Search PubMed⌕ Search

Biomedical subjects

I Yahara

Publications and source records attributed to I Yahara.

At least 37 records · Page 2Linked to original sources
PubMed1998

A novel 66-kDa stress protein, p66, associated with the process of cyst formation of Physarum polycephalum is a Physarum homologue of a yeast actin-interacting protein, AIP1.

When exposed to various stresses including heat shock, myxoamoebae, growing haploid cells of Physarum polycephalum, show marked morphological changes and consequently become disk-shaped microcysts. We have found that p66 is induced exclusively in the course of microcyst formation and has an actin-binding activity. In this study, we purified p66 to homogeneity and isolated a p66 cDNA. The deduced protein sequence contained 601 amino acids and showed 31% identity to a yeast actin-interacting protein, AIP1. Northern blot analysis revealed that the amount of p66 mRNA was significantly increased by heat shock in myxoamoebae but not in plasmodia. Thus, p66 seems to be a developmentally-expressed stress protein which regulates the rearrangement of actin organization during microcyst formation in P. polycephalum.

Actins↗
PubMed1997

Phosphorylation of the immunosuppressant FK506-binding protein FKBP52 by casein kinase II: regulation of HSP90-binding activity of FKBP52.

FKBP52 (HSP56, p59, HBI) is the 59-kDa immunosuppressant FK506-binding protein and has peptidyl prolyl isomerase as well as a chaperone-like activity in vitro. FKBP52 associates with the heat shock protein HSP90 and is included in the steroid hormone receptor complexes in vivo. FKBP52 possesses a well conserved phosphorylation site for casein kinase II (CK2) that was previously shown to be associated with HSP90. Here we examined whether FKBP52 is phosphorylated by CK2 both in vivo and in vitro. Recombinant rabbit FKBP52 was phosphorylated by purified CK2. We expressed and purified deletion mutants of FKBP52 to determine the site(s) phosphorylated by CK2. Thr-143 in the hinge I region was identified as the major phosphorylation site for CK2. A synthetic peptide corresponding to this region was phosphorylated by CK2, and the peptide competitively inhibited the phosphorylation of other substrates by CK2. The [32P]phosphate labeling of FKBP52-expressing cells revealed that the same site is also phosphorylated in vivo. FK506 binding to FKBP52 did not affect the phosphorylation by CK2 and, conversely, the FK506-binding activity of FKBP52 was not affected by the phosphorylation. Most importantly, CK2-phosphorylated FKBP52 did not bind to HSP90. These results indicate that CK2 phosphorylates FKBP52 both in vitro and in vivo and thus may regulate the protein composition of chaperone-containing complexes such as those of steroid receptors and certain protein kinases.

Amino Acid Sequence↗
PubMed1997

Cdc37 is a molecular chaperone with specific functions in signal transduction.

Cdc37 is required for cyclin-dependent kinase activation and is genetically linked with the activity of several other kinases, including oncogenic v-Src, casein kinase II, MPS-1 kinase, and sevenless. Strikingly, many pathways involving Cdc37 also involve the protein chaperone Hsp90. The identification of Cdc37 as the 50-kD protein in several Hsp90-kinase complexes, together with other data, led to the recent suggestion that Cdc37 is a kinase-targeting "subunit" of Hsp90. We directly examined the effect of Cdc37 on Hsp90 functions. Rather than simply acting as an accessory factor for Hsp90, Cdc37 is itself a protein chaperone with properties remarkably similar to those of Hsp90. In vitro, Cdc37 maintains denatured beta-galactosidase in an activation-competent state without reactivating it and stabilizes mature, but unstable, casein kinase II. In vivo, Cdc37 overexpression can compensate for decreased Hsp90 function, but the proteins are not interchangeable. Cdc37 can compensate for Hsp90 in maintaining the activity of v-Src kinase but does not maintain the activity of the glucocorticoid receptor. Thus, the very similar chaperone activities of the two proteins, uncovered through in vitro analysis, diverge in vivo in specific signal transduction pathways.

Cell Cycle Proteins↗
PubMed1997

Isolation and partial characterization of cytoplasmic granules of LAK cells from mice showing side effects of recombinant human IL-2.

