minichromosome maintenance

File:Homology shared within the Mcm2-7 protein family.jpg

The minichromosome maintenance proteins were named after a yeast genetics screen for mutants defective in the regulation of DNA replication initiation.{{cite journal | vauthors = Maine GT, Sinha P, Tye BK | title = Mutants of S. cerevisiae defective in the maintenance of minichromosomes | journal = Genetics | volume = 106 | issue = 3 | pages = 365–85 | date = March 1984 | doi = 10.1093/genetics/106.3.365 | pmid = 6323245 | pmc = 1224244 }} The rationale behind this screen was that if replication origins were regulated in a manner analogous to transcription promoters, where transcriptional regulators showed promoter specificity, then replication regulators should also show origin specificity. Since eukaryotic chromosomes contain multiple replication origins and the plasmids contain only one, a slight defect in these regulators would have a dramatic effect on the replication of plasmids but little effect on chromosomes. In this screen, mutants conditional for plasmid loss were identified. In a secondary screen, these conditional mutants were selected for defects in plasmid maintenance against a collection of plasmids each carrying a different origin sequence. Two classes of mcm mutants were identified: Those that affected the stability of all minichromosomes and others that affected the stability of only a subset of the minichromosomes. The former were mutants defective in chromosome segregation such as mcm16, mcm20 and mcm21. Among the latter class of origin-specific mutants were mcm1, mcm2, mcm3, mcm5 and mcm10. Later on, others identified Mcm4, Mcm6 and Mcm7 in yeasts and other eukaryotes based on homology to Mcm2p, Mcm3p and Mcm5p expanding the MCM family to six, subsequently known as the Mcm2-7 family.{{cite journal | vauthors = Tye BK | title = MCM proteins in DNA replication | journal = Annual Review of Biochemistry | volume = 68 | issue = 1 | pages = 649–86 | date = June 1999 | pmid = 10872463 | doi = 10.1146/annurev.biochem.68.1.649 }} In archaea, the heterohexamer ring is replaced by a homohexamer made up of a single type mcm protein, pointing at a history of gene duplication and diversification.{{cite journal | vauthors = Ausiannikava D, Allers T | title = Diversity of DNA Replication in the Archaea | journal = Genes | volume = 8 | issue = 2 | pages = 56 | date = January 2017 | pmid = 28146124 | pmc = 5333045 | doi = 10.3390/genes8020056 | doi-access = free }}

Mcm1{{cite journal | vauthors = Passmore S, Elble R, Tye BK | title = A protein involved in minichromosome maintenance in yeast binds a transcriptional enhancer conserved in eukaryotes | journal = Genes & Development | volume = 3 | issue = 7 | pages = 921–35 | date = July 1989 | pmid = 2673922 | doi = 10.1101/gad.3.7.921 | doi-access = free }}{{cite journal | vauthors = Chang VK, Fitch MJ, Donato JJ, Christensen TW, Merchant AM, Tye BK | title = Mcm1 binds replication origins | journal = The Journal of Biological Chemistry | volume = 278 | issue = 8 | pages = 6093–100 | date = February 2003 | pmid = 12473677 | doi = 10.1074/jbc.M209827200 | doi-access = free }} and Mcm10{{cite journal | vauthors = Merchant AM, Kawasaki Y, Chen Y, Lei M, Tye BK | title = A lesion in the DNA replication initiation factor Mcm10 induces pausing of elongation forks through chromosomal replication origins in Saccharomyces cerevisiae | journal = Molecular and Cellular Biology | volume = 17 | issue = 6 | pages = 3261–71 | date = June 1997 | pmid = 9154825 | pmc = 232179 | doi = 10.1128/MCB.17.6.3261 }}{{cite journal | vauthors = Homesley L, Lei M, Kawasaki Y, Sawyer S, Christensen T, Tye BK | title = Mcm10 and the MCM2-7 complex interact to initiate DNA synthesis and to release replication factors from origins | journal = Genes & Development | volume = 14 | issue = 8 | pages = 913–26 | date = April 2000 | pmid = 10783164 | pmc = 316538 | doi=10.1101/gad.14.8.913}} are also involved in DNA replication, directly or indirectly, but have no sequence homology to the Mcm2-7 family.

