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

Expression and prognostic significance of MCM-3 and MCM-7 in salivary adenoid cystic carcinoma

This study sought to investigate minichromosome maintenance protein 3 (MCM3) and minichromosome maintenance protein 7 (MCM7) expression in salivary adenoid cystic carcinoma (SACC) samples, and to evaluate the relationship between clinicopathological characteristics and prognosis. The expressions of MCM3 and MCM7 were evaluated using immunohistochemistry of tissue sections from SACC patients, and statistical analyses were performed to evaluate the associations between MCM expression and clinicopathological variables and to analyze the disease-free survival (DFS) and prognostic factors. The positive expression rates of MCM3 and MCM7 in SACC were 98.8% and 96.6%, respectively. MCM3 expression correlated with T-stage and nerve invasion. MCM7 expression correlated with T-stage, adjacent tissue invasion, nerve invasion, and prognosis, and was negatively associated with DFS. However, there was no significant correlation between MCM3 expression and DFS. A kappa analysis demonstrated that MCM3 was closely associated with MCM7. MCM7 may be a favorable prognosis indicator in SACC.

Microscopy techniques to examine DNA replication in fission yeast

Temporal and spatial visualization of replication proteins and associated structures within the narrow confines of a yeast nucleus is technically challenging. Choosing the appropriate method depends upon the parameters of the experiment, the nature of the molecules to be observed, and the hypothesis to be tested. In this chapter, we review three broad types of visualization: whole-cell fluorescence or immunofluorescence, which is useful for questions of timing and chromatin association; nuclear spreads, which provide greater resolution within the chromatin for co-localization and region-specific effects; and chromatin fibers, which allow observation of labeled proteins and newly synthesized DNA on a linear chromosome. We also suggest a mounting procedure for live fission yeast with fluorescent proteins. We discuss applications of these protocols and some considerations for choosing methods and fluorophores.

Origin licensing requires ATP binding and hydrolysis by the MCM replicative helicase

Loading of the six related Minichromosome Maintenance (MCM) proteins as head-to-head double hexamers during DNA replication origin licensing is crucial for ensuring once-per-cell-cycle DNA replication in eukaryotic cells. Assembly of these prereplicative complexes (pre-RCs) requires the Origin Recognition Complex (ORC), Cdc6, and Cdt1. ORC, Cdc6, and MCM are members of the AAA+ family of ATPases, and pre-RC assembly requires ATP hydrolysis. Here we show that ORC and Cdc6 mutants defective in ATP hydrolysis are competent for origin licensing. However, ATP hydrolysis by Cdc6 is required to release nonproductive licensing intermediates. We show that ATP binding stabilizes the wild-type MCM hexamer. Moreover, by analyzing MCM containing mutant subunits, we show that ATP binding and hydrolysis by MCM are required for Cdt1 release and double hexamer formation. This work alters our view of how ATP is used by licensing factors to assemble pre-RCs.

Rapid regulation of nuclear proteins by rapamycin-induced translocation in fission yeast

Genetic analysis of protein function requires a rapid means of inactivating the gene under study. Typically, this exploits temperature-sensitive mutations or promoter shut-off techniques. We report the adaptation to Schizosaccharomyces pombe of the anchor-away technique, originally designed in budding yeast by Laemmli lab. This method relies on a rapamycin-mediated interaction between the FRB- and FKBP12-binding domains to relocalize nuclear proteins of interest to the cytoplasm. We demonstrate a rapid nuclear depletion of abundant proteins as proof of principle.

