Biomedical subjects

H Masai

Publications and source records attributed to H Masai.

At least 19 recordsLinked to original sources

PubMed2001

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.

Amino Acid Sequence↗

PubMed2001

Bipartite binding of a kinase activator activates Cdc7-related kinase essential for S phase.

Dfp1/Him1 protein of fission yeast, Schizosaccharomyces pombe, encodes the regulatory subunit for Hsk1 kinase, a homologue of budding yeast Cdc7 kinase essential for initiation and progression of the S phase of the cell cycle. This protein binds and activates Hsk1 kinase, which phosphorylates the MCM2 protein. Comparison of the amino acid sequences of the Cdc7 regulatory subunits from various eukaryotes revealed the presence of three small stretches of conserved amino acid sequences, namely Dbf4 motifs N, M, and C. We report here that the Dbf4 motif M, a unique proline-rich motif, and the Dbf4 motif C, a C(2)H(2)-type zinc finger motif, are essential for mitotic functions of Dfp1/Him1 protein as well as for full-level activation of Hsk1 kinase. In vitro, a small segment containing the Dbf4 motif M or C alone binds to and partially activates Hsk1. Co-expression of these two segments augments the extent of activation. Furthermore, a fused polypeptide containing only Dbf4 motifs M and C without any spacer can activate Hsk1 and is capable of rescuing the growth defect of him1 null cells. Insertion of a long stretch of amino acids between the motif M and motif C can be tolerated for mitotic functions. On the other hand, internal deletion of Dbf4 motif N, which has some similarity with the BRCA C-terminal domain motif, results in a defect in hydroxyurea-induced checkpoint responses and sensitivity to methyl methane sulfonate, yet mitotic functions and kinase activation are intact. In one-hybrid assays with budding yeast Dbf4, motif N mutants exhibit reduced interaction with a replication origin. Our observations suggest the molecular architecture of Cdc7.Dbf4-related kinase complexes at the origins, in which they are tethered to replication machinery through Dbf4 motif N and the catalytic subunits are activated through bipartite binding of Dbf4 motifs M and C of the regulatory subunits.

Amino Acid Motifs↗

PubMed2001

Essential role of Sna41/Cdc45 in loading of DNA polymerase alpha onto minichromosome maintenance proteins in fission yeast.

Assembly of replication complexes at the replication origins is strictly regulated. Cdc45p is known to be a part of the active replication complexes. In Xenopus egg extracts, Cdc45p was shown to be required for loading of DNA polymerase alpha onto chromatin. The fission yeast cdc45 homologue was identified as a suppressor for nda4 and named sna41. Nevertheless, it is not known how Cdc45p facilitates loading of DNA polymerase alpha onto chromatin, particularly to prereplicative complexes. To gain novel insight into the function of this protein in fission yeast, we characterized the fission yeast Cdc45 homologue, Sna41p. We have constructed C-terminally epitope-tagged Sna41p and Pol alpha p and replaced the endogenous genes with the corresponding tagged genes. Analyses of protein-protein interactions in vivo by the use of these tagged strains revealed the following: Sna41p interacts with Pol alpha p throughout the cell cycle, whereas it interacts with Mis5p/Mcm6p in the chromatin fractions at the G(1)-S boundary through S phase. In an initiation-defective sna41 mutant, sna41(goa1), interaction of Pol alpha p with Mis5p is not observed, although Pol alpha p loading onto the chromatin that occurs before G(1) START is not affected. These results show that fission yeast Sna41p facilitates the loading of Pol alpha p onto minichromosome maintenance proteins. Our results are consistent with a model in which loading of Pol alpha p onto replication origins occurs through two steps, namely, loading onto chromatin at preSTART and association with prereplicative complexes at G(1)-S through Sna41p, which interacts with minichromosome maintenance proteins in a cell cycle-dependent manner.

Carrier Proteins↗

PubMed2001

Sna41goa1, a novel mutation causing G1/S arrest in fission yeast, is defective in a CDC45 homolog and interacts genetically with polalpha.