MCM2-7 is required for both DNA replication initiation and elongation; its regulation at each stage is a central feature of eukaryotic DNA replication. During G1 phase, the two head-to-head Mcm2-7 rings serve as the scaffold for the assembly of the bidirectional replication initiation complexes at the replication origin. During S phase, the Mcm2-7 complex forms the catalytic core of the Cdc45-MCM-GINS helicase - the DNA unwinding engine of the replisome.

Site selection for replication origins is carried out by the Origin Recognition Complex (ORC), a six subunit complex (Orc1-6).{{cite journal | vauthors = Bell SP, Stillman B | title = ATP-dependent recognition of eukaryotic origins of DNA replication by a multiprotein complex | journal = Nature | volume = 357 | issue = 6374 | pages = 128–34 | date = May 1992 | pmid = 1579162 | doi = 10.1038/357128a0 | bibcode = 1992Natur.357..128B | s2cid = 4346767 }}{{cite journal | vauthors = Li N, Lam WH, Zhai Y, Cheng J, Cheng E, Zhao Y, Gao N, Tye BK | title = Structure of the origin recognition complex bound to DNA replication origin | journal = Nature | volume = 559 | issue = 7713 | pages = 217–222 | date = July 2018 | pmid = 29973722 | doi = 10.1038/s41586-018-0293-x | bibcode = 2018Natur.559..217L | s2cid = 49577101 }} During the G1 phase of the cell cycle, Cdc6 is recruited by ORC to form a launching pad for the loading of two head-to-head Mcm2-7 hexamers, also known as the pre-replication complex (pre-RC).{{cite journal | vauthors = Diffley JF, Cocker JH, Dowell SJ, Harwood J, Rowley A | title = Stepwise assembly of initiation complexes at budding yeast replication origins during the cell cycle | journal = Journal of Cell Science. Supplement | volume = 19 | pages = 67–72 | date = 1995 | pmid = 8655649 | doi = 10.1242/jcs.1995.supplement_19.9 | doi-access = free }} There is genetic and biochemical evidence that the recruitment of the double hexamer may involve either one{{cite journal | vauthors = Ticau S, Friedman LJ, Ivica NA, Gelles J, Bell SP | title = Single-molecule studies of origin licensing reveal mechanisms ensuring bidirectional helicase loading | journal = Cell | volume = 161 | issue = 3 | pages = 513–525 | date = April 2015 | pmid = 25892223 | pmc = 4445235 | doi = 10.1016/j.cell.2015.03.012 }} or two{{cite journal | vauthors = Coster G, Diffley JF | title = Bidirectional eukaryotic DNA replication is established by quasi-symmetrical helicase loading | journal = Science | volume = 357 | issue = 6348 | pages = 314–318 | date = July 2017 | pmid = 28729513 | pmc = 5608077 | doi = 10.1126/science.aan0063 | bibcode = 2017Sci...357..314C }} ORCs. Soluble Mcm2-7 hexamer forms a flexible left-handed open-ringed structure stabilised by Cdt1 prior to its loading onto chromatin,{{cite journal | vauthors = Frigola J, He J, Kinkelin K, Pye VE, Renault L, Douglas ME, Remus D, Cherepanov P, Costa A, Diffley JF | title = Cdt1 stabilizes an open MCM ring for helicase loading | journal = Nature Communications | volume = 8 | pages = 15720 | date = June 2017 | pmid = 28643783 | pmc = 5490006 | doi = 10.1038/ncomms15720 | bibcode = 2017NatCo...815720F }} one at a time.{{cite journal | vauthors = Ticau S, Friedman LJ, Champasa K, Corrêa IR, Gelles J, Bell SP | title = Mechanism and timing of Mcm2-7 ring closure during DNA replication origin licensing | journal = Nature Structural & Molecular Biology | volume = 24 | issue = 3 | pages = 309–315 | date = March 2017 | pmid = 28191892 | pmc = 5336523 | doi = 10.1038/nsmb.3375 }} The structure of the ORC-Cdc6-Cdt1-MCM (OCCM) intermediate formed after the loading of the first Cdt1-Mcm2-7 heptamer indicates that the winged helix domain at the C-terminal extensions (CTE) of the Mcm2-7 complex firmly anchor onto the surfaces created by the ORC-Cdc6 ring structure around origin DNA.{{cite journal | vauthors = Yuan Z, Riera A, Bai L, Sun J, Nandi S, Spanos C, Chen ZA, Barbon M, Rappsilber J, Stillman B, Speck C, Li H | title = Structural basis of Mcm2-7 replicative helicase loading by ORC-Cdc6 and Cdt1 | journal = Nature Structural & Molecular Biology | volume = 24 | issue = 3 | pages = 316–324 | date = March 2017 | pmid = 28191893 | pmc = 5503505 | doi = 10.1038/nsmb.3372 }} The fusion of the two head-to-head Mcm2-7 hexamers is believed to be facilitated by the removal of Cdt1, leaving the NTDs of the two MCM hexamers flexible for inter-ring interactions.{{cite journal | vauthors = Zhai Y, Li N, Jiang H, Huang X, Gao N, Tye BK | title = Unique Roles of the Non-identical MCM Subunits in DNA Replication Licensing | journal = Molecular Cell | volume = 67 | issue = 2 | pages = 168–179 | date = July 2017 | pmid = 28732205 | doi = 10.1016/j.molcel.2017.06.016 | doi-access = free }} The loading of MCM2-7 onto DNA is an active process that requires ATP hydrolysis by both Orc1-6 and Cdc6.{{cite journal | vauthors = Randell JC, Bowers JL, Rodríguez HK, Bell SP | title = Sequential ATP hydrolysis by Cdc6 and ORC directs loading of the Mcm2-7 helicase | journal = Molecular Cell | volume = 21 | issue = 1 | pages = 29–39 | date = January 2006 | pmid = 16387651 | doi = 10.1016/j.molcel.2005.11.023 | doi-access = free }} This process is coined "Replication Licensing" as it is a prerequisite for DNA replication initiation in every cell division cycle.{{cite journal | vauthors = Tye BK | title = The MCM2-3-5 proteins: are they replication licensing factors? | journal = Trends in Cell Biology | volume = 4 | issue = 5 | pages = 160–6 | date = May 1994 | pmid = 14731643 | doi = 10.1016/0962-8924(94)90200-3 }}{{cite journal | vauthors = Thömmes P, Kubota Y, Takisawa H, Blow JJ | title = The RLF-M component of the replication licensing system forms complexes containing all six MCM/P1 polypeptides | journal = The EMBO Journal | volume = 16 | issue = 11 | pages = 3312–9 | date = June 1997 | pmid = 9214646 | pmc = 1169947 | doi = 10.1093/emboj/16.11.3312 }}

