase in a manner suggesting they in fact are required for DNA replication in the meiotic cycle. This is confirmed by analysis of a degron mcm4 mutant, which cannot carry out premeiotic DNA replication. Later in meiosis, Mcm4 chromatin association is blocked between meiotic nuclear divisions, presumably accounting for the absence of a second round of DNA replication. Together, these results emphasize similarity between replication mechanisms in mitotic and meiotic cell cycles.

Biochemical activities associated with mouse Mcm2 protein

Mcm2, a member of the Mcm2-7 protein family essential for the initiation of DNA replication, has several biochemical activities including the ability to inhibit the Mcm4,6,7 helicase. In this study, we characterized the activities associated with Mcm2 and determined the region required for them. It was found that Mcm2 deleted at an amino-terminal portion is able to bind to an Mcm4,6,7 hexameric complex and to inhibit its DNA helicase activity. The same deletion mutant of Mcm2 and the carboxyl-terminal half of Mcm2 were both able to bind to Mcm4, suggesting that the carboxyl-half of Mcm2 binds to Mcm4 to disassemble the Mcm4,6,7 hexamer. Phosphorylation of Mcm2,4,6,7 complexes with Cdc7 kinase showed that the amino-terminal region of Mcm2 is required for the phosphorylation, and it contains major Cdc7-mediated phosphorylation sites. We also found that Mcm2 itself can assemble a nucleosome-like structure in vitro in the presence of H3/H4 histones. The amino-terminal region of Mcm2 was required for the activity where a histone-binding domain is located. Finally, we identified a region required for the nuclear localization of Mcm2. The function of Mcm2 is discussed based on these biochemical characteristics.

MCM2-7 proteins are essential components of prereplicative complexes that accumulate cooperatively in the nucleus during G1-phase and are required to establish, but not maintain, the S-phase checkpoint

A prereplicative complex (pre-RC) of proteins is assembled at budding yeast origins of DNA replication during the G1-phase of the cell cycle, as shown by genomic footprinting. The proteins responsible for this prereplicative footprint have yet to be identified but are likely to be involved in the earliest stages of the initiation step of chromosome replication. Here we show that MCM2-7 proteins are essential for both the formation and maintenance of the pre-RC footprint at the origin ARS305. It is likely that pre-RCs contain heteromeric complexes of MCM2-7 proteins, since degradation of Mcm2, 3, 6, or 7 during G1-phase, after pre-RC formation, causes loss of Mcm4 from the nucleus. It has been suggested that pre-RCs on unreplicated chromatin may generate a checkpoint signal that inhibits premature mitosis during S-phase. We show that, although mitosis does indeed occur in the absence of replication if MCM proteins are degraded during G1-phase, anaphase is prevented if MCMs are degraded during S-phase. Our data indicate that pre-RCs do not play a direct role in checkpoint control during chromosome replication.

Characterization of Schizosaccharomyces pombe mcm7(+) and cdc23(+) (MCM10) and interactions with replication checkpoints

MCM proteins are required for the proper regulation of DNA replication. We cloned fission yeast mcm7(+) and showed it is essential for viability; spores lacking mcm7(+) begin S phase later than wild-type cells and arrest with an apparent 2C DNA content. We isolated a novel temperature-sensitive allele, mcm7-98, and also characterized two temperature-sensitive alleles of the fission yeast homolog of MCM10, cdc23(+). mcm7-98 and both cdc23ts alleles arrest with damaged chromosomes and an S phase delay. We find that mcm7-98 is synthetically lethal with the other mcmts mutants but does not interact genetically with either cdc23ts allele. However, cdc23-M36 interacts with mcm4ts. Unlike other mcm mutants or cdc23, mcm7-98 is synthetically lethal with checkpoint mutants Deltacds1, Deltachk1, or Deltarad3, suggesting chromosomal defects even at permissive temperature. Mcm7p is a nuclear protein throughout the cell cycle, and its localization is dependent on the other MCM proteins. Our data suggest that the Mcm3p-Mcm5p dimer interacts with the Mcm4p-Mcm6p-Mcm7p core complex through Mcm7p.