Proteins involved in the initiation of DNA replication play critical roles in the assembly and loading of replication complexes at replication origins. To gain further insight into the regulation of initiation, we screened in fission yeast for temperature-sensitive mutants which arrested at the G1/S boundary, and isolated nine mutants which arrested with a 1C DNA content at 36 degrees C. By linkage analysis, two complementation groups were identified which were not allelic to known G1 arrest mutations. One of the mutants isolated, sna41goul, arrested with a G1 DNA content and expressed a pleiomorphic phenotype, i.e., a mixture of cut and cdc phenotypes, at 36 degrees C. The point of arrest was identified as after START but before the hydroxyurea-induced block, by taking advantage of the mutant rad26.a14, which has a defect in an early S phase-specific checkpoint, and by performing reciprocal shift experiments. sna41 goal is allelic to sna41+, which is homologous to the CDC45 gene of budding yeast, and the mutation lies in a motif that is highly conserved in Cdc45-related proteins. The temperature sensitivity of the sna41goal mutant can be suppressed to some extent by ts mutations in polalpha. Our genetic results are consistent with a model in which Cdc45 plays crucial roles in the assembly of the replication apparatus at replication origins.

Carrier Proteins↗

PubMed2001

Regulation of initiation of S phase, replication checkpoint signaling, and maintenance of mitotic chromosome structures during S phase by Hsk1 kinase in the fission yeast.

Hsk1, Saccharomyces cerevisiae Cdc7-related kinase in Shizosaccharomyces pombe, is required for G1/S transition and its kinase activity is controlled by the regulatory subunit Dfp1/Him1. Analyses of a newly isolated temperature-sensitive mutant, hsk1-89, reveal that Hsk1 plays crucial roles in DNA replication checkpoint signaling and maintenance of proper chromatin structures during mitotic S phase through regulating the functions of Rad3 (ATM)-Cds1 and Rad21 (cohesin), respectively, in addition to expected essential roles for initiation of mitotic DNA replication through phosphorylating Cdc19 (Mcm2). Checkpoint defect in hsk1-89 is indicated by accumulation of cut cells at 30 degrees C. hsk1-89 displays synthetic lethality in combination with rad3 deletion, indicating that survival of hsk1-89 depends on Rad3-dependent checkpoint pathway. Cds1 kinase activation, which normally occurs in response to early S phase arrest by nucleotide deprivation, is largely impaired in hsk1-89. Furthermore, Cds1-dependent hyperphosphorylation of Dfp1 in response to hydroxyurea arrest is eliminated in hsk1-89, suggesting that sufficient activation of Hsk1-Dfp1 kinase is required for S phase entry and replication checkpoint signaling. hsk1-89 displays apparent defect in mitosis at 37 degrees C leading to accumulation of cells with near 2C DNA content and with aberrant nuclear structures. These phenotypes are similar to those of rad21-K1 and are significantly enhanced in a hsk1-89 rad21-K1 double mutant. Consistent with essential roles of Rad21 as a component for the cohesin complex, sister chromatid cohesion is partially impaired in hsk1-89, suggesting a possibility that infrequent origin firing of the mutant may affect the cohesin functions during S phase.

Cell Cycle Proteins↗

PubMed2000

Human Cdc7-related kinase complex. In vitro phosphorylation of MCM by concerted actions of Cdks and Cdc7 and that of a criticial threonine residue of Cdc7 bY Cdks.