In late G1/early S phase, the pre-RC is activated for DNA unwinding by the cyclin-dependent kinases (CDKs) and DDK. This facilitates the loading of additional replication factors (e.g., Cdc45, MCM10, GINS, and DNA polymerases) and unwinding of the DNA at the origin. Once pre-RC formation is complete, Orc1-6 and Cdc6 are no longer required for MCM2-7 retention at the origin, and they are dispensable for subsequent DNA replication.

Upon entry into S phase, the activity of the CDKs and the Dbf4-dependent kinase (DDK) Cdc7 promotes the assembly of replication forks, likely in part by activating MCM2-7 to unwind DNA. Following DNA polymerase loading, bidirectional DNA replication commences.

During S phase, Cdc6 and Cdt1 are degraded or inactivated to block additional pre-RC formation, and bidirectional DNA replication ensues. When the replication fork encounters lesions in the DNA, the S-phase checkpoint response slows or stops fork progression and stabilizes the association of MCM2-7 with the replication fork during DNA repair.{{cite journal | vauthors = Kamimura Y, Tak YS, Sugino A, Araki H | title = Sld3, which interacts with Cdc45 (Sld4), functions for chromosomal DNA replication in Saccharomyces cerevisiae | journal = The EMBO Journal | volume = 20 | issue = 8 | pages = 2097–107 | date = April 2001 | pmid = 11296242 | pmc = 125422 | doi = 10.1093/emboj/20.8.2097 }}

The replication licensing system acts to ensure that the no section of the genome is replicated more than once in a single cell cycle.{{cite journal | vauthors = Tada S, Blow JJ | title = The replication licensing system | journal = Biological Chemistry | volume = 379 | issue = 8–9 | pages = 941–9 | date = August 1998 | pmid = 9792427 | pmc = 3604913 | doi = 10.1515/bchm.1998.379.8-9.941 }}