Fission yeast Aip3p (spAip3p) is required for an alternative actin-directed polarity program

Aip3p is an actin-interacting protein that regulates cell polarity in budding yeast. The Schizosaccharomyces pombe-sequencing project recently led to the identification of a homologue of Aip3p that we have named spAip3p. Our results confirm that spAip3p is a true functional homologue of Aip3p. When expressed in budding yeast, spAip3p localizes similarly to Aip3p during the cell cycle and complements the cell polarity defects of an aip3Delta strain. Two-hybrid analysis shows that spAip3p interacts with actin similarly to Aip3p. In fission yeast, spAip3p localizes to both cell ends during interphase and later organizes into two rings at the site of cytokinesis. spAip3p localization to cell ends is dependent on microtubule cytoskeleton, its localization to the cell middle is dependent on actin cytoskeleton, and both patterns of localization require an operative secretory pathway. Overexpression of spAip3p disrupts the actin cytoskeleton and cell polarity, leading to morphologically aberrant cells. Fission yeast, which normally rely on the microtubule cytoskeleton to establish their polarity axis, can use the actin cytoskeleton in the absence of microtubule function to establish a new polarity axis, leading to the formation of branched cells. spAip3p localizes to, and is required for, branch formation, confirming its role in actin-directed polarized cell growth in both Schizosaccharomyces pombe and Saccharomyces cerevisiae.

The art and design of genetic screens: yeast

Understanding the biology of complex systems is facilitated by comparing them with simpler organisms. Budding and fission yeasts provide ideal model systems for eukaryotic cell biology. Although they differ from one another in terms of a range of features, these yeasts share powerful genetic and genomic tools. Classical yeast genetics remains an essential element in discovering and characterizing the genes that make up a eukaryotic cell.

Control of DNA rereplication via Cdc2 phosphorylation sites in the origin recognition complex

Cdc2 kinase is a master regulator of cell cycle progression in the fission yeast Schizosaccharomyces pombe. Our data indicate that Cdc2 phosphorylates replication factor Orp2, a subunit of the origin recognition complex (ORC). Cdc2 phosphorylation of Orp2 appears to be one of multiple mechanisms by which Cdc2 prevents DNA rereplication in a single cell cycle. Cdc2 phosphorylation of Orp2 is not required for Cdc2 to activate DNA replication initiation. Phosphorylation of Orp2 appears first in S phase and becomes maximal in G(2) and M when Cdc2 kinase activity is required to prevent reinitiation of DNA replication. A mutant lacking Cdc2 phosphorylation sites in Orp2 (orp2-T4A) allowed greater rereplication of DNA than congenic orp2 wild-type strains when the limiting replication initiation factor Cdc18 was deregulated. Thus, Cdc2 phosphorylation of Orp2 may be redundant with regulation of Cdc18 for preventing reinitiation of DNA synthesis. Since Cdc2 phosphorylation sites are present in Orp2 (also known as Orc2) from yeasts to metazoans, we propose that cell cycle-regulated phosphorylation of the ORC provides a safety net to prevent DNA rereplication and resulting genetic instability.

Fission yeast Rad17 associates with chromatin in response to aberrant genomic structures

Fission yeast checkpoint protein Rad17 is required for the DNA integrity checkpoint responses. A fraction of Rad17 is chromatin bound independent of the other checkpoint proteins throughout the cell cycle. Here we show that in response to DNA damage induced by either methyl methanesulfonate treatment or ionizing radiation, increased levels of Rad17 bind to chromatin. Following S-phase stall induced by hydroxyurea or a cdc22 mutation, the chromatin-bound Rad17 progressively dissociates from the chromatin. After S-phase arrest by hydroxyurea in cds1Delta or rad3Delta cells or by replication mutants, Rad17 remains chromatin bound. Rad17 is able to complex in vivo with an Rfc small subunit, Rfc2, but not with Rfc1. Furthermore, cells with rfc1Delta are checkpoint proficient, suggesting that Rfc1 does not have a role in checkpoint function. A checkpoint-defective mutant protein, Rad17(K118E), which has similar nuclear localization to that of the wild type, is unable to bind ATP and has reduced ability in chromatin binding. Mutant Rad17(K118E) protein also has reduced ability to complex with Rfc2, suggesting that Lys(118) of Rad17 plays a role in Rad17-Rfc small-subunit complex formation and chromatin association. However, in the rad17.K118E mutant cells, Cds1 can be activated by hydroxyurea. Together, these results suggest that Rad17 binds to chromatin in response to an aberrant genomic structure generated from DNA damage, replication mutant arrest, or hydroxyurea arrest in the absence of Cds1. Rad17 is not required to bind chromatin when genomic structures are protected by hydroxyurea-activated Cds1. The possible checkpoint events induced by chromatin-bound Rad17 are discussed.