huCdc7 encodes a catalytic subunit for Saccharomyces cerevisae Cdc7-related kinase complex of human. ASK, whose expression is cell cycle-regulated, binds and activates huCdc7 kinase in a cell cycle-dependent manner (Kumagai, H., Sato, N., Yamada, M., Mahony, D. , Seghezzi, W., Lees, E., Arai, K., and Masai, H. (1999) Mol. Cell. Biol. 19, 5083-5095). We have expressed huCdc7 complexed with ASK regulatory subunit using the insect cell expression system. To facilitate purification of the kinase complex, glutathione S-transferase (GST) was fused to huCdc7 and GST-huCdc7-ASK complex was purified. GST-huCdc7 protein is inert as a kinase on its own, and phosphorylation absolutely depends on the presence of the ASK subunit. It autophosphorylates both subunits in vitro and phosphorylates a number of replication proteins to different extents. Among them, MCM2 protein, either in a free form or in a MCM2-4-6-7 complex, serves as an excellent substrate for huCdc7-ASK kinase complex in vitro. MCM4 and MCM6 are also phosphorylated by huCdc7 albeit to less extent. MCM2 and -4 in the MCM2-4-6-7 complex are phosphorylated by Cdks as well, and prior phosphorylation of the MCM2-4-6-7 complex by Cdks facilitates phosphorylation of MCM2 by huCdc7, suggesting collaboration between Cdks and Cdc7 in phosphorylation of MCM for initiation of S phase. huCdc7 and ASK proteins can also be phosphorylated by Cdks in vitro. Among four possible Cdk phosphorylation sites of huCdc7, replacement of Thr-376, corresponding to the activating threonine of Cdk, with alanine (T376A mutant) dramatically reduces kinase activity, indicative of kinase activation by phosphorylation of this residue. In vitro, Cdk2-Cyclin E, Cdk2-Cyclin A, and Cdc2-Cyclin B, but not Cdk4-Cyclin D1, phosphorylates the Thr-376 residue of huCdc7, suggesting possible regulation of huCdc7 by Cdks.

Antigens, Viral, Tumor↗

PubMed2000

Dbf4 motifs: conserved motifs in activation subunits for Cdc7 kinases essential for S-phase.

Dbf4 and its related molecules were originally identified as cyclin-like partners for Cdc7 kinases, essential for S-phase. Recent reports and database search indicate the presence of multiple Dbf4-related molecules with distinct functions. We have identified three stretches of amino acids which are conserved in various Dbf4-related molecules and possibly play distinct functions in binding to and activation of the catalytic subunits as well as in interactions with other proteins. Discovery of conserved motifs for this possible new protein family would serve as a useful framework for future identification of new members of this family as well as for probing their functions.

Amino Acid Motifs↗

PubMed2000

A Cdc7p-Dbf4p protein kinase activity is conserved from yeast to humans.

DBF4 and CDC7 were identified as budding yeast cell cycle mutants that arrest immediately before S phase. The Dbf4p and Cdc7p proteins interact to form a protein kinase, Cdc7p being the catalytic subunit and Dbf4p is a cyclin-like molecule that activates the kinase in late G1. Dbf4p also targets Cdc7p to origins of replication where likely substrates include the Mcm proteins. Dbf4p and Cdc7p related proteins occur in the fission yeast and in metazoans. These also phosphorylate Mcm proteins and preliminary evidence indicates a similar function to Dbf4p/Cdc7p in budding yeast. The Dbf4p/Cdc7p activity will therefore very likely be conserved in all eukaryotes.

Cell Cycle↗

PubMed2000

Regulation of DNA replication during the cell cycle: roles of Cdc7 kinase and coupling of replication, recombination, and repair in response to replication fork arrest.

