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S Y Chun

Publications and source records attributed to S Y Chun.

50 records · Page 3Linked to original sources
PubMed1994

In vivo studies of the role of SecA during protein export in Escherichia coli.

SecA is found in Escherichia coli both tightly associated with the cytoplasmic membrane where it functions as a translocation ATPase during protein export and free in the cytosol (R. J. Cabelli, K. M. Dolan, L. Qian, and D. B. Oliver, J. Biol. Chem. 266:24420-24427, 1991; D. B. Oliver and J. Beckwith, Cell 30:311-319, 1982; W. Wickner, A. J. M. Driessen, and F.-U. Hartl, Annu. Rev. Biochem. 60:101-124, 1991). Here we show that SecA can be immunoprecipitated from the cytosol in complex with both fully elongated and nascent species of the precursor of maltose-binding protein, an exported, periplasmic protein. In addition, under conditions in which the distribution of SecA between the cytosolic and membrane-bound states changes from that normally observed, the distribution of precursor maltose-binding protein changes in parallel. These results support the idea that cytosolic SecA plays a role in export. With the aim of determining the roles of the multiple binding sites for ATP on SecA, we compared the export defect in a culture of E. coli expressing a temperature-sensitive allele of secA with the defect in a culture treated with sodium azide. The results indicate that the mutational change and treatment with sodium azide inhibit export by affecting different steps in the cycle of ATP binding and hydrolysis by SecA.

ATP-Binding Cassette Transporters↗
PubMed1994

Gonadotropin suppression of apoptosis in cultured preovulatory follicles: mediatory role of endogenous insulin-like growth factor I.

Although the majority of ovarian follicles undergo atresia through a mechanism involving apoptotic cell death, in vivo studies concerning the hormonal regulation of atresia have been difficult due to the presence of heterogeneous population of follicles in the ovary. In the present study, the regulation of follicle apoptosis by gonadotropins, insulin-like growth factor I (IGF-I), and IGF-binding protein 3 (IGFBP-3) was examined using a serum-free culture of preovulatory follicles. Immature rats at 26 days of age received a single dose of PMSG. Two days later, the largest preovulatory follicles were collected for in vitro culture with or without hormones. After 24 h of culture, follicular apoptotic DNA fragmentation was analyzed by autoradiography of size-fractionated DNA labeled at 3'-ends by [32P]dideoxy-ATP. A spontaneous increase in apoptotic DNA fragmentation occurred after 24 h of culture in the absence of hormones, whereas treatment with human CG (hCG) or FSH suppressed follicular apoptosis in a dose-dependent manner, with 0.1 microgram/ml causing maximal suppression by 60-62%. Cotreatment with hCG and FSH had no additional effect. Like gonadotropins, treatment with IGF-I and insulin also suppressed the spontaneous onset of apoptosis, with IGF-I being more effective than insulin. Cotreatment with IGFBP-3 and hCG dose-dependently reversed the suppressive effect of hCG on apoptosis by 42%, suggesting a mediatory role of endogenously produced IGF-I. The addition of IGFBP-3 also blocked the suppressive action of IGF-I by 49%, whereas it did not affect the suppressive action of an IGF-I agonist or insulin. Treatment with IGFBP-3 alone had no effect on apoptotic DNA fragmentation. Estrogen and progesterone production by the cultured follicles were also analyzed by RIA. Gonadotropin treatment resulted in a marked stimulation of the production of both steroid productions. In contrast, treatment with IGF-I caused a small increase in estrogen but decreased progesterone production. Although treatment with IGFBP-3 alone decreased both estrogen and progesterone production, cotreatment with IGFBP-3 and hCG resulted in a slight decrease in estrogen production but an increase in progesterone production. Furthermore, IGFBP-3 did not affect IGF-I action on steroid production. To further substantiate the hypothesis that IGFBP-3 blocks the suppressive effect of hCG on apoptosis by neutralizing endogenously produced IGF-I, solution hybridization analysis was performed, and hCG treatment was shown to increase IGF-I messenger RNA levels in cultured follicles by 1.9-fold.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1993

Folding of maltose-binding protein. Evidence for the identity of the rate-determining step in vivo and in vitro.

