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  • 1
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 283 (1980), S. 545-549 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] An 18-Å resolution map of the ‘gap junction’ has been obtained by electron microscopy. The protein oligomer in the junctional membranes, the ‘connexon’, is a cylinder composed of six subunits which are tilted around its axis. Analysis of two different subunit ...
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  • 2
    ISSN: 1432-1424
    Keywords: Freeze-fracture ; Plasma membranes ; Heterologous expression
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Abstract The Xenopus laevis oocyte is widely used to express exogenous channels and transporters and is well suited for functional measurements including currents, electrolyte and nonelectrolyte fluxes, water permeability and even enzymatic activity. It is difficult, however, to transform functional measurements recorded in whole oocytes into the capacity of a single channel or transporter because their number often cannot be estimated accurately. We describe here a method of estimating the number of exogenously expressed channels and transporters inserted in the plasma membrane of oocytes. The method is based on the facts that the P (protoplasmic) face in water-injected control oocytes exhibit an extremely low density of endogenous particles (212±48 particles/μm2, mean, sd) and that exogenously expressed channels and transporters increased the density of particles (up to 5,000/μm2) only on the P face. The utility and generality of the method were demonstrated by estimating the “gating charge” per particle of the Na+/ glucose cotransporter (SGLT1) and a nonconducting mutant of the Shaker K+ channel proteins, and the single molecule water permeability of CHIP (Channel-like Intramembrane Protein) and MIP (Major Intrinsic Protein). We estimated a “gating charge” of ∼3.5 electronic charges for SGLT1 and ∼9 for the mutant Shaker K+ channel from the ratio of Q max to density of particles measured on the same oocytes. The “gating charges” were 3-fold larger than the “effective valences” calculated by fitting a Boltzmann equation to the same charge transfer data suggesting that the charge movement in the channel and cotransporter occur in several steps. Single molecule water permeabilities (p f s) of 1.4 × 10−14 cm3/ sec for CHIP and of 1.5 × 10−16 cm3/sec for MIP were estimated from the ratio of the whole-oocyte water permeability (P f ) to the density of particles. Therefore, MIP is a water transporter in oocytes, albeit ∼100-fold less effective than CHIP.
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  • 3
    ISSN: 1432-1424
    Keywords: Key words: Epithelia — Cell/cell junctions — Tight junctions — Apical/basolateral polarity — Transepithelial electrical resistance — Lipid composition — Occludin — Paracellular
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Abstract. Tight junctions (TJs) are cell-to-cell contacts made of strands, which appear as ridges on P faces and complementary furrows on E faces on freeze fracture replicas. Evidences and opinions on whether these strands are composed of either membrane-bound proteins or lipid micelles are somewhat varied. In the present work we alter the lipid composition of Madin-Darby canine kidney monolayers using a novel approach, while studying (i) their transepithelial electrical resistance, a parameter that depends on the degree of sealing of the TJs; (ii) the apical-to-basolateral flux of 4 kD fluorescent dextran (JDEX), that reflects the permeability of the intercellular spaces; (iii) the ability of TJs to restrict apical-to-basolateral diffusion of membrane lipids; and (iv) the pattern of distribution of endogenous and transfected occludin, the sole membrane protein presently known to form part of the TJs. We show that changing the total composition of phospholipids, sphingolipids, cholesterol and the content of fatty acids, does not alter TER nor the structure of the strands. Interestingly, enrichment with linoleic acid increases the JDEX by 631%. The fact that this increase is not reflected in a decrease of TER, suggests that junctional strands do not act as simple resistive elements but may contain mobile translocating mechanisms.
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  • 4
    ISSN: 1432-1424
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary It is generally accepted that the mechanism for electrotonic coupling involves the presence of hydrophilic channels connecting the cytoplasm of neighboring cells. These channels are presumed to be water filled holes. To test this hypothesis, we measured the temperature dependence of coupling parameters and calculated the specific resistance of junctional synapses of crayfish segmented axons. Results demostrate that: (i) low temperature increases the junctional resistance in a manner that depends on the time course of cooling; (ii) the specific junctional resistance is, at most, 1–20 Ω cm2. These results are consistent with a hypothesis of cell communication based on hydrophilic channels and suggest the presence of a temperature-dependent component of these channels.