The side effects of recombinant human interleukin-2 (rhIL-2) in mice are considered to be mediated by lymphokine-activated killer (LAK) cells that have cytoplasmic granules and infiltrate into target organs. In cloned cytotoxic lymphocytes (CLs) including LAK cells in vitro, granule exocytosis is one of the mechanisms in target cell lysis, and hemolytic and BLT-serine esterase (BLT-SE) activities detected in the granules are important to the cytotoxicity of the CLs in vitro. However, no information is available on these activities of LAK granules in vivo in mice showing the side effects of rhIL-2. Therefore, the present study was designed to isolate the LAK granules from the homogenate of LAK cell-rich subcutaneous tissue in mice treated with continuous infusion of a dose of rhIL-2 that induces known toxicity. High activities in hemolysis and BLT-SE were detected in Percoll fractions from the tissue homogenate. Electron microscopic observation of these fractions revealed almost only LAK granules. These results indicate that hemolytic and BLT-SE activities are detectable in the LAK granules in vivo, suggesting that the cells have potency for cytotoxicity via their granules and are involved in the toxicity of rhIL-2 in mice. In addition, perforin, which has been identified as a hemolytic protein in CL granules in vitro, was not detected by immunoblotting in the granule fraction of the LAK cells in vivo, indicating that the hemolytic activity of the LAK granules in vivo is not due to perforin.

Animals↗
PubMed1997

Binding affinity of proteins to hsp90 correlates with both hydrophobicity and positive charges. A surface plasmon resonance study.

The 90 kDa heat shock protein (hsp90) is a major cytoplasmic molecular chaperone associating with numerous other proteins including steroid receptors. Here we provide the first numerical analysis of hsp90-target associations using surface plasmon resonance. Binding affinities of histones, the "native molten globule", casein and calmodulin to hsp90 decrease in the order of Kd = 70 +/- 24, 220 +/- 70 and 1800 +/- 600 nM, respectively. Analysis of the structure of binding proteins revealed that their binding affinity depends on both hydrophobicity and positive charges making the discriminative features of hsp90 similar to those of other molecular chaperones.

Adenosine Triphosphate↗
PubMed1997

Live dynamics of Dictyostelium cofilin suggests a role in remodeling actin latticework into bundles.

Cofilin, an indispensable, actin-regulating protein represents the 'cofilin family' of actin-binding proteins existing in a wide variety of organisms. Our previous and other in vitro studies have implied that cofilin can accelerate transformation of filamentous (F)-actin and (alpha)-actinin latticework into bundles, and overexpression of cofilin induces formation of F-actin bundles in Dictyostelium. Here we expressed an Aequorea green fluorescent protein (GFP)-Dictyostelium cofilin fusion protein in Dictyostelium, and observed the live dynamics to examine the physiological function of cofilin. We show that purified GFP-cofilin binds to actin filaments and decreases the apparent viscosity of actin solution in a similar manner to authentic Dictyostelium cofilin. Expressed GFP-cofilin exhibits normal actin-binding activities in the cytoplasm as represented by incorporation into the actin rods induced with dimethyl sulfoxide. Free moving cells form a crown-like cortical structure on the dorsal surface, and GFP-cofilin exhibits dynamic assembly into actin bundles being formed beneath the cortex. During phagocytosis, GFP-cofilin accumulates into actin bundles formed in the region underlying the phagocytic cups. In cells chemotactically activated with cyclic AMP, GFP-cofilin exhibits a high level of accumulation in projecting leading edges. When the chemo-attraction is experimentally changed, the redistribution of GFP-cofilin towards the new pseudopod occurs in a matter of 30-60 seconds. These results demonstrate that cofilin plays a crucial role in vivo in rapid remodeling of the cortical actin meshwork into bundles.

Actin Depolymerizing Factors↗
PubMed1997

A green fluorescent protein-actin fusion protein dominantly inhibits cytokinesis, cell spreading, and locomotion in Dictyostelium.