Reconstitution of the 26S proteasome reveals functional asymmetries in its AAA+ unfoldase

The 26S proteasome is the major eukaryotic ATP-dependent protease, yet the detailed mechanisms used by the proteasomal heterohexameric AAA+ unfoldase to drive substrate degradation remain poorly understood. To perform systematic mutational analyses of individual ATPase subunits, we heterologously expressed the unfoldase subcomplex from Saccharomyces cerevisiae in Escherichia coli and reconstituted the proteasome in vitro. Our studies demonstrate that the six ATPases have distinct roles in degradation, corresponding to their positions in the spiral staircases adopted by the AAA+ domains in the absence or presence of substrate. ATP hydrolysis in subunits at the top of the staircases is critical for substrate engagement and translocation. Whereas the unfoldase relies on this vertical asymmetry for substrate processing, interaction with the peptidase exhibits three-fold symmetry with contributions from alternate subunits. These diverse functional asymmetries highlight how the 26S proteasome deviates from simpler, homomeric AAA+ proteases.

Genome wide comparative comprehensive analysis of Plasmodium falciparum MCM family with human host

Mini chromosome maintenance (MCM) proteins 2-7, a subgroup of the large AAA ATPase family are critically required for eukaryotic DNA replication. These proteins are most likely responsible for unwinding DNA at the replication forks. Besides this function, some MCMs are also involved in other chromosome transactions such as transcription, chromatin remodeling and genome stability. All the MCMs contain a conserved region of ~200 amino acids responsible for nucleotide binding. The importance of MCM proteins is evident by the fact that deregulation of the activity of MCM family of proteins appears to be directly linked to human carcinogenesis. This article will focus on members of this important family of proteins from the malaria parasite Plasmodium falciparum and their comparison with the human host.

Effect of an MCM4 mutation that causes tumours in mouse on human MCM4/6/7 complex formation

It has been reported that a point mutation of minichromosome maintenance (MCM)4 causes mammary carcinoma, and it deregulates DNA replication to produce abnormal chromosome structures. To understand the effect of this mutation at level of MCM2-7 interaction, we examined the effect of the same mutation of human MCM4 on the complex formation with MCM6 and MCM7 in insect cells. Human MCM4/6/7 complexes containing the mutated MCM4 were formed, but the hexameric complex formation was not evident in comparison with those containing wild-type MCM4. In binary expression of MCM4 and MCM6, decreased levels of MCM6 were recovered with the mutated MCM4, compared with wild-type MCM4. These results suggest that this mutation of MCM4 perturbs proper interaction with MCM6 to affect complex formation of MCM4/6/7 that is a core structure of MCM2-7 complex. Consistent with this notion, nuclear localization and MCM complex formation of forcedly expressed MCM4 in human cells are affected by this mutation. Thus, the defect of this mutant MCM4 in interacting with MCM6 may generate a decreased level of chromatin binding of MCM2-7 complex.

Schizosaccharomyces pombe minichromosome maintenance-binding protein (MCM-BP) antagonizes MCM helicase

The minichromosome maintenance (MCM) complex, a replicative helicase, is a heterohexamer essential for DNA duplication and genome stability. We identified Schizosaccharomyces pombe mcb1(+) (Mcm-binding protein 1), an apparent orthologue of the human MCM-binding protein that associates with a subset of MCM complex proteins. mcb1(+) is an essential gene. Deletion of mcb1(+) caused cell cycle arrest after several generations with a cdc phenotype and disrupted nuclear structure. Mcb1 is an abundant protein, constitutively present across the cell cycle. It is widely distributed in cytoplasm and nucleoplasm and bound to chromatin. Co-immunoprecipitation suggested that Mcb1 interacts robustly with Mcm3-7 but not Mcm2. Overproduction of Mcb1 disrupted the association of Mcm2 with other MCM proteins, resulting in inhibition of DNA replication, DNA damage, and activation of the checkpoint kinase Chk1. Thus, Mcb1 appears to antagonize the function of MCM helicase.

