Interaction of human minichromosome maintenance protein-binding protein with minichromosome maintenance 2-7

The MCM2-7 Complex: Roles beyond DNA Unwinding

The MCM2-7 complex is a hexameric protein complex that serves as a DNA helicase. It unwinds the DNA double helix during DNA replication, thereby providing the single-stranded replication template. In recent years, it has become clear that the MCM2-7 complex has additional functions that extend well beyond its role in DNA replication. Through physical and functional interactions with different pathways, it impacts other nuclear events and activities, including folding of the genome, histone inheritance, chromosome segregation, DNA damage sensing and repair, and gene transcription. Collectively, the diverse roles of the MCM2-7 complex suggest it plays a critical role in maintaining genome integrity by integrating the regulation of DNA replication with other pathways in the nucleus.

Rejuvenating Aged Bone Repair through Multihierarchy Reactive Oxygen Species-Regulated Hydrogel

Aging exacerbates the dysfunction of tissue regeneration at multiple levels and gradually diminishes individual's capacity to withstand stress, damage, and disease. The excessive accumulation of reactive oxygen species (ROS) is considered a hallmark feature of senescent stem cells, which causes oxidative stress, deteriorates the host microenvironment, and eventually becomes a critical obstacle for aged bone defect repair. Till now, the strategies cannot synchronously and thoroughly regulate intracellular and extracellular ROS in senescent cells. Herein, a multihierarchy ROS scavenging system for aged bone regeneration is developed by fabricating an injectable PEGylated poly(glycerol sebacate) (PEGS-NH2 )/poly(γ-glutamic acid) (γ-PGA) hydrogel containing rapamycin-loaded poly(diselenide-carbonate) nanomicelles (PSeR). This PSeR hydrogel exhibits highly sensitive ROS responsiveness to the local aged microenvironment and dynamically releases drug-loaded nanomicelles to scavenge the intracellular ROS accumulated in senescent bone mesenchymal stem cells. The PSeR hydrogel effectively tunes the antioxidant function and delays senescence of bone mesenchymal stem cells by safeguarding DNA replication in an oxidative environment, thereby promoting the self-renewal ability and enhancing the osteogenic capacity for aged bone repair in vitro and in vivo. Thus, this multihierarchy ROS-regulated hydrogel provides a new strategy for treating degenerative diseases.

The assembly of the MCM2-7 hetero-hexamer and its significance in DNA replication

The mini-chromosome maintenance proteins 2-7 (MCM2-7) hexamer is a protein complex that is key for eukaryotic DNA replication, which occurs only once per cell cycle. To achieve DNA replication, eukaryotic cells developed multiple mechanisms that control the timing of the loading of the hexamer onto chromatin and its activation as the replicative helicase. MCM2-7 is highly abundant in proliferating cells, which confers resistance to replication stress. Thus, the presence of an excess of MCM2-7 is important for maintaining genome integrity. However, the mechanism via which high MCM2-7 levels are achieved, other than the transcriptional upregulation of the MCM genes in the G1 phase, remained unknown. Recently, we and others reported that the MCM-binding protein (MCMBP) plays a role in the maintenance of high MCM2-7 levels and hypothesized that MCMBP functions as a chaperone in the assembly of the MCM2-7 hexamer. In this review, we discuss the roles of MCMBP in the control of MCM proteins and propose a model of the assembly of the MCM2-7 hexamer. Furthermore, we discuss a potential mechanism of the licensing checkpoint, which arrests the cells in the G1 phase when the levels of chromatin-bound MCM2-7 are reduced, and the possibility of targeting MCMBP as a chemotherapy for cancer.

MCMBP promotes the assembly of the MCM2-7 hetero-hexamer to ensure robust DNA replication in human cells

The MCM2–7 hetero-hexamer is the replicative DNA helicase that plays a central role in eukaryotic DNA replication. In proliferating cells, the expression level of the MCM2–7 hexamer is kept high, which safeguards the integrity of the genome. However, how the MCM2–7 hexamer is assembled in living cells remains unknown. Here, we revealed that the MCM-binding protein (MCMBP) plays a critical role in the assembly of this hexamer in human cells. MCMBP associates with MCM3 which is essential for maintaining the level of the MCM2–7 hexamer. Acute depletion of MCMBP demonstrated that it contributes to MCM2–7 assembly using nascent MCM3. Cells depleted of MCMBP gradually ceased to proliferate because of reduced replication licensing. Under this condition, p53-positive cells exhibited arrest in the G1 phase, whereas p53-null cells entered the S phase and lost their viability because of the accumulation of DNA damage, suggesting that MCMBP is a potential target for killing p53-deficient cancers.