We transformed Dictyostelium discoideum cells by a vector for expression of a chimerical fusion protein consisting of Aequorea Victoria green fluorescent protein (GFP) and D. discoideum actin at its amino- and carboxy-terminal, respectively. The amount of expressed GFP-actin was about 3% of total actin molecules in the transformed cells. The expression of GFP-actin in D. discoideum completely inhibited cytokinesis in suspension culture. The expression decreased the rate of random cell locomotion to about a half of that of control cells. The expression also caused the cells to round up. These phenotypic observations suggested that GFP-actin acts as a dominant negative form of actin in the cells. The rounding up by expression of GFP-actin was suppressed by genetical elimination of myosin II heavy chain. This result suggested that myosin II is necessary for the rounding up of GFP-actin expressing cells. GFP-actin constructed cortical actin filament architectures together with intrinsic actin in the cells. Purified GFP-actin polymerized and de-polymerized repetitively according to the solution conditions in vitro. The critical concentration of GFP-actin for polymerization is the same as that of actin. The GFP-actin filaments was able to bind to coverglass surfaces coated with myosin head fragments. However, the GFP-actin filaments did not slide at all on the coverglass by addition of ATP. This indicates that the GFP-actin filaments form rigor complex with myosin II in vitro even in the presence of ATP. The formation of rigor complex may cause the cells to round up.

Actins↗
PubMed1996

Redistribution of phosphatidylethanolamine at the cleavage furrow of dividing cells during cytokinesis.

Ro09-0198 is a tetracyclic polypeptide of 19 amino acids that recognizes strictly the structure of phosphatidylethanolamine (PE) and forms a tight equimolar complex with PE on biological membranes. Using the cyclic peptide coupled with fluorescence-labeled streptavidin, we have analyzed the cell surface localization of PE in dividing Chinese hamster ovary cells. We found that PE was exposed on the cell surface specifically at the cleavage furrow during the late telophase of cytokinesis. PE was exposed on the cell surface only during the late telophase and no alteration in the distribution of the plasma membrane-bound cyclic peptide was observed during the cytokinesis, suggesting that the surface exposure of PE reflects the enhanced scrambling of PE at the cleavage furrow. Furthermore, cell surface immobilization of PE induced by adding the cyclic peptide coupled with streptavidin to prometaphase cells effectively blocked the cytokinesis at late telophase. The peptide-streptavidin complex treatment had no effect on furrowing, rearrangement of microtubules, and nuclear reconstitution, but specifically inhibited both actin filament disassembly at the cleavage furrow and subsequent membrane fusion. These results suggest that the redistribution of the plasma membrane phospholipids is a crucial step for cytokinesis and the cell surface PE may play a pivotal role in mediating a coordinate movement between the contractile ring and plasma membrane to achieve successful cell division.

Animals↗
PubMed1996

Tertiary structure of destrin and structural similarity between two actin-regulating protein families.

Destrin is an isoprotein of cofilin that regulates actin cytoskeleton in various eukaryotes. We determined the tertiary structure of destrin by triple-resonance multidimensional nuclear magnetic resonance. In spite of there being no significant amino acid sequence homology, we found that the folding of destrin was strikingly similar to that of repeated segments in the gelsolin family, which resulted in a new protein fold group. Sequential dissimilarity of the actin-binding helix of destrin to that of gelsolin explains the Ca2+-independent actin-binding of destrin. Possible mechanisms of phosphorylation-sensitive phosphoinositide-competitive actin binding, of pH-dependent filament severing, and of nuclear translocation with actin in response to stresses, are discussed on the basis of the tertiary structure.

Actin Depolymerizing Factors↗
PubMed1996

Heat-induced chaperone activity of HSP90.

The 90-kDa stress protein, HSP90, is a major cytosolic protein ubiquitously distributed in all species. Using two substrate proteins, dihydrofolate reductase (DHFR) and firefly luciferase, we demonstrate here that HSP90 newly acquires a chaperone activity when incubated at temperatures higher than 46 degrees C, which is coupled with self-oligomerization of HSP90. While chemically denatured DHFR refolds spontaneously upon dilution from denaturant, oligomerized HSP90 bound DHFR during the process of refolding and prevented it from renaturation. DHFR was released from the complex with HSP90 by incubating with GroEL/ES complexes in an ATP-dependent manner and refolded into the native form. alpha-Casein inhibited the binding of DHFR to HSP90 and also chased DHFR from the complex with HSP90. These results suggest that HSP90 binds substrates to maintain them in a folding-competent structure. Furthermore, we found that HSP90 prevents luciferase from irreversible thermal denaturation and enables it to refold when postincubated with reticulocyte lysates. This heat-induced chaperone activity of HSP90 associated with its oligomerization may have a pivotal role in protection of cells from thermal damages.

