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L A Staehelin

Publications and source records attributed to L A Staehelin.

144 records · Page 8Linked to original sources
PubMed1972

Three types of gap junctions interconnecting intestinal epithelial cells visualized by freeze-etching.

Gap junctions are specialized membrane regions that seem to mediate intercellular communication. They appear to contain closely packed arrays of equally sized particles all of which, upon freeze-cleaving, remain attached to one membrane leaflet and not to the other. One gap junction cleavage face, therefore, always exhibits a closely packed array of particles, while the other features a corresponding array of pits. By using these morphological criteria, we have been able to distinguish three different types of gap junctions interlinking adjacent epithelial cells of the small intestine. All three types may be found in close proximity to each other, and in all cases, the particles remain attached to the cleavage face of the cytoplasmic membrane leaflet (A-face). The most frequently encountered type-I gap junctions, which have already been observed in many other tissues, possess 8- to 9-nm (80- to 90-A) particles with a center-to-center spacing of 9-10 nm (90-100 A) when packed in a hexagonal lattice. Type-II gap junctions are always found in close association with type-I junctions. They can be distinguished from the type-I junctions by the greater size [10-11 nm (100-110 A) in diameter] and the greater spacing (190-200 A) of their hexagonally arrayed particles. In contrast, the particles of the type-III gap junctions are arranged in very small rectilinear arrays with a spacing of only 6-8 nm (60-80 A). Gap junctions may be involved in the control of intercellular flow of different types of regulatory molecules.

Animals↗
PubMed1972

Lumenal plasma membrane of the urinary bladder. I. Three-dimensional reconstruction from freeze-etch images.

To determine the three-dimensional structure of the lumenal membrane of transitional epithelium, a study was made of sectioned, negatively stained, and freeze-etched specimens from intact epithelium and membrane fractions from rabbit urinary bladder. Particulate membrane components are confined to plaque regions within which the unit membrane is asymmetric, having a thicker outer leaflet. Transversely fractured freeze-etched plaques display a thick ( approximately 80 A), particulate lumenal leaflet and a thin ( approximately 40 A) cytoplasmic one. Four different faces of the two leaflets can be distinguished: two complementary, split, inner membrane faces exposed by freeze-cleaving the bilayer and two external (lumenal and cytoplasmic) membrane surfaces revealed by deep-etching. On the split, inner face of the lumenal leaflet appear polygonal plaques of hexagonally arranged particles. These fit into holes observed on the complementary, split, innerface of the cytoplasmic leaflet. The particles, which have a center-to-center spacing of approximately 160 A, also seem to protrude from the external surface of the lumenal leaflet, where their subunits ( approximately 50 A in diameter) are revealed by freeze-etching and negative staining. The plaques are separated from each other by smooth-surfaced regions, which cleave like simple lipid bilayers. Since the array of plaque particles covers only approximately 73% of the membrane surface area, whereas 27% is taken up by particle-free interplaque regions, the presence of particles cannot in itself entirely account for the permeability barrier of the lumenal membrane. Although no particles are observed protruding from the cytoplasmic surface of the membrane, cytoplasmic filaments are attached to it by short, cross-bridge-like filaments that seem to contact the particles within the membrane. These long cytoplasmic filaments cross-link adjacent plaques. Therefore, we suggest that at least one function of the particles is to serve as anchoring sites for cytoplasmic filaments, which limit the expansion of the lumenal membrane during distention of the bladder, thereby preventing it from rupturing. The particle-free interplaque regions probably function as hinge areas between the stiff plaques, allowing the membrane to fold up when the bladder is contracted.

Animals↗
PubMed1972

Lumenal plasma membrane of the urinary bladder. II. Isolation and structure of membrane components.

A technique has been devised for isolation of lumenal plasma membranes from transitional epithelial cells lining the urinary bladder in rabbits and for subsequent separation of particle-bearing plaque regions from particle-free areas of the membranes. The success of the procedures employed and their effects on the isolates were assessed by electron microscopy of conventional plastic sections, negatively stained preparations, and freeze-etch replicas. When bladders are distended with a solution of 0.01 M thioglycolic acid, which reduces sulfhydryl bridges, cytoplasmic filaments are disrupted, and large segments of the lumenal membranes rupture and float free into the lumen. A centrifugation procedure was developed for isolating a fraction enriched with the large fragments. A comparison of membranes isolated in the presence of thioglycolate with those isolated from epithelial cells homogenized in sucrose medium indicates that thioglycolate has little effect on their fine structure except for the removal of filaments which are normally associated with their cytoplasmic surface. The curved plaques of hexagonally arrayed particles and the particle-free interplaque regions, both characteristic of membranes before exposure to thioglycolate, are well preserved. Subsequent treatment of thioglycolate-isolated lumenal membranes with 