DNA replication is central to cell growth, development, and generation of tissues and organs. Recent advances in understanding replication machinery have revealed striking conservation of components involved in the processes of DNA replication, from yeasts to human. The conservation extends even to bacteria for some basic components of replication apparatus. Eukaryotic DNA replication is regulated at various stages to ensure strict regulation during cell cycle. We have identified a novel mammalian kinase, Cdc7-ASK (Activator of S phase Kinase), that plays a key role at the entry into S phase as a molecular switch for DNA replication. This kinase is specifically activated during S phase and triggers the firing of DNA replication by phosphorylating an essential DNA helicase component of the replication complex. Environmental stresses such as DNA damages or depletion of essential nutrients for DNA synthesis lead to unscheduled arrest of DNA replication forks. In bacteria, this leads to induction of altered modes of DNA replication, which may repair DNA damages, facilitate reassembly of replication machinery at the stalled replication fork, or do both. In eukaryotes, blocking replication forks usually induces both checkpoint responses, which prevent premature progression of cell cycle events before precise completion of the preceding cell cycle stage, and the recombinational repair system for the lesions. Possible common bases in recognition of stalled replication forks in bacteria and eukaryotes will be discussed. Furthermore, we will discuss the potential of replication and checkpoint proteins as targets of anticancer agents as well as possible novel technology for stem cell amplification through manipulation of DNA replication.

Animals↗

PubMed2000

Use of electroporation to accelerate the skin permeability enhancing action of oleic acid.

Rat skin permeability after treatment by electroporation (newly developed frog type electrode, 100V, 10 pulses), oleic acid/propylene glycol (PG) and a combination of both were investigated using Fourier transformed infrared attenuated total reflectance (FT-IR/ATR) analysis. Electroporation immediately disordered the stratum corneum lipid structure up to a certain threshold level. This action lasted throughout the experiment. This may be attributed to the formation of long lifetime of metastable lipid structures, which may allow molecules to pass to the inside of the stratum corneum due to the electroporation-induced fluidized lipid membranes. Electroporation also altered the protein structure of the stratum corneum. When electroporation was combined with 0.05 M oleic acid/PG, uptake of oleic acid and PG into the stratum corneum was remarkably accelerated compared to the application of only 0.05 M oleic acid/PG to the skin. This indicates that electroporation enables oleic acid and PG to penetrate the stratum corneum easily by disrupting the structure of the latter. PG transfer into the dermis from the epidermis was accelerated, not because of the direct action of electroporation on the dermis, but because electroporation induced the rapidly disordering action of oleic acid on the stratum corneum. Lipid-soluble indomethacin permeated the skin more rapidly when the skin was treated with electroporation plus oleic acid/PG than with 0.05 M oleic acid/PG in vitro.

Animals↗

PubMed2000

Prolapsing gyrus rectus as a cause of progressive optic neuropathy.

The pathogenesis of optic neuropathy caused by neurovascular compression or by similar mechanisms is unclear. Thin-slice magnetic resonance (MR) imaging was performed in 69 patients with optic neuropathy without demonstrable ophthalmological lesions (57.0 +/- 17.1 years of age) and 102 normal subjects (57.7 +/- 13.9 years of age). The MR imaging features were classified into "no compression" by the internal carotid artery (ICA), "compression" by the ICA, "no contact" with the anterior cerebral artery (ACA) or the gyrus rectus, "contact" with either or both, "compression" by the ACA, and "compression" by the gyrus rectus. The Spearman correlation coefficients were calculated between patients or controls, the MR classification, and the age, and the number of patients in each MR classification were evaluated by the chi 2 test. Five of the 69 patients with rapidly progressive symptoms were operated on via the frontotemporal approach. The MR imaging feature of "compression" by the gyrus rectus was the best predictor of optic neuropathy (Spearman correlation coefficients rho = -0.23646, p < 0.0018). This MR imaging feature was observed in 38 of 69 patients and in 32 of 102 controls (p = 0.002). Compression of the nerve by the gyrus rectus or the ACA was confirmed in all five operated cases. Decompression of the nerve was fully achieved in four of the five patients, and their symptoms have not progressed since then. Optic neuropathies due to compression by the prolapsing gyrus rectus are not well understood. Such neuropathies may be detected by MR imaging.

Adolescent↗

PubMed1999

CDC7 kinase complex as a molecular switch for DNA replication.