Different dynamics in nuclear entry of subunits of the repair/transcription factor TFIIH

We report here the different ways in which four subunits of the basal transcription/repair factor TFIIH (XPB, XPD, p62 and p44) and the damage recognition XPC repair protein can enter the nucleus. We examined their nuclear localization by transiently expressing the gene products tagged with the enhanced green fluorescent protein (EGFP) in transfected 3T3 cells. In agreement with the identification of more than one putative nuclear localization signal (NLS) in their protein sequences, XPB, XPC, p62 and p44 chimeras were rapidly sorted to the nucleus. In contrast, the XPD-EGFP chimeras appeared mainly localized in the cytoplasm, with a minor fraction of transfectants showing the EGFP-based fluorescence also in the nucleus. The ability of the XPD chimeras to enter the nucleus was confirmed by western blotting on fractionated cell extracts and by functional complementation of the repair defect in the UV5 rodent cells, mutated in the XPD homologous gene. By deletion mutagenesis, we were unable to identify any sequence specific for nuclear localization. In particular, deletion of the putative NLS failed to affect subcellular localization and, conversely, the C-terminal part of XPD containing the putative NLS showed no specific nuclear accumulation. These findings suggest that the nuclear entry of XPD depends on its complexation with other proteins in the cytoplasm, possibly other components of the TFIIH complex.

Regulation of chromosome replication

The initiation of DNA replication in eukaryotic cells is tightly controlled to ensure that the genome is faithfully duplicated once each cell cycle. Genetic and biochemical studies in several model systems indicate that initiation is mediated by a common set of proteins, present in all eukaryotic species, and that the activities of these proteins are regulated during the cell cycle by specific protein kinases. Here we review the properties of the initiation proteins, their interactions with each other, and with origins of DNA replication. We also describe recent advances in understanding how the regulatory protein kinases control the progress of the initiation reaction. Finally, we describe the checkpoint mechanisms that function to preserve the integrity of the genome when the normal course of genome duplication is perturbed by factors that damage the DNA or inhibit DNA synthesis.

Fission yeast myosin-I, Myo1p, stimulates actin assembly by Arp2/3 complex and shares functions with WASp

Fission yeast myo1(+) encodes a myosin-I with all three tail homology domains (TH1, 2, 3) found in typical long-tailed myosin-Is. Myo1p tail also contains a COOH-terminal acidic region similar to the A-domain of WASp/Scar proteins and other fungal myosin-Is. Our analysis shows that Myo1p and Wsp1p, the fission yeast WASp-like protein, share functions and cooperate in controlling actin assembly. First, Myo1p localizes to cortical patches enriched at tips of growing cells and at sites of cell division. Myo1p patches partially colocalize with actin patches and are dependent on an intact actin cytoskeleton. Second, although deletion of myo1(+) is not lethal, Deltamyo1 cells have actin cytoskeletal defects, including loss of polarized cell growth, delocalized actin patches, and mating defects. Third, additional disruption of wsp1(+) is synthetically lethal, suggesting that these genes may share functions. In mapping the domains of Myo1p tail that share function with Wsp1p, we discovered that a Myo1p construct with just the head and TH1 domains is sufficient for cortical localization and to rescue all Deltamyo1 defects. However, it fails to rescue the Deltamyo1 Deltawsp1 lethality. Additional tail domains, TH2 and TH3, are required to complement the double mutant. Fourth, we show that a recombinant Myo1p tail binds to Arp2/3 complex and activates its actin nucleation activity.

Schizosaccharomyces pombe Hsk1p is a potential cds1p target required for genome integrity

The fission yeast Hsk1p kinase is an essential activator of DNA replication. Here we report the isolation and characterization of a novel mutant allele of the gene. Consistent with its role in the initiation of DNA synthesis, hsk1(ts) genetically interacts with several S-phase mutants. At the restrictive temperature, hsk1(ts) cells suffer abnormal S phase and loss of nuclear integrity and are sensitive to both DNA-damaging agents and replication arrest. Interestingly, hsk1(ts) mutants released to the restrictive temperature after early S-phase arrest in hydroxyurea (HU) are able to complete bulk DNA synthesis but they nevertheless undergo an abnormal mitosis. These findings indicate a second role for hsk1 subsequent to HU arrest. Consistent with a later S-phase role, hsk1(ts) is synthetically lethal with Deltarqh1 (RecQ helicase) or rad21ts (cohesin) mutants and suppressed by Deltacds1 (RAD53 kinase) mutants. We demonstrate that Hsk1p undergoes Cds1p-dependent phosphorylation in response to HU and that it is a direct substrate of purified Cds1p kinase in vitro. These results indicate that the Hsk1p kinase is a potential target of Cds1p regulation and that its activity is required after replication initiation for normal mitosis.

