The permeability to cytochalasin B of the new unpigmented surface in the first cleavage furrow of the newt's egg.
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Two of 10 mug/ml cytochalasin B (CB) caused retraction of the first cleavage furrow in Triturus eggs, a spreading of the unpigmented surface from the furrow region and a flattening of the whole egg. CB appears to act against the contractility of the microfilamentous band at mid-cleavage so as to relax the furrow and also to weaken unpigmented surface to allow the egg to flatten. Uncleaved eggs and the initial formation of the cleavage groove were unaffected by CB. A fully-retracted first cleavage furrow reformed itself on transfer of the egg to normal medium but only at the time of second cleavage. Initiation of second cleavage depended upon there being sufficient of the original pigmented surface on the animal hemisphere. Tritium-labelled CB of high specific activty was prepared and used to study its ability to penetrate the surface of newt eggs during cleavage. Scintillation couting of whole eggs showed that CB was not taken into the newt egg until mid-cleavage (about 17 min after the double stripe stage) when new surface began to spread in the cleavage furrow. Fixation in glutaraldehyde and osmium tetroxide retained radioactivity in the egg, but more CB was retained after fixation in osmium tetroxide alone than after double fixation. Most of the retained radioactivity was in yolk platelets. Autoradiographs were prepared to sectioned eggs which had been fixed at late cleavage after [3H]CB had flattend the furrow. These showed that CB entered the egg through the unpigmented surface which formed in the furrow but it could not enter through the pigmented surface. The impermeability of the pigmented surface explains the observations that CB does not prevent initial furrowing at cleavage. Once inside the egg CB is transported slowly. CB penetrates to a limited extent beneath the pigmented surface from its border with the unpigmented surface in the first cleavage furrow and this seems insufficient in some circumstances to suppress the contractile phase of second cleavage. (+info)
An aqueous channel for filamentous phage export.
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Filamentous phage f1 exits its Escherichia coli host without killing the bacterial cell. It has been proposed that f1 is secreted through the outer membrane via a phage-encoded channel protein, pIV. A functional pIV mutant was isolated that allowed E. coli to grow on large maltodextrins and rendered E. coli sensitive to large hydrophilic antibiotics that normally do not penetrate the outer membrane. In planar lipid bilayers, both mutant and wild-type pIV formed highly conductive channels with similar permeability characteristics but different gating properties: the probability of the wild-type channel being open was much less than that of the mutant channel. The high conductivity of pIV channels suggests a large-diameter pore, thus implicating pIV as the outer membrane phage-conducting channel. (+info)
Characterization of novel antimicrobial peptoids.
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Peptoids differ from peptides in that peptoids are composed of N-substituted rather than alpha-carbon-substituted glycine units. In this paper we report the in vitro and in vivo antibacterial activities of several antibacterial peptoids discovered by screening combinatorial chemistry libraries for bacterial growth inhibition. In vitro, the peptoid CHIR29498 and some of its analogues were active in the range of 3 to 12 microg/ml against a panel of gram-positive and gram-negative bacteria which included isolates which were resistant to known antibiotics. Peptoid antimicrobial activity against Staphylococcus aureus was rapid, bactericidal, and independent of protein synthesis. beta-Galactosidase and propidium iodide leakage assays indicated that the membrane is the most likely target of activity. Positional isomers of an active peptoid were also active, consistent with a mode of action, such as membrane disruption, that does not require a specific fit between the molecule and its target. In vivo, CHIR29498 protected S. aureus-infected mice in a simple infection model. (+info)
Outer membrane permeability barrier in Escherichia coli mutants that are defective in the late acyltransferases of lipid A biosynthesis.
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The tight packing of six fatty acids in the lipid A constituent of lipopolysaccharide (LPS) has been proposed to contribute to the unusually low permeability of the outer membrane of gram-negative enteric bacteria to hydrophobic antibiotics. Here it is shown that the Escherichia coli msbB mutant, which elaborates defective, penta-acylated lipid A, is practically as resistant to a representative set of hydrophobic solutes (rifampin, fusidic acid, erythromycin, clindamycin, and azithromycin) as the parent-type control strain. The susceptibility index, i.e., the approximate ratio between the MIC for the msbB mutant and that for the parent-type control, was maximally 2.7-fold. In comparison, the rfa mutant defective in the deep core oligosaccharide part of LPS displayed indices ranging from 20 to 64. The lpxA and lpxD lipid A mutants had indices higher than 512. Furthermore, the msbB mutant was resistant to glycopeptides (vancomycin, teicoplanin), whereas the rfa, lpxA, and lpxD mutants were susceptible. The msbB htrB double mutant, which elaborates even-more-defective, partially tetra-acylated lipid A, was still less susceptible than the rfa mutant. These findings indicate that hexa-acylated lipid A is not a prerequisite for the normal function of the outer membrane permeability barrier. (+info)
Mapping an interface of SecY (PrlA) and SecE (PrlG) by using synthetic phenotypes and in vivo cross-linking.