Cdc7 kinase and its activator Dbf4 protein, originally identified in budding yeast Saccharomyces cerevisiae, are widely conserved in eukaryotes including fission yeast and human. Dbf4-related activators bind and stimulate kinase activity of Cdc7-like catalytic subunit. Its kinase activity is cell cycle-regulated, mainly through availability of the activation subunit whose level increases at G1/S boundary and is maintained at a high level throughout S phase. MCM2 protein is among physiologically important substrates. Genetic studies in fission yeast indicate that Cdc7-related kinase complex also functions in meiosis, uninduced mutagenesis, DNA replication checkpoint signaling and maintenance of chromatin structures during S phase.

Animals↗

PubMed1999

First the CDKs, now the DDKs.

In budding yeast, Dbf4p and Cdc7p control initiation of DNA synthesis. They form a protein kinase - Cdc7p being the catalytic subunit and Dbf4p a cyclin-like molecule that activates the kinase in late G1 phase. Dbf4p also targets Cdc7p to origins of replication, where probable substrates include certain Mcm proteins. Recent studies have identified Dbf4p- and Cdc7p-related proteins in fission yeast and metazoans. These homologues also phosphorylate Mcm proteins and could have a similar function to that of Dbf4p-Cdc7p in budding yeast. Thus, it seems likely that, like the cyclin-dependent kinases (CDKs), the Dbf4p-Cdc7p activity is conserved in all eukaryotes.

Animals↗

PubMed1999

A novel growth- and cell cycle-regulated protein, ASK, activates human Cdc7-related kinase and is essential for G1/S transition in mammalian cells.

A novel human protein, ASK (activator of S phase kinase), was identified on the basis of its ability to bind to human Cdc7-related kinase (huCdc7). ASK forms an active kinase complex with huCdc7 that is capable of phosphorylating MCM2 protein. ASK appears to be the major activator of huCdc7, since immunodepletion of ASK protein from the extract is accompanied by the loss of huCdc7-dependent kinase activity. Expression of ASK is regulated by growth factor stimulation, and levels oscillate through the cell cycle, reaching a peak during S phase. Concomitantly, the huCdc7-dependent kinase activity significantly increases when cells are in S phase. Furthermore, we have demonstrated that ASK serves an essential function for entry into S phase by showing that microinjection of ASK-specific antibodies into mammalian cells inhibited DNA replication. Our data show that ASK is a novel cyclin-like regulatory subunit of the huCdc7 kinase complex and that it plays a pivotal role in G1/S transition in mammalian cells.

Amino Acid Sequence↗

PubMed1999

A fission yeast gene, him1(+)/dfp1(+), encoding a regulatory subunit for Hsk1 kinase, plays essential roles in S-phase initiation as well as in S-phase checkpoint control and recovery from DNA damage.

Saccharomyces cerevisiae CDC7 encodes a serine/threonine kinase required for G(1)/S transition, and its related kinases are present in fission yeast as well as in higher eukaryotes, including humans. Kinase activity of Cdc7 protein depends on the regulatory subunit, Dbf4, which also interacts with replication origins. We have identified him1(+) from two-hybrid screening with Hsk1, a fission yeast homologue of Cdc7 kinase, and showed that it encodes a regulatory subunit of Hsk1. Him1, identical to Dfp1, previously identified as an associated molecule of Hsk1, binds to Hsk1 and stimulates its kinase activity, which phosphorylates both catalytic and regulatory subunits as well as recombinant MCM2 protein in vitro. him1(+) is essential for DNA replication in fission yeast cells, and its transcription is cell cycle regulated, increasing at middle M to late G(1). The protein level is low at START in G(1), increases at the G(1)/S boundary, and is maintained at a high level throughout S phase. Him1 protein is hyperphosphorylated at G(1)/S through S during the cell cycle as well as in response to early S-phase arrest induced by nucleotide deprivation. Deletion of one of the motifs conserved in regulatory subunits for Cdc7-related kinases as well as alanine substitution of three serine and threonine residues present in the same motif resulted in a defect in checkpoint regulation normally induced by hydroxyurea treatment. The alanine mutant also showed growth retardation after UV irradiation and the addition of methylmethane sulfonate. In keeping with this result, a database search indicates that him1(+) is identical to rad35(+). Our results reveal a novel function of the Cdc7/Dbf4-related kinase complex in S-phase checkpoint control as well as in growth recovery from DNA damage in addition to its predicted essential function in S-phase initiation.