The inactivation of any of at least five of the six MCM subunits during S phase quickly blocks ongoing elongation. As a critical mechanism to ensure only a single round of DNA replication, the loading of additional MCM2-7 complexes into pre-RCs is inactivated by redundant means after passage into S phase. {{cite journal | vauthors = Neves H, Kwok HF | title = In sickness and in health: The many roles of the minichromosome maintenance proteins | journal = Biochimica et Biophysica Acta (BBA) - Reviews on Cancer | volume = 1868 | issue = 1 | pages = 295–308 | date = August 2017 | pmid = 28579200 | doi = 10.1016/j.bbcan.2017.06.001 }}

MCM2-7 activity can also be regulated during elongation. The loss of replication fork integrity, an event precipitated by DNA damage, unusual DNA sequence, or insufficient deoxyribonucleotide precursors, can lead to the formation of DNA double-strand breaks and chromosome rearrangements. Normally, these replication problems trigger an S-phase checkpoint that minimizes genomic damage by blocking further elongation and physically stabilizing protein-DNA associations at the replication fork until the problem is fixed. This stabilization of the replication fork requires the physical interaction of MCM2-7 with Mrc1, Tof1, and Csm3 (M/T/C complex).{{cite journal | vauthors = Katou Y, Kanoh Y, Bando M, Noguchi H, Tanaka H, Ashikari T, Sugimoto K, Shirahige K | title = S-phase checkpoint proteins Tof1 and Mrc1 form a stable replication-pausing complex | language = En | journal = Nature | volume = 424 | issue = 6952 | pages = 1078–83 | date = August 2003 | pmid = 12944972 | doi = 10.1038/nature01900 | bibcode = 2003Natur.424.1078K | s2cid = 4330982 }} In the absence of these proteins, dsDNA unwinding and replisome movement powered by MCM2-7 continue, but DNA synthesis stops. At least part of this stop is due to the dissociation of polymerase ε from the replication fork.

Each subunit in the MCM structure contains two large N- and C-terminal domains. The N-terminal domain consists of three small sub-domains and appears to be used mainly for structural organization.{{cite journal | vauthors = Liu W, Pucci B, Rossi M, Pisani FM, Ladenstein R | title = Structural analysis of the Sulfolobus solfataricus MCM protein N-terminal domain | journal = Nucleic Acids Research | volume = 36 | issue = 10 | pages = 3235–43 | date = June 2008 | pmid = 18417534 | pmc = 2425480 | doi = 10.1093/nar/gkn183 }} The N-domain can coordinate with a neighboring subunit's C-terminal AAA+ helicase domain through a long and conserved loop.{{cite journal | vauthors = Brewster AS, Chen XS | title = Insights into the MCM functional mechanism: lessons learned from the archaeal MCM complex | journal = Critical Reviews in Biochemistry and Molecular Biology | volume = 45 | issue = 3 | pages = 243–56 | date = June 2010 | pmid = 20441442 | pmc = 2953368 | doi = 10.3109/10409238.2010.484836 }} This conserved loop, named the allosteric control loop, has been shown to play a role in regulating interactions between N- and C-terminal regions by facilitating communication between the domains in response to ATP hydrolysis [10]. The N-domain also establishes the in vitro 3′→5′ directionality of MCM. {{cite journal | vauthors = Barry ER, McGeoch AT, Kelman Z, Bell SD | title = Archaeal MCM has separable processivity, substrate choice and helicase domains | journal = Nucleic Acids Research | volume = 35 | issue = 3 | pages = 988–98 | date = 2007-02-01 | pmid = 17259218 | pmc = 1807962 | doi = 10.1093/nar/gkl1117 }}{{cite journal | vauthors = Georgescu R, Yuan Z, Bai L, de Luna Almeida Santos R, Sun J, Zhang D, Yurieva O, Li H, O'Donnell ME | display-authors = 6 | title = Structure of eukaryotic CMG helicase at a replication fork and implications to replisome architecture and origin initiation | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 114 | issue = 5 | pages = E697-E706 | date = January 2017 | pmid = 28096349 | pmc = 5293012 | doi = 10.1073/pnas.1620500114 | doi-access = free }}

Regarding the physical mechanism of how a hexameric helicase unwinds DNA, two models have been proposed based on in vivo and in vitro data. In the "steric" model, the helicase tightly translocates along one strand of DNA while physically displacing the complementary strand. In the "pump" model, pairs of hexameric helicases unwind duplex DNA by either twisting it apart or extruding it through channels in the complex.