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  • 5
    ISSN: 1432-1424
    Keywords: ion channels ; phosphorylation ; modulation of ion channels ; lens fibers ; reconstitution ; intercellular junctions ; major intrinsic protein ; gap junctions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Major intrinsic polypeptide (MIP), a 28-kDa protein isolated from lens fiber cell membranes, forms large, nonselective channels when reconstituted into lipid bilayers. MIP channels are regulated by voltage, such that these channels close when the potential across the membrane is greater than 30 mV. We have investigated the modulation of the voltage-dependent closure of MIP channels by phosphorylation. In this report, we describe the isolation of two isomers of MIP from lens fiber cell membranes. These isomers differ by a single phosphate at a protein kinase A phosphorylation site. The phosphorylated isomer produces channels that close in response to applied voltages when reconstituted into bilayers. The nonphosphorylated isomer produces voltage-independent hannels. Direct phosphorylation with protein kinase A converts voltage-independent channels to voltage-dependent channels in situ. Analyses of macroscopic and single channel currents suggest that phosphorylation increases the voltage-dependent closure of MIP channels by increasing closed channel lifetimes and the rate of channel closure following the application of voltage.
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  • 6
    ISSN: 1432-1424
    Keywords: lens ; embryonic ; gap junctions ; ion channels
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Ion channels are believed to play an important role in the maintenance of lens transparency. In order to ascribe junctional and nonjunctional permeability properties to specific lens cell types, embryonic chick lenses were enzymatically dissociated into cell clusters, cell pairs and single cells, and both cell-to-cell and single-membrane permeability properties were characterized with the patch-clamp technique. Double patch-clamp experiments and single patch-clamp experiments with Lucifer yellow in the pipette demonstrated that the cells in the dissociated preparation were well coupled, the average conductance between pairs being 42 ± 27 nS. Double patch-clamp experiments also revealed single cell-to-cell channel events with a predominant unitary conductance of 286 ± 38 pS. Whole-cell measurements of surface membrane conductance indicate heterogeneity within the population of dissociated embryonic chick lens cells: 63% of the cells have a voltage-independent leak current, 14% of the cells have a potassium-selective inward rectifier current, and 23% of the cells have a current which turns off with positive voltage on a time scale on the order of seconds. The time constant for this turnoff is voltage dependent.
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  • 7
    ISSN: 1432-1424
    Keywords: plasma membrane connections ; kidney loop of Henle
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary The tubular cells from the thick ascending limb of the loop of Henle in rabbit kidney medulla contain in their basallateral surfaces a complex system of interdigitations. Within these interdigitations, the plasma membranes are separated by extracellular spaces of relatively constant width that contain a previously undescribed fibrillar system. The structural organization and distribution of this intercellular fibrillar skeleton was studied using freeze-fracture etch and then section electron microscopy. The skeleton is comprised of discrete strands with a density of 300 to 400 per μm2 evenly distributed along the entire basal-lateral region. Each strand has the shape of a brace and it is constructed from up to eight finer filaments each having a width of about 2 nm. The filaments are tightly joined together along their shafts for about 30 nm but they separate at both ends for about 10 nm before contacting the external surface of the plasma membrane. We propose that this intercellular fibrillar skeleton is responsible for maintaining the wide (about 50 nm) and uniform plasma membrane separation along the entire length of the basallateral region of the tubular cells of the thick ascending limb.
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  • 8
    ISSN: 1573-4935
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
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  • 9
    ISSN: 0003-9861
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Biology , Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
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  • 10
    ISSN: 0005-2736
    Keywords: (Canine kidney) ; (Na^+ + K^+)-ATPase crystal ; Electron microscopy ; Freeze-fracture
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Biology , Chemistry and Pharmacology , Medicine , Physics
    Type of Medium: Electronic Resource
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