Nuclear import of the retrotransposon Tf1 is governed by a nuclear localization signal that possesses a unique requirement for the FXFG nuclear pore factor Nup124p

Retroviruses, such as human immunodeficiency virus, that infect nondividing cells generate integration precursors that must cross the nuclear envelope to reach the host genome. As a model for retroviruses, we investigated the nuclear entry of Tf1, a long-terminal-repeat-containing retrotransposon of the fission yeast Schizosaccharomyces pombe. Because the nuclear envelope of yeasts remains intact throughout the cell cycle, components of Tf1 must be transported through the envelope before integration can occur. The nuclear localization of the Gag protein of Tf1 is different from that of other proteins tested in that it has a specific requirement for the FXFG nuclear pore factor, Nup124p. Using extensive mutagenesis, we found that Gag contained three nuclear localization signals (NLSs) which, when included individually in a heterologous protein, were sufficient to direct nuclear import. In the context of the intact transposon, mutations in the NLS that mapped to the first 10 amino acid residues of Gag significantly impaired Tf1 retrotransposition and abolished nuclear localization of Gag. Interestingly, this NLS activity in the heterologous protein was specifically dependent upon the presence of Nup124p. Deletion analysis of heterologous proteins revealed the surprising result that the residues in Gag with the NLS activity were independent from the residues that conveyed the requirement for Nup124p. In fact, a fragment of Gag that lacked NLS activity, residues 10 to 30, when fused to a heterologous protein, was sufficient to cause the classical NLS of simian virus 40 to require Nup124p for nuclear import. Within the context of the current understanding of nuclear import, these results represent the novel case of a short amino acid sequence that specifies the need for a particular nuclear pore complex protein.

The intrinsic DNA helicase activity of Methanobacterium thermoautotrophicum delta H minichromosome maintenance protein

Minichromosome maintenance proteins (MCMs) form a family of conserved molecules that are essential for initiation of DNA replication. All eukaryotes contain six orthologous MCM proteins that function as heteromultimeric complexes. The sequencing of the complete genomes of several archaebacteria has shown that MCM proteins are also present in archaea. The archaea Methanobacterium thermoautotrophicum contains a single MCM-related sequence. Here we report on the expression and purification of the recombinant M. thermoautotrophicum MCM protein (MtMCM) in both Escherichia coli and baculovirus-infected cells. We show that purified MtMCM protein assembles in large macromolecular complexes consistent in size with being double hexamers. We demonstrate that MtMCM contains helicase activity that preferentially uses dATP and DNA-dependent dATPase and ATPase activities. The intrinsic helicase activity of MtMCM is abolished when a conserved lysine in the helicase domain I/nucleotide binding site is mutated. MtMCM helicase unwinds DNA duplexes in a 3' --> 5' direction and can unwind up to 500 base pairs in vitro. The kinetics, processivity, and directionality of MtMCM support its role as a replicative helicase in M. thermoautotrophicum. This strongly suggests that this function is conserved for MCM proteins in eukaryotes where a replicative helicase has yet to be identified.

Chromatin binding of the fission yeast replication factor mcm4 occurs during anaphase and requires ORC and cdc18

We describe an in situ technique for studying the chromatin binding of proteins in the fission yeast Schizosaccharomyces pombe. After tagging the protein of interest with green fluorescent protein (GFP), chromatin-associated protein is detected by GFP fluorescence following cell permeabilization and washing with a non-ionic detergent. Cell morphology and nuclear structure are preserved in this procedure, allowing structures such as the mitotic spindle to be detected by indirect immunofluorescence. Cell cycle changes in the chromatin association of proteins can therefore be determined from individual cells in asynchronous cultures. We have applied this method to the DNA replication factor mcm4/cdc21, and find that chromatin association occurs during anaphase B, significantly earlier than is the case in budding yeast. Binding of mcm4 to chromatin requires orc1 and cdc18 (homologous to Cdc6 in budding yeast). Release of mcm4 from chromatin occurs during S phase and requires DNA replication. Upon overexpressing cdc18, we show that mcm4 is required for re-replication of the genome in the absence of mitosis and is associated with chromatin in cells undergoing re-replication.