Pre-replication complex proteins assemble at regions of low nucleosome occupancy within the Chinese hamster dihydrofolate reductase initiation zone

Genome-scale mapping of pre-replication complex proteins has not been reported in mammalian cells. Poor enrichment of these proteins at specific sites may be due to dispersed binding, poor epitope availability or cell cycle stage-specific binding. Here, we have mapped sites of biotin-tagged ORC and MCM protein binding in G1-synchronized populations of Chinese hamster cells harboring amplified copies of the dihydrofolate reductase (DHFR) locus, using avidin-affinity purification of biotinylated chromatin followed by high-density microarray analysis across the DHFR locus. We have identified several sites of significant enrichment for both complexes distributed throughout the previously identified initiation zone. Analysis of the frequency of initiations across stretched DNA fibers from the DHFR locus confirmed a broad zone of de-localized initiation activity surrounding the sites of ORC and MCM enrichment. Mapping positions of mononucleosomal DNA empirically and computing nucleosome-positioning information in silico revealed that ORC and MCM map to regions of low measured and predicted nucleosome occupancy. Our results demonstrate that specific sites of ORC and MCM enrichment can be detected within a mammalian intitiation zone, and suggest that initiation zones may be regions of generally low nucleosome occupancy where flexible nucleosome positioning permits flexible pre-RC assembly sites.

The Mcm complex: unwinding the mechanism of a replicative helicase

The Mcm2-7 complex serves as the eukaryotic replicative helicase, the molecular motor that both unwinds duplex DNA and powers fork progression during DNA replication. Consistent with its central role in this process, much prior work has illustrated that Mcm2-7 loading and activation are landmark events in the regulation of DNA replication. Unlike any other hexameric helicase, Mcm2-7 is composed of six unique and essential subunits. Although the unusual oligomeric nature of this complex has long hampered biochemical investigations, recent advances with both the eukaryotic as well as the simpler archaeal Mcm complexes provide mechanistic insight into their function. In contrast to better-studied homohexameric helicases, evidence suggests that the six Mcm2-7 complex ATPase active sites are functionally distinct and are likely specialized to accommodate the regulatory constraints of the eukaryotic process.

Microscopy techniques to examine DNA replication in fission yeast

Temporal and spatial visualization of replication proteins and associated structures within the narrow confines of a yeast nucleus is technically challenging. Choosing the appropriate method depends upon the parameters of the experiment, the nature of the molecules to be observed, and the hypothesis to be tested. In this chapter, we review three broad types of visualization: whole cell fluorescence or immunofluorescence, which is useful for questions of timing and chromatin association; nuclear spreads, which provide greater resolution within the chromatin for colocalization and region-specific effects; and chromatin fibers, which allow observation of labeled proteins and newly synthesized DNA on a linear chromosome. We discuss applications of these protocols and some considerations for choosing methods and fluorophores.

Subunit organization of Mcm2-7 and the unequal role of active sites in ATP hydrolysis and viability

The Mcm2-7 (minichromosome maintenance) complex is a toroidal AAA(+) ATPase and the putative eukaryotic replicative helicase. Unlike a typical homohexameric helicase, Mcm2-7 contains six distinct, essential, and evolutionarily conserved subunits. Precedence to other AAA(+) proteins suggests that Mcm ATPase active sites are formed combinatorially, with Walker A and B motifs contributed by one subunit and a catalytically essential arginine (arginine finger) contributed by the adjacent subunit. To test this prediction, we used copurification experiments to identify five distinct and stable Mcm dimer combinations as potential active sites; these subunit associations predict the architecture of the Mcm2-7 complex. Through the use of mutant subunits, we establish that at least three sites are active for ATP hydrolysis and have a canonical AAA(+) configuration. In isolation, these five active-site dimers have a wide range of ATPase activities. Using Walker B and arginine finger mutations in defined Mcm subunits, we demonstrate that these sites similarly make differential contributions toward viability and ATP hydrolysis within the intact hexamer. Our conclusions predict a structural discontinuity between Mcm2 and Mcm5 and demonstrate that in contrast to other hexameric helicases, the six Mcm2-7 active sites are functionally distinct.