Animals↗
PubMed1996

Phosphorylation of Ser-3 of cofilin regulates its essential function on actin.

BACKGROUND: Cofilin is a low-molecular weight actin-modulating protein, and is structurally and functionally conserved in eucaryotes from yeast to mammals. The functions of cofilin appear to be regulated by phosphorylation and dephosphorylation. RESULTS: A proteolytic study of phosphorylated porcine cofilin and expression of a mutated cofilin in cultured cells revealed that Ser-3 is the unique phosphorylation site. Phosphorylated cofilin was found not to bind to either F- or G-actin while unphosphorylated cofilin binds to both. S3D-cofilin, in which Ser-3 was replaced with Asp, did not bind in vitro to actin while S3A-cofilin did. The transient over-expression of wild-type or S3A-cofilin in cultured cells caused disruption of preexisting actin structures and induced cytoplasmic actin bundles. Heat shock-induced nuclear or NaCl buffer-induced cytoplasmic actin/cofilin rods contained the expressed cofilin. In contrast, the over-expression of S3D-cofilin did not alter the actin structures. Induced actin rods did not contain S3D-cofilin. S3D-porcine cofilin did not complement the lethality associated with delta cof1 mutations in Saccharomyces cerevisiae while wild-type and S3A-cofilin did. Furthermore, we found that S2A/S4D- and S2D/S4D-yeast cofilin mutants were not viable. CONCLUSION: We conclude that the function of cofilin is negatively regulated in vivo by phosphorylation of Ser-3 and that cells require the function of unphosphorylated cofilin for viability.

Actin Depolymerizing Factors↗
PubMed1996

Overexpression of cofilin stimulates bundling of actin filaments, membrane ruffling, and cell movement in Dictyostelium.

Cofilin is a low molecular weight actin-modulating protein whose structure and function are conserved among eucaryotes. Cofilin exhibits in vitro both a monomeric actin-sequestering activity and a filamentous actin-severing activity. To investigate in vivo functions of cofilin, cofilin was overexpressed in Dictyostelium discoideum cells. An increase in the content of D. discoideum cofilin (d-cofilin) by sevenfold induced a co-overproduction of actin by threefold. In cells over-expressing d-cofilin, the amount of filamentous actin but not that of monomeric actin was increased. Overexpressed d-cofilin co-sedimented with actin filaments, suggesting that the sequestering activity of d-cofilin is weak in vivo. The overexpression of d-cofilin increased actin bundles just beneath ruffling membranes where d-cofilin was co-localized. The overexpression of d-cofilin also stimulated cell movement as well as membrane ruffling. We have demonstrated in vitro that d-cofilin transformed latticework of actin filaments cross-linked by alpha-actinin into bundles probably by severing the filaments. D. discoideum cofilin may sever actin filaments in vivo and induce bundling of the filaments in the presence of cross-linking proteins so as to generate contractile systems involved in membrane ruffling and cell movement.

Actin Depolymerizing Factors↗
PubMed1996

A role of cofilin/destrin in reorganization of actin cytoskeleton in response to stresses and cell stimuli.

1. Cofilin is an essential actin-regulating protein widely distributed in all eucaryotes. The structure and function of cofilin are conserved during evolution. 2. Cofilin depolymerizes F-actin in vitro at alkaline pH and severs F-actin in vitro at pH lower than 7.3. Overexpression of cofilin in viable cells induced bundles of actin filaments suggesting that the severing activity rather than the actin-depolymerizing or monomeric actin-sequestering activity is physiologically significant in vivo. 3. The actin bundle formation induced by overexpression of cofilin is accompanied with an increase in cell motility of Dictyostelium cells. 4. In higher vertebrates, the actin-binding activity of cofilin is negatively regulated by phosphorylation on its Ser-3 residue. The actin-binding activity is essential for yeast cells to grow. 5. Stresses and various cell stimuli activate cofilin by inducing dephosphorylation of cofilin in resting vertebrate cells. 6. Cofilin has an nuclear localization signal sequence and translocates into the nucleus together with actin in response to various stresses. Functional roles of cofilin/actin in the nucleus remain to be elucidated. 7. Tertiary structure of destrin (cofilin) resembles that of gelsolin segment 1 and well explains its functions such as Ca(2+)-independent actin binding activity.