The folding of maltose-binding protein, a periplasmic protein in Escherichia coli, was shown to proceed through the same rate-limiting step whether folding occurred in the cell under physiological conditions or in vitro in the absence of other proteins. Four species of maltose-binding protein containing aminoacyl substitutions identified as decreasing the rate of folding of the protein in vivo were purified, and their denaturant-induced folding transitions were analyzed by monitoring the intrinsic fluorescence of tryptophan. In all four cases the rate of folding in vitro was slower than that of the wild-type maltose-binding protein; thus the same step determines the rate of folding in vivo and in vitro. Furthermore, examination of the three-dimensional structure of maltose-binding protein as determined by x-ray crystallography (F. Quiocho, personal communication; Spurlino, J. C., Lu, G.-Y., and Quiocho, F. A. (1991) J. Biol. Chem. 266, 5202-5219) indicates that all 4 of the residues identified as crucial to folding lie in one structural element of the native protein. We conclude that the rate-limiting step both in vivo and in vitro involves formation of this element of structure.

ATP-Binding Cassette Transporters↗
PubMed1993

Severe leukocyte depletion does not affect follicular rupture in the rat.

This study was initiated in order to examine the involvement of leukocytes in follicular rupture in the rat. To evaluate changes in ovarian neutrophil population, ovaries from eCG-primed (15 IU s.c. on Days 25-26) rats were collected 0, 3, 6, and 9 h after hCG (4 IU) administration, and ovarian content of neutrophils was estimated by assaying myeloperoxidase (MPO) activity. The stimulation of hCG increased ovarian MPO activity within 6 h (p < 0.01). Coadministration of inhibitors of eicosanoid synthesis into the ovarian bursa (0.5 mg/bursa) markedly augmented the action of hCG on ovarian MPO activity (p < 0.0001). To examine the possible participation of leukocytes in the process of follicular rupture, peripheral leukocytes were depleted by a single i.v. injection of vinblastine sulfate or cyclophosphamide 4 days before hCG treatment. In spite of a severe depletion in the number of circulating neutrophils or total leukocytes, ovulation rate remained normal. The hCG-stimulated increase in ovarian MPO activity reflects influx of neutrophils into the ovaries during the periovulatory period, and inhibitors of eicosanoid synthesis, which suppress ovulation, further enhance this increase. Nevertheless, the periovulatory rise in ovarian neutrophil content does not seem to be obligatory for follicular rupture. Thus, inhibitors of eicosanoid synthesis block ovulation in the face of an increase in ovarian neutrophil content. Likewise, severe depletion of peripheral neutrophil or total leukocyte counts did not prevent ovulation. The observed influx of neutrophils into the ovary seems to be a consequence of vascular changes associated with the ovulatory response, rather than an obligatory requirement for follicular rupture.

Animals↗
PubMed1992

Localization of preovulatory expression of plasminogen activator inhibitor type-1 and tissue inhibitor of metalloproteinase type-1 mRNAs in the rat ovary.

Proteinases and their inhibitors control follicular connective tissue remodeling associated with follicular rupture. We examined the regulation and cellular localization of plasminogen activator inhibitor type-1 (PAI-1) and tissue inhibitor of metalloproteinase type-1 (TIMP-1) mRNAs by in situ hybridization. [35S]UTP-labeled RNA probes were hybridized to ovarian sections of eCG-primed immature rats treated with hCG. Before hCG stimulation of ovulation, very low expression of PAI-1 mRNA was observed in theca cells. After hCG administration, expression of PAI-1 mRNA was increased in theca cells of most antral follicles, whereas expression in granulosa cells was limited to preovulatory follicles and only to areas where the basal membrane was dissociated. Before hCG treatment, low expression of TIMP-1 mRNA was observed in theca cells, but not in granulosa cells. After hCG treatment, TIMP-1 mRNA was greatly stimulated in theca cells irrespective of follicle size, while the expression in granulosa cells was limited to large antral follicles. The present study demonstrates cell-specific expression of PAI-1 and TIMP-1 mRNAs in the LH/hCG-stimulated ovary, thus confirming the localized control of preovulatory proteolysis by coexpression of both enzymes and their respective inhibitors.