Schizosaccharomyces pombe histone acetyltransferase Mst1 (KAT5) is an essential protein required for damage response and chromosome segregation

Schizosaccharomyces pombe Mst1 is a member of the MYST family of histone acetyltransferases and is the likely ortholog of Saccharomyces cerevisiae Esa1 and human Tip60 (KAT5). We have isolated a temperature-sensitive allele of this essential gene. mst1 cells show a pleiotropic phenotype at the restrictive temperature. They are sensitive to a variety of DNA-damaging agents and to the spindle poison thiabendazole. mst1 has an increased frequency of Rad22 repair foci, suggesting endogenous damage. Two-hybrid results show that Mst1 interacts with a number of proteins involved in chromosome integrity and centromere function, including the methyltransferase Skb1, the recombination mediator Rad22 (Sc Rad52), the chromatin assembly factor Hip1 (Sc Hir1), and the Msc1 protein related to a family of histone demethylases. mst1 mutant sensitivity to hydroxyurea suggests a defect in recovery following HU arrest. We conclude that Mst1 plays essential roles in maintenance of genome stability and recovery from DNA damage.

Schizosaccharomyces pombe Rad4/Cut5 protein modification and chromatin binding changes in DNA damage

The Schizosaccharomyces pombe Rad4/Cut5 protein is essential for DNA replication and checkpoint control. We have analyzed the behavior of the protein during unperturbed DNA replication, in different replication and checkpoint mutant backgrounds and in response to DNA-damaging agents. In an unperturbed cell cycle, Rad4 is chromatin bound and the mobility of the protein is not altered. Rad4 protein level and thus chromatin binding are dependent on a functional DNA polymerase epsilon. In response to replication arrest and DNA damage, the protein is modified in a Rad3-dependent manner. These data indicate that Rad4 undergoes diverse forms of regulation that are distinct in both DNA replication and checkpoint response.

Differences in the single-stranded DNA binding activities of MCM2-7 and MCM467: MCM2 and MCM5 define a slow ATP-dependent step

The MCM2-7 complex, a hexamer containing six distinct and essential subunits, is postulated to be the eukaryotic replicative DNA helicase. Although all six subunits function at the replication fork, only a specific subcomplex consisting of the MCM4, 6, and 7 subunits (MCM467) and not the MCM2-7 complex exhibits DNA helicase activity in vitro. To understand why MCM2-7 lacks helicase activity and to address the possible function of the MCM2, 3, and 5 subunits, we have compared the biochemical properties of the Saccharomyces cerevisiae MCM2-7 and MCM467 complexes. We demonstrate that both complexes are toroidal and possess a similar ATP-dependent single-stranded DNA (ssDNA) binding activity, indicating that the lack of helicase activity by MCM2-7 is not due to ineffective ssDNA binding. We identify two important differences between them. MCM467 binds dsDNA better than MCM2-7. In addition, we find that the rate of MCM2-7/ssDNA association is slow compared with MCM467; the association rate can be dramatically increased either by preincubation with ATP or by inclusion of mutations that ablate the MCM2/5 active site. We propose that the DNA binding differences between MCM2-7 and MCM467 correspond to a conformational change at the MCM2/5 active site with putative regulatory significance.

Phosphorylation of MCM4 by Cdc7 kinase facilitates its interaction with Cdc45 on the chromatin

Cdc7 kinase, conserved from yeasts to human, plays important roles in DNA replication. However, the mechanisms by which it stimulates initiation of DNA replication remain largely unclear. We have analyzed phosphorylation of MCM subunits during cell cycle by examining mobility shift on SDS-PAGE. MCM4 on the chromatin undergoes specific phosphorylation during S phase. Cdc7 phosphorylates MCM4 in the MCM complexes as well as the MCM4 N-terminal polypeptide. Experiments with phospho-amino acid-specific antibodies indicate that the S phase-specific mobility shift is due to the phosphorylation at specific N-terminal (S/T)(S/T)P residues of the MCM4 protein. These specific phosphorylation events are not observed in mouse ES cells deficient in Cdc7 or are reduced in the cells treated with siRNA specific to Cdc7, suggesting that they are mediated by Cdc7 kinase. The N-terminal phosphorylation of MCM4 stimulates association of Cdc45 with the chromatin, suggesting that it may be an important phosphorylation event by Cdc7 for activation of replication origins. Deletion of the N-terminal non-conserved 150 amino acids of MCM4 results in growth inhibition, and addition of amino acids carrying putative Cdc7 target sequences partially restores the growth. Furthermore, combination of MCM4 N-terminal deletion with alanine substitution and deletion of the N-terminal segments of MCM2 and MCM6, respectively, which contain clusters of serine/threonine and are also likely targets of Cdc7, led to an apparent nonviable phenotype. These results are consistent with the notion that the N-terminal phosphorylation of MCM2, MCM4, and MCM6 may play functionally redundant but essential roles in initiation of DNA replication.