Equilibrium between nascent and parental MCM proteins protects replicating genomes

Minichromosome maintenance proteins (MCMs) are DNA-dependent ATPases that bind to replication origins and license them to support a single round of DNA replication. A large excess of MCM2-7 assembles on chromatin in G1 phase as pre-replication complexes (pre-RCs), of which only a fraction become the productive CDC45-MCM-GINS (CMG) helicases that are required for genome duplication^1-4. It remains unclear why cells generate this surplus of MCMs, how they manage to sustain it across multiple generations, and why even a mild reduction in the MCM pool compromises the integrity of replicating genomes^5,6. Here we show that, for daughter cells to sustain error-free DNA replication, their mother cells build up a nuclear pool of MCMs both by recycling chromatin-bound (parental) MCMs and by synthesizing new (nascent) MCMs. Although all MCMs can form pre-RCs, it is the parental pool that is inherently stable and preferentially matures into CMGs. By contrast, nascent MCM3-7 (but not MCM2) undergo rapid proteolysis in the cytoplasm, and their stabilization and nuclear translocation require interaction with minichromosome-maintenance complex-binding protein (MCMBP), a distant MCM paralogue^7,8. By chaperoning nascent MCMs, MCMBP safeguards replicating genomes by increasing chromatin coverage with pre-RCs that do not participate on replication origins but adjust the pace of replisome movement to minimize errors during DNA replication. Consequently, although the paucity of pre-RCs in MCMBP-deficient cells does not alter DNA synthesis overall, it increases the speed and asymmetry of individual replisomes, which leads to DNA damage. The surplus of MCMs therefore increases the robustness of genome duplication by restraining the speed at which eukaryotic cells replicate their DNA. Alterations in physiological fork speed might thus explain why even a minor reduction in MCM levels destabilizes the genome and predisposes to increased incidence of tumour formation.

Function of the amino-terminal region of human MCM4 in helicase activity

The amino-terminal region of eukaryotic MCM4 is characteristic of the presence of a number of phosphorylation sites for CDK and DDK, suggesting that the region plays regulatory roles in the MCM2-7 helicase function. However, the roles are not fully understood. We analyzed the role of the amino-terminal region of human MCM4 by using MCM4/6/7 helicase as a model for MCM2-7 helicase. First we found that deletion of 35 amino acids at the amino-terminal end resulted in inhibition of DNA helicase activity of the MCM4/6/7 complex. Conversion of arginine at amino acid no. 10 and 11 to alanine had similar effect to the deletion mutant of Δ1-35, suggesting that these arginine play a role in the DNA helicase activity. The data suggest that expression of these mutant MCM4 in HeLa cells perturbed the progression of the S phase. Substitution of six CDK phosphorylation sites (3, 7, 19, 32, 54 and 110) in the amino-terminal region by phospho-mimetic glutamic acids affected the hexamer formation of the MCM4/6/7 complex. MCM4 phosphorylation by CDK may play a role in DNA replication licensing system, and the present results suggest that the phosphorylation interferes MCM function by lowering stability of MCM complex.

Regulation of MCM2-7 function

Recently published structural and functional analyses of the CMG complex have provided insight into the mechanism of its DNA helicase function and into the distinct roles of its central six component proteins MCM2-MCM7 (MCM2-7). To activate CMG helicase, the two protein kinases CDK and DDK, as well as MCM10, are required. In addition to the initiation of DNA replication, MCM function must be regulated at the DNA replication steps of elongation and termination. Polyubiquitylation of MCM7 is involved in terminating MCM function. Reinitiation of DNA replication in a single cell cycle, which is prevented mainly by CDK, is understood at the molecular level. MCM2-7 gene expression is regulated during cellular aging and the cell cycle, and the expression depends on oxygen concentration. These regulatory processes have been described recently. Genomic structural alteration, which is an essential element in cancer progression, is mainly generated by disruptions of DNA replication fork structures. A point mutation in MCM4 that disturbs MCM2-7 function results in genomic instability, leading to the generation of cancer cells. In this review, I focus on the following points: 1) function of the MCM2-7 complex, 2) activation of MCM2-7 helicase, 3) regulation of MCM2-7 function, 4) MCM2-7 expression, and 5) the role of MCM mutation in cancer progression.