Animals↗
PubMed1992

Differentiated structure and function of cultures from human tracheal epithelium.

Here we describe the conditions which allow cultured human tracheal epithelial cells to retain the ion transport properties and ultrastructure of the original tissue. The order of potency of growth supports and media additives in elevating baseline short-circuit current (Isc) and responses to mediators were vitrogen gel (VIT) greater than extracellular matrix from bovine corneal endothelial cells (ECM) greater than human placental collagen (HPC), and 2% Ultroser G serum substitute (USG) greater than 5% fetal calf serum (FCS) greater than defined growth factors (GF). For all combinations of medium and growth supports, an air interface (AIR) gave better electrical properties than immersion feeding (IMM). As opposed to our earlier conditions (HPC/FCS/IMM), the best new combination (VIT/USG/AIR) produced higher baseline Isc (58.0 +/- 10.6 vs. 5.1 +/- 1.0 microA/cm2) and increased Isc responses to isoproterenol (6.1 +/- 1.5 vs. 0.8 +/- 0.3 microA/cm2) and bradykinin (9.6 +/- 2.0 vs. 1.0 +/- 0.2 microA/cm2), while retaining high transepithelial resistance (227 +/- 5 omega.cm2). VIT/USG/AIR led to the appearance of cilia, an increase in the depth of the cell sheets (50 vs. 10 microns), longer and more frequent apical microvilli, and increased interdigitations of the basolateral membrane. Protein and DNA content were also significantly increased. Secretory granules were present which stained with antibody to goblet cells, but not to serous or mucous gland cells. CF cells grown in VIT/USG/AIR showed high baseline Isc (69 +/- 18 microA/cm2) and a proportionately larger inhibition of Isc by amiloride (70 +/- 10 vs. 34 +/- 3%). Isc did not respond to isoproterenol, and the response to bradykinin was 22% normal.

Amiloride↗
PubMed1991

Preovulatory changes in ovarian expression of collagenases and tissue metalloproteinase inhibitor messenger ribonucleic acid: role of eicosanoids.

The preovulatory surge of gonadotropins activates a cascade of proteolytic enzymes resulting in the rupture of the follicular wall and the release of a fertilizable ovum during ovulation. In the rat the process is initiated by a rise in follicular tissue-type plasminogen activator, produced predominantly in granulosa cells. Recent studies revealed a preovulatory increase in ovarian collagenolytic activity in vivo and an increase in activatable collagenase in vitro. In view of the complicated control of mammalian collagenase synthesis and activity by local inhibitors and activators, we examined the expression of ovarian interstitial and type IV collagenases and tissue inhibitor of metalloproteinase (TIMP) mRNA after an ovulatory stimulus. Ovarian mRNA was isolated from immature PMSG-treated rats 3, 6, and 9 h after hCG stimulation. Northern blot analyses revealed a mRNA of 1.7 kilobases (kb) hybridizing with the human interstitial collagenase cDNA probe. The levels of this mRNA showed a 25-fold increase between 3-6 h after hCG stimulation. The human cDNA probe of collagenase IV hybridized with a mRNA of 3.1 kb, which showed only a 4-fold increase 9 h after hCG treatment. The interstitial collagenase mRNA was expressed in both granulosa cells of preovulatory follicles and the residual ovarian tissue, whereas the expression of collagenase IV mRNA was limited to the residual tissue. Inhibitors of eicosanoid synthesis, previously shown to block ovulation and the LH/hCG-induced rise in ovarian collagenolysis, suppressed the gonadotropic stimulation of interstitial collagenase mRNA, but slightly stimulated that of collagenase IV. The mouse cDNA probe of TIMP hybridized with a 0.9-kb mRNA, which was stimulated by hCG to reach a maximum (7- to 8-fold increase) between 6-9 h after stimulation. TIMP was expressed and stimulated in both the granulosa cells and the residual tissue. Inhibitors of eicosanoid synthesis did not affect the gonadotropic stimulation of TIMP mRNA. These data support the suggested role of interstitial collagenase in follicle rupture and the essential role of eicosanoids in the mediation of gonadotropic stimulation of interstitial collagenase production and action. The observed stimulation of TIMP mRNA expression by the gonadotropin and the lack of any effect of eicosanoid synthesis inhibitors on this action of LH/hCG offer an additional mechanism by which these inhibitors may block ovulation. Thus, the suppression of ovulation by inhibitors of eicosanoid synthesis may result from selective inhibition of interstitial collagenase expression and undisturbed gonadotropin-stimulated TIMP expression.