The ATPase activity of MCM2-7 is dispensable for pre-RC assembly but is required for DNA unwinding

Eukaryotes have six minichromosome maintenance (MCM) proteins that are essential for DNA replication. The contribution of ATPase activity of MCM complexes to their function in replication is poorly understood. We have established a cell-free system competent for replication in which all MCM proteins are supplied by purified recombinant Xenopus MCM complexes. Recombinant MCM2-7 complex was able to assemble onto chromatin, load Cdc45 onto chromatin, and restore DNA replication in MCM-depleted extracts. Using mutational analysis in the Walker A motif of MCM6 and MCM7 of MCM2-7, we show that ATP binding and/or hydrolysis by MCM proteins is dispensable for chromatin loading and pre-replicative complex (pre-RC) assembly, but is required for origin unwinding during DNA replication. Moreover, this ATPase-deficient mutant complex did not support DNA replication in MCM-depleted extracts. Altogether, these results both demonstrate the ability of recombinant MCM proteins to perform all replication roles of MCM complexes, and further support the model that MCM2-7 is the replicative helicase. These data establish that mutations affecting the ATPase activity of the MCM complex uncouple its role in pre-RC assembly from DNA replication.

Schizosaccharomyces pombe mst2+ encodes a MYST family histone acetyltransferase that negatively regulates telomere silencing

Histone acetylation and deacetylation are associated with transcriptional activity and the formation of constitutively silent heterochromatin. Increasingly, histone acetylation is also implicated in other chromosome transactions, including replication and segregation. We have cloned the only Schizosaccharomyces pombe MYST family histone acetyltransferase genes, mst1(+) and mst2(+). Mst1p, but not Mst2p, is essential for viability. Both proteins are localized to the nucleus and bound to chromatin throughout the cell cycle. Deltamst2 genetically interacts with mutants that affect heterochromatin, cohesion, and telomere structure. Mst2p is a negative regulator of silencing at the telomere but does not affect silencing in the centromere or mating type region. We generated a census of proteins and histone modifications at wild-type telomeres. A histone acetylation gradient at the telomeres is lost in Deltamst2 cells without affecting the distribution of Taz1p, Swi6p, Rad21p, or Sir2p. We propose that the increased telomeric silencing is caused by histone hypoacetylation and/or an increase in the ratio of methylated to acetylated histones. Although telomere length is normal, meiosis is aberrant in Deltamst2 diploid homozygote mutants, suggesting that telomeric histone acetylation contributes to normal meiotic progression.

Conserved locus-specific silencing functions of Schizosaccharomyces pombe sir2+

In Schizosaccharomyces pombe, three genes, sir2(+), hst2(+), and hst4(+), encode members of the Sir2 family of conserved NAD(+)-dependent protein deacetylases. The S. pombe sir2(+) gene encodes a nuclear protein that is not essential for viability or for resistance to treatment with UV or a microtubule-destabilizing agent. However, sir2(+) is essential for full transcriptional silencing of centromeres, telomeres, and the cryptic mating-type loci. Chromatin immunoprecipitation results suggest that the Sir2 protein acts directly at these chromosomal regions. Enrichment of Sir2p at silenced regions does not require the HP1 homolog Swi6p; instead, Swi6-GFP localization to telomeres depends in part on Sir2p. The phenotype of sir2 swi6 double mutants supports a model whereby Sir2p functions prior to Swi6p at telomeres and the silent mating-type loci. However, Sir2p does not appear to be essential for the localization of Swi6p to centromeric foci. Cross-complementation experiments showed that the Saccharomyces cerevisiae SIR2 gene can function in place of S. pombe sir2(+), suggesting overlapping deacetylation substrates in both species. These results also suggest that, despite differences in most of the other molecules required, the two distantly related yeast species share a mechanism for targeting Sir2p homologs to silent chromatin.