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SecY and SecE are integral cytoplasmic membrane proteins that form an essential part of the protein translocation machinery in Escherichia coli. Sites of direct contact between these two proteins have been suggested by the allele-specific synthetic phenotypes exhibited by pairwise combinations of prlA and prlG signal sequence suppressor mutations in these genes. We have introduced cysteine residues within the first periplasmic loop of SecY and the second periplasmic loop of SecE, at a specific pair of positions identified by this genetic interaction. The expression of the cysteine mutant pair results in a dominant lethal phenotype that requires the presence of DsbA, which catalyzes the formation of disulfide bonds. A reducible SecY-SecE complex is also observed, demonstrating that these amino acids must be sufficiently proximal to form a disulfide bond. The use of cysteine-scanning mutagenesis enabled a second contact site to be discovered. Together, these two points of contact allow the modeling of a limited region of quaternary structure, establishing the first characterized site of interaction between these two proteins. This study proves that actual points of protein-protein contact can be identified by using synthetic phenotypes. (+info)
Effects of interferon alpha-2b on barrier function and junctional complexes of renal proximal tubular LLC-PK1 cells.
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BACKGROUND: Interferon alpha-2b (IFNalpha) treatment of diseases can be accompanied by impaired renal function and capillary leak syndrome. To explore potential mechanisms of IFNalpha-induced renal dysfunction, an in vitro cell culture model system was established to investigate the effects of IFNalpha on barrier function and junctional complexes. METHODS: LLC-PK1 cells were cultured on microporous membranes. Transepithelial resistance (TER) was measured, and the dose- and time-dependent effects of IFNalpha were assessed. The expression patterns of junctional proteins were examined by Western blot analysis and by confocal immunofluorescence microscopy. RESULTS: IFNalpha produced a dose- and time-dependent decrease in TER. The effect was reversible on removal of IFNalpha at doses up to 5 x 103 U/ml. Tyrphostin, an inhibitor of phosphotyrosine kinases, ameliorated the IFNalpha-induced decrease in TER. Increased expression of occludin and E-cadherin was detected by Western blot analysis after IFNalpha treatment. Immunofluorescence confocal microscopy revealed a broader staining of occludin and E-cadherin following IFNalpha treatment, with prominent staining at the basal cell pole in addition to localization at the junctional region. A marked increase in phosphotyrosine staining along the apico-lateral cell border was detected after IFNalpha treatment. CONCLUSIONS: These findings provide evidence that IFNalpha can directly affect barrier function in renal epithelial cells. The mechanisms involve enhanced tyrosine phosphorylation and overexpression and possibly displacement or missorting of the junctional proteins occludin and E-cadherin. (+info)
Nitric oxide enhances paracellular permeability of opossum kidney cells.
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BACKGROUND: Nitric oxide (NO) has been shown to be a paracrine/autocrine regulator of proximal tubular transport. In this study, we investigated the effect of NO on the paracellular permeability of opossum kidney (OK) cells, a proximal tubule cell line that possesses a leaky paracellular pathway resembling that of the in vivo proximal tubule. METHODS: Paracellular permeability of OK cells cultured on permeable supports was measured as the apparent paracellular permeability coefficient (Papp) for 3[H]-D-mannitol. Changes in cell viability, cellular adenosine triphosphate (ATP) content, cGMP levels, and lipid peroxidation were assessed. RESULTS: Incubation with 2 mM sodium nitroprusside (SNP), an NO donor, for 24 hours significantly enhanced the Papp of OK cell sheets by 30.6 +/- 7.0% (N = 8, P < 0.05). This effect was largely blunted by hemoglobin, a NO scavenger. Cell viability was not compromised. This effect of SNP was concomitant with a moderate reduction of cellular ATP content, an increase in lipid peroxidation, and an increase in cellular cGMP levels. The antioxidant superoxide dismutase (SOD) significantly attenuated the effect of SNP on cellular ATP content and blunted the increase in Papp caused by SNP. A soluble guanylate cyclase inhibitor did not affect the effect of SNP on the Papp. CONCLUSIONS: NO enhances the paracellular permeability of OK cells possibly via mechanisms involving decreases in cellular ATP content. (+info)
Cytosolic components are required for proteasomal degradation of newly synthesized apolipoprotein B in permeabilized HepG2 cells.
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Recent studies have proposed that post-translational degradation of apolipoprotein B100 (apoB) involves the cytosolic ubiquitin-proteasome pathway. In this study, immunocytochemistry indicated that endoplasmic reticulum (ER)-associated proteasome molecules were concentrated in perinuclear regions of digitonin-permeabilized HepG2 cells. Signals produced by antibodies that recognize both alpha- and beta-subunits of the proteasome co-localized in the ER with specific domains of apoB. The mechanism of apoB degradation in the ER by the ubiquitin-proteasome pathway was studied using pulse-chase labeling and digitonin-permeabilized cells. ApoB in permeabilized cells incubated at 37 degrees C in buffer alone was relatively stable. When permeabilized cells were incubated with both exogenous ATP and rabbit reticulocyte lysate (RRL) as a source of ubiquitin-proteasome factors, >50% of [3H]apoB was degraded in 30 min. The degradation of apoB in the intact ER of permeabilized cells was much more rapid than that of extracted [3H]apoB incubated with RRL and ATP in vitro. The degradation of apoB was reduced by clasto-lactacystin beta-lactone, a potent proteasome inhibitor, and by ubiquitin K48R mutant protein, an inhibitor of polyubiquitination. ApoB in HepG2 cells was ubiquitinated, and polyubiquitination of apoB was stimulated by incubation of permeabilized cells with RRL. These results suggest that newly synthesized apoB in the ER is accessible to the cytoplasmic ubiquitin-proteasome pathway and that factors in RRL stimulate polyubiquitination of apoB, leading to rapid degradation of apoB in permeabilized cells. (+info)