Actin Depolymerizing Factors↗
PubMed1995

Interaction between casein kinase II and the 90-kDa stress protein, HSP90.

Purified casein kinase II (CKII) aggregates and loses activity under physiological salt conditions and within the range of physiological temperatures. In accord with our previous report [Miyata, Y., & Yahara, I. (1992) J. Biol. Chem. 267, 7042-7047], we report here that HSP90 protects CKII from the aggregation and inactivation by forming soluble CKII-HSP90 complexes. Surface plasmon resonance (SPR) measurements revealed that CKII binds to immobilized HSP90 within minutes. The KD of the binding is approximately 10(-7) M. ATP does not influence the interaction. The membrane-overlay method revealed that HSP90 binds to the catalytic CKII alpha subunit. Heparin, which binds to CKII alpha, inhibited the binding of CKII to HSP90-Sepharose. In addition, HSP90 competed with DNA for binding to CKII. Finally, SPR experiments showed that a peptide corresponding to the heparin and DNA binding site of CKII alpha binds to immobilized HSP90. These results indicate that HSP90, DNA, and heparin compete with each other for binding to a common site of CKII alpha. If the binding of CKII to DNA is biologically significant, it could be possibly regulated also by HSP90.

Amino Acid Sequence↗
PubMed1995

Role of the protein chaperone YDJ1 in establishing Hsp90-mediated signal transduction pathways.

The substrate-specific protein chaperone Hsp90 (heat shock protein 90) from Saccharomyces cerevisiae functions in diverse signal transduction pathways. A mutation in YDJ1, a member of the DnaJ chaperone family, was recovered in a synthetic-lethal screen with Hsp90 mutants. In an otherwise wild-type background, the ydj1 mutation exerted strong and specific effects on three Hsp90 substrates, derepressing two (the estrogen and glucocorticoid receptors) and reducing the function of the third (the tyrosine kinase p60v-src). Analysis of one of these substrates, the glucocorticoid receptor, indicated that Ydj1 exerts its effects through physical interaction with Hsp90 substrates.

Base Sequence↗
PubMed1995

Identification, characterization, and intracellular distribution of cofilin in Dictyostelium discoideum.

We identified and purified an actin monomer-binding protein of apparent molecular weight of 15,000 from Dictyostelium discoideum. The 15-kDa protein depolymerized actin filaments in a pH-dependent manner. The protein also had an activity to decrease apparent viscosity of actin solutions in a dose-dependent manner. This activity was inhibited by phosphatidyl inositides. Molecular cloning of genes encoding this protein revealed that the protein is 42% identical in its primary sequence to yeast cofilin. We concluded that the 15-kDa protein is cofilin of this organism. D. discoideum cells contain two cofilin genes (DCOF1 and DCOF2) whose nucleotide sequences were entirely identical in their exsons while the promoter and intron regions were different. Promoter assay experiments revealed that DCOF1 is expressed both in vegetative and differentiating cells and that DCOF2 is not expressed under any conditions examined. Gene disruption experiments suggested that DCOF1 might be essential for the proliferation of D. discoideum cells whereas the disruption of DCOF2 was proven not to alter any phenotypes. Indirect immunofluorescence microscopic observations showed that cofilin is distributed diffusely throughout cytoplasm in vegetative cells. In flattened cells under starvation stress, cofilin localized at dramatically reorganizing actin-cytoskeletons in ruffling membranes of the leading edge, but not at rigid actin meshwork in focal adhesion plaques. These results suggest that cofilin may be involved in dynamic reorganization of membranous actin cytoskeletons.

Actin Depolymerizing Factors↗