A screen for Schizosaccharomyces pombe mutants defective in rereplication identifies new alleles of rad4+, cut9+ and psf2+

Fission yeast mutants defective in DNA replication have widely varying morphological phenotypes. We designed a screen for temperature-sensitive mutants defective in the process of replication regardless of morphology by isolating strains unable to rereplicate their DNA in the absence of cyclin B (Cdc13). Of the 42 rereplication-defective mutants analyzed, we were able to clone complementing plasmids for 10. This screen identified new alleles of the APC subunit cut9(+), the initiation/checkpoint factor rad4(+)/cut5(+), and the first mutant allele of psf2(+), a subunit of the novel GINS replication complex. Other genes identified are likely to play general roles in gene expression and protein localization.

Schizosaccharomyces pombe replication protein Cdc45/Sna41 requires Hsk1/Cdc7 and Rad4/Cut5 for chromatin binding

Cdc45 is a conserved protein required for firing of replication origins and processive DNA replication. We used an in situ chromatin-binding assay to determine factors required for fission yeast Cdc45p chromatin binding. Assembly of the pre-replicative complex is essential for Cdc45p chromatin binding, but pre-replicative complex assembly occurs independently of Cdc45p. Fission yeast Cdc45p associates with MCM proteins in asynchronously growing cells and cells arrested in S phase by hydroxyurea, but not in cells arrested at the G2/M transition. Both hsk1+ (the fission yeast CDC7 homologue) and rad4+/ cut5+ (the fission yeast DPB11 homologue) are required for Cdc45p chromatin binding. Cdc45p also remains chromatin-bound in mutants that fail to recover from replication arrest. In summary, Cdc45p chromatin binding requires an intact pre-replicative complex as well as signaling from both the Dbf4-dependent kinase and cyclin-dependent kinases.

Novel and deviant Walker A ATP-binding motifs in bacteriophage large terminase-DNA packaging proteins

Bacteriophage terminases constitute a very interesting class of viral-coded multifunctional ATPase "motors" that apparently drive directional translocation of DNA into an empty viral capsid. A common Walker A motif and other conserved signatures of a critical ATPase catalytic center are identified in the N-terminal half of numerous large terminase proteins. However, several terminases, including the well-characterized lambda and SPP1 terminases, seem to lack the classic Walker A in the N-terminus. Using sequence alignment approaches, we discovered the presence of deviant Walker A motifs in these and many other phage terminases. One deviation, the presence of a lysine at the beginning of P-loop, may represent a 3D equivalent of the universally conserved lysine in the Walker A GKT/S signature. This and other novel putative Walker A motifs that first came to light through this study help define the ATPase centers of phage and viral terminases as well as elicit important insights into the molecular functioning of this fundamental motif in biological systems.

Saccharomyces cerevisiae CSM1 gene encoding a protein influencing chromosome segregation in meiosis I interacts with elements of the DNA replication complex

We present the characteristics of the Csm1 (Spo86) protein of Saccharomyces cerevisiae that are important for meiotic division. The level of Csm1p does not change throughout the cell cycle, but this protein is absent in mature spores. Deletion of CSM1 causes incorrect spore formation and meiotic chromosome missegregation together with increased sensitivity of vegetative cells to benomyl and manganese. In a two-hybrid analysis with Csm1p as bait, we detected interactions with three members of the Mcm2-7 family of proteins involved in the initiation of DNA replication, and with Clf1p also implicated in replication. The Csm1p-Mcm3, Mcm5 and Mcm7p interactions were confirmed by co-immunoprecipitation. Three other interacting proteins, Mgs1p, Ulp2, and Plp2, participate in chromosome assembling and segregation, whereas the function of two others has not been established. Genetic experiments showed that the two-hybrid isolates MGS1, CLF1, MCM3, 5, 7 (CDC47), and YDL089w, when overexpressed, partially suppress the csm1Delta/csm1Delta sporulation defect. We propose that, besides its other functions, Csm1p may be involved in premeiotic DNA replication.