Amino Acid Sequence↗

PubMed1998

Growth regulation of the expression of mouse cDNA and gene encoding a serine/threonine kinase related to Saccharomyces cerevisiae CDC7 essential for G1/S transition. Structure, chromosomal localization, and expression of mouse gene for s. cerevisiae Cdc7-related kinase.

Saccharomyces cerevisiae CDC7 encodes a serine/threonine kinase required for G1/S transition of the yeast cells. We previously reported human and Xenopus cDNAs encoding CDC7-related kinases and suggested the possibility that chromosomal replication of higher eukaryotes may be regulated through conserved mechanisms involving Cdc7-related kinases. Here we report a murine cDNA and gene (muCdc7) encoding a serine/threonine kinase related to CDC7. The predicted coding frame for the longest cDNA for muCdc7 consists of 564 amino acids, which shares 46, 77, and 93% identity, respectively, with those of budding yeast, Xenopus, and human in kinase conserved domains. The chromosomal gene for muCdc7, located at the band 5E5 on the mouse chromosome 5, consists of 12 exons, and its exon/intron organization shares some similarity with that of other protein kinases including Cdk and cAMP-dependent kinase. Transcription of muCdc7, initiated at multiple sites over the 370-base pair promoter region, is repressed in the resting state and is induced at the G1/S boundary after growth factor stimulation in a growth factor-dependent cell line. Transient transfection assays indicated that a 231-base pair segment of the muCdc7 promoter containing three putative E2F binding sites and one Sp1 site but lacking TATA sequence is sufficient for response to growth stimulation.

Amino Acid Sequence↗

PubMed1998

Inhibition of NMDA receptors and nitric oxide synthase reduces ischemic injury of the retina.

This study was performed to examine the roles of body temperature, NMDA receptors and nitric oxide (NO) synthase in post-ischemic retinal injury in rats. Cell loss in the ganglion cell layer and thinning of the inner plexiform layer were observed 7 days after ischemia. Cell loss in the ganglion cell layer but not thinning of the inner plexiform layer was reduced by hypothermia during ischemia. Intravenous injection of dizocilpine (MK-801) or Nomega-nitro-L-arginine methyl ester (L-NAME) prior to ischemia ameliorated retinal injury. These results suggest that activation of NO synthase following NMDA receptor stimulation is involved in ischemia-induced retinal injury.

Animals↗

PubMed1998

Mechanism of the pathogenesis of glutamate neurotoxicity in retinal ischemia.

PURPOSE: This study was carried out to examine the involvement of glutamate and nitric oxide neurotoxicity in ischemia/reperfusion-induced retinal injury in vivo. METHODS: We monitored glutamate release from in vivo cat retina during and after pressure-induced ischemia using a microdialysis technique. Morphometric studies were performed to study the effects of MK-801 (dizocilpine), L-NAME (N omega-nitro-L-arginine methyl ester), and D-NAME (N omega-nitro-D-arginine methyl ester) on the histological changes in the rat retina induced by ischemia or intravitreal injection of NMDA (N-methyl-D-aspartate; 200 nmol). RESULTS: A large release of glutamate occurred during ischemia, followed by a marked release after reperfusion. Histological changes occurred selectively in the inner part of the retina after ischemia as well as intravitreal injection of NMDA. Pretreatment with intravenous injection of MK-801 or L-NAME significantly inhibited the ischemic injury of the inner retina. Intravitreal injection of L-NAME inhibited NMDA-induced neurotoxicity in the retina. CONCLUSION: These findings indicate that nitric oxide mediates neurotoxic actions of glutamate which are responsible for ischemic injury in the retina.

Animals↗