Global transcriptome analysis and characterization of Dryopteris fragrans (L.) Schott sporangium in different developmental stages

BACKGROUND

Dryopteris fragrans (D. fragrans) is a potential medicinal fern distributed in volcanic magmatic rock areas under tough environmental condition. Sporangia are important organs for fern reproduction. This study was designed to characterize the transcriptome characteristics of the wild D. fragrans sporangia in three stages (stage A, B, and C) with the aim of uncovering its molecular mechanism of growth and development.

RESULTS

Using a HiSeq 4000, 79.81 Gb clean data (each sample is at least 7.95 GB) were obtained from nine samples, with three being supplied from each period, and assembled into 94,705 Unigenes, among which 44,006 Unigenes were annotated against public protein databases (NR, Swiss-Prot, KEGG, COG, KOG, GO, eggNOG and Pfam). Furthermore, we observed 7126 differentially expressed genes (DEG) (Fold Change > 4, FDR < 0.001), 349,885 SNP loci, and 10,584 SSRs. DEGs involved in DNA replication and homologous recombination were strongly expressed in stage A, and several DEGs involved in cutin, suberin and wax biosynthesis had undergone dramatic changes during development, which was consistent with morphological observations. DEGs responsible for secondary metabolism and plant hormone signal transduction changed clearly in the last two stages. DEGs homologous to those known genes associated with the development of reproductive organs of flowering plants have also been validated and discussed, such as AGL61, AGL62, ONAC010. In particular, a Unigene encoding TFL1, an important flower-development regulator in flowering plants, was identified and exhibited the highest expression level in stage B in D. fragrans sporangia.

CONCLUSIONS

This study is the first report on global transcriptome analysis in the development of sporangia of wild D. fragrans. DEGs related to development and homologous to flower-seed development in flowering plants were discussed. All DEGs involved in DNA replication and homologous recombination were consistent with morphological observations of paraffin slices. The results of this study provide rare resources for further investigation of the D. fragrans sporangium development, stress resistance and secondary metabolism.

Effect of daidzein and equol on DNA replication in MCF-7 cells

It has been reported that daidzein and equol stimulate DNA replication and proliferation of MCF-7 cells. However, their molecular mechanisms of action are still unclear. We examined the effects of daidzein and equol on DNA replication of MCF-7 cells, focusing on MCM2-7 proteins, which function as the replicative helicase. In the presence of either 1 μM of daidzein or equol, the number of cells in S-phase, which was determined by detecting bromodeoxyuridine incorporated into replicated DNA, almost doubled. The total amounts of MCM7 protein and chromatin-bound MCM7 protein increased in the presence of daidzein. The data suggest that phytoestrogens facilitate cell cycle progression in G1-phase by increasing the level of MCM proteins. In the presence of phytoestrogens, phosphorylation of Rb and levels of MCM2, 3 and 7 mRNA increased, suggesting that stimulation of MCM2-7 transcription is involved in the cell cycle progression. Under the same conditions, double-stranded DNA breakage in logarithmically growing MCF-7 cells, which was detected using anti-γ-H2AX antibodies, did not increase in the presence of equol.

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.

G364R mutation of MCM4 detected in human skin cancer cells affects DNA helicase activity of MCM4/6/7 complex

A number of gene mutations are detected in cells derived from human cancer tissues, but roles of these mutations in cancer cell development are largely unknown. We examined G364R mutation of MCM4 detected in human skin cancer cells. Formation of MCM4/6/7 complex is not affected by the mutation. Consistent with this notion, the binding to MCM6 is comparable between the mutant MCM4 and wild-type MCM4. Nuclear localization of this mutant MCM4 expressed in HeLa cells supports this conclusion. Purified MCM4/6/7 complex containing the G364R MCM4 exhibited similar levels of single-stranded DNA binding and ATPase activities to the complex containing wild-type MCM4. However, the mutant complex showed only 30-50% of DNA helicase activity of the wild-type complex. When G364R MCM4 was expressed in HeLa cells, it was fractionated into nuclease-sensitive chromatin fraction, similar to wild-type MCM4. These results suggest that this mutation does not affect assembly of MCM2-7 complex on replication origins but it interferes some step at function of MCM2-7 helicase. Thus, this mutation may contribute to cancer cell development by disturbing DNA replication.