Animals↗
PubMed1988

Bacterial motility: membrane topology of the Escherichia coli MotB protein.

The MotB protein of Escherichia coli is an essential component of the force generators that couple proton movement across the cytoplasmic membrane to rotation of the flagellar motors. The membrane topology of MotB was examined to explore the possibility that it might form a proton channel. MotB--alkaline phosphatase fusion proteins were constructed to identify likely periplasmic domains of the MotB molecule. Fusions distal to a putative membrane-spanning segment near the amino terminus of MotB exhibited alkaline phosphatase activity, indicating that an extensive carboxyl-terminal portion of MotB may be located on the periplasmic side of the membrane. Protease treatment of MotB in spheroplasts confirmed this view. The simple transmembrane organization of MotB is difficult to reconcile with a role as a proton conductor.

Alkaline Phosphatase↗
PubMed1988

Charged-particle transport in one dimension.

A numerical solution to high-energy charged-particle transport is found by evaluation of the integral equations obtained by inverting the Boltzmann differential operator. An algorithm has been written for either continuous or discrete spectra at the boundary, which allows efficient and accurate evaluation of this rather complicated problem. Present results are compared with analytic solutions based on the perturbation theory.

Algorithms↗
PubMed1984

Experience of posterior surgery in atlanto-axial instability.

Twenty patients with atlanto-axial instability treated by posterior spinal fusion were reviewed. Patients with atlanto-axial instability due to congenital causes usually presented late with chronic myelopathy and treatment in these patients was associated with poor surgical results. Atlanto-axial fusion for fracture non-unions offers immediate stability, reliability, few complications, and good range of neck movement after surgery. Occipito-cervical fusion is indicated whenever atlanto-axial fusion is surgically not feasible.

Adolescent↗
PubMed1983

Long-term results of lumbar intervertebral disc prolapse.

A series of 40 patients is presented at least ten years following anterior spinal fusion for intervertebral disc prolapse. The proportion of patients who had complete relief or only isolated transient symptoms at the five-year review (67.5%) showed only a very slight fall over the years (65.0%). The union rate was shown to improve with time. There was no clear evidence that anterior spinal fusion led to accelerated degeneration of adjacent intervertebral discs. The significance of posterior extension of the fusion is discussed.

Adult↗
PubMedDate not supplied

Gonadal cell apoptosis: hormone-regulated cell demise.

It has become evident that apoptosis, an active form of cell 'suicide', plays an important role in the normal function of all tissues. A balance of cell proliferation and apoptosis is maintained in a healthy individual and any imbalance of the two processes could lead to pathological changes. In both sexes, massive apoptosis accounts for the demise of a majority of gonadal cells (ovarian granulosa cells and male germ cells) during reproductive life. Recent studies have indicated the important role of gonadotrophins as survival factors in both the ovary and the testis. Furthermore, intra-gonadal survival. factors in the ovary (oestrogens, insulin-like growth factor I, epidermal growth factor, basic fibroblast growth factor, interleukin-1 beta, nitric oxide, etc.) and testis (androgens) have been shown to act in concert with the gonadotrophins. In contrast, several apoptotic factors (androgens, gonadotrophin-releasing hormone-like peptide and interleukin-6) may be important in inducing the demise of ovarian follicles. Understanding of the hormonal and cellular mechanisms responsible for gonadal cell apoptosis will provide new approaches for the treatment of gonadal degenerative conditions such as premature ovarian failure and cryptorchidism, as well as for the design of new contraceptive approaches.

Animals↗