Eukaryotic MCM proteins: beyond replication initiation

The minichromosome maintenance (or MCM) protein family is composed of six related proteins that are conserved in all eukaryotes. They were first identified by genetic screens in yeast and subsequently analyzed in other experimental systems using molecular and biochemical methods. Early data led to the identification of MCMs as central players in the initiation of DNA replication. More recent studies have shown that MCM proteins also function in replication elongation, probably as a DNA helicase. This is consistent with structural analysis showing that the proteins interact together in a heterohexameric ring. However, MCMs are strikingly abundant and far exceed the stoichiometry of replication origins; they are widely distributed on unreplicated chromatin. Analysis of mcm mutant phenotypes and interactions with other factors have now implicated the MCM proteins in other chromosome transactions including damage response, transcription, and chromatin structure. These experiments indicate that the MCMs are central players in many aspects of genome stability.

Mcm4,6,7 uses a "pump in ring" mechanism to unwind DNA by steric exclusion and actively translocate along a duplex

Mcm4,6,7 is a ring-shaped heterohexamer and the putative eukaryotic replication fork helicase. In this study, we examine the mechanism of Mcm4,6,7. Mcm4,6,7 binds to only one strand of a duplex during unwinding, corresponding to the leading strand of a replication fork. Mcm4,6,7 unwinding stops at a nick in either strand. The Mcm4,6,7 ring also actively translocates along duplex DNA, enabling the protein to drive branch migration of Holliday junctions. The Mcm4,6,7 mechanism is very similar to DnaB, except the proteins translocate with opposite polarity along DNA. Mcm4,6,7 and DnaB have different structural folds and evolved independently; thus, the similarity in mechanism is surprising. We propose a "pump in ring" mechanism for both Mcm4,6,7 and DnaB, wherein a single-stranded DNA pump is situated within the central channel of the ring-shaped helicase, and unwinding is the result of steric exclusion. In this example of convergent evolution, the "pump in ring" mechanism was probably selected by eukaryotic and bacterial replication fork helicases in order to restrict unwinding to replication fork structures, stop unwinding when the replication fork encounters a nick, and actively translocate along duplex DNA to accomplish additional activities such as DNA branch migration.

Human Mcm proteins at a replication origin during the G1 to S phase transition

Previous work with yeast cells and with Xenopus egg extracts had shown that eukaryotic pre-replication complexes assemble on chromatin in a step-wise manner whereby specific loading factors promote the recruitment of essential Mcm proteins at pre-bound origin recognition complexes (ORC with proteins Orc1p-Orc6p). While the order of assembly--Mcm binding follows ORC binding--seems to be conserved in cycling mammalian cells in culture, it has not been determined whether mammalian Mcm proteins associate with ORC-bearing chromatin sites. We have used a chromatin immunoprecipitation approach to investigate the site of Mcm binding in a genomic region that has previously been shown to contain an ORC-binding site and an origin of replication. Using chromatin from HeLa cells in G1 phase, antibodies against Orc2p as well as antibodies against Mcm proteins specifically immunoprecipitate chromatin enriched for a DNA region that includes a replication origin. However, with chromatin from cells in S phase, only Orc2p-specific antibodies immunoprecipitate the origin-containing DNA region while Mcm-specific antibodies immunoprecipitate chromatin with DNA from all parts of the genomic region investigated. Thus, human Mcm proteins first assemble at or adjacent to bound ORC and move to other sites during genome replication.