The steric model hypothesizes that the helicase encircles dsDNA and, after local melting of the duplex DNA at the origin, translocates away from the origin, dragging a rigid proteinaceous "wedge" (either part of the helicase itself or another associated protein) that separates the DNA strands.{{cite journal | vauthors = Patel SS, Picha KM | title = Structure and function of hexameric helicases | journal = Annual Review of Biochemistry | volume = 69 | issue = 1 | pages = 651–97 | date = 2000-06-01 | pmid = 10966472 | doi = 10.1146/annurev.biochem.69.1.651 }}

The pump model postulates that multiple helicases load at replication origins, translocate away from one another, and in some manner eventually become anchored in place. They then rotate dsDNA in opposite directions, resulting in the unwinding of the double helix in the intervening region.{{cite journal | vauthors = Laskey RA, Madine MA | title = A rotary pumping model for helicase function of MCM proteins at a distance from replication forks | journal = EMBO Reports | volume = 4 | issue = 1 | pages = 26–30 | date = January 2003 | pmid = 12524516 | pmc = 1315806 | doi = 10.1038/sj.embor.embor706 }} The pump model has also been proposed to be restricted to the melting of origin DNA while the Mcm2-7 complexes are still anchored at the origin just before replication initiation.

Various MCMs have been shown to promote cell proliferation in vitro and in vivo especially in certain types of cancer cell lines. The association between MCMs and proliferation in cancer cell lines is mostly attributed to its ability to enhance DNA replication. The roles of MCM2 and MCM7 in cell proliferation have been demonstrated in various cellular contexts and even in human specimens. 

MCM2 has been shown to be frequently expressed in proliferating premalignant lung cells. Its expression was associated with cells having a higher proliferation potential in non-dysplastic squamous epithelium, malignant fibrous histiocytomas, and endometrial carcinoma, while MCM2 expression was also correlated higher mitotic index in breast cancer specimens. {{cite journal | vauthors = Gonzalez MA, Pinder SE, Callagy G, Vowler SL, Morris LS, Bird K, Bell JA, Laskey RA, Coleman N | title = Minichromosome maintenance protein 2 is a strong independent prognostic marker in breast cancer | journal = Journal of Clinical Oncology | volume = 21 | issue = 23 | pages = 4306–13 | date = December 2003 | pmid = 14645419 | doi = 10.1200/jco.2003.04.121 }}

Similarly, many research studies have shown the link between MCM7 expression and cell proliferation. Expression of MCM7 was significantly correlated with the expression of Ki67 in choriocarcinomas, lung cancer, papillary urothelial neoplasia, esophageal cancer, and endometrial cancer. Its expression was also associated with a higher proliferative index in prostatic intraepithelial neoplasia and cancer.{{cite journal | vauthors = Guan B, Wang X, Yang J, Zhou C, Meng Y | title = Minichromosome maintenance complex component 7 has an important role in the invasion of papillary urothelial neoplasia | journal = Oncology Letters | volume = 10 | issue = 2 | pages = 946–950 | date = August 2015 | pmid = 26622601 | pmc = 4509410 | doi = 10.3892/ol.2015.3333 }}

• Human genes encoding MCM proteins include:{{cite journal | vauthors = Cortez D, Glick G, Elledge SJ | title = Minichromosome maintenance proteins are direct targets of the ATM and ATR checkpoint kinases | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 101 | issue = 27 | pages = 10078–83 | date = July 2004 | pmid = 15210935 | pmc = 454167 | doi = 10.1073/pnas.0403410101 | doi-access = free }}
• MCM2
• MCM3
• MCM4
• MCM5
• MCM6
• MCM7
• MCM8
• MCM9
• MCM10

{{Reflist}}

• {{cite web |url= http://focus.hms.harvard.edu/2001/Jan12_2001/pathology1.html |title= Molecular Bureaucrat Tied to Replication Approval Process |author= Humphries C |date= 2001-01-12 |work= Focus |publisher= Harvard Medical, Dental, and Public Health Schools |archive-url= https://web.archive.org/web/20070831032723/http://focus.hms.harvard.edu/2001/Jan12_2001/pathology1.html |archive-date= 2007-08-31 |access-date= 2008-10-03 |url-status= dead }}
• [http://www.pdbe.org/emsearch/mcm%20OR%20minichromosom*3D macromolecular structures of MCM at the EM Data Bank(EMDB)]

{{DNA replication}}

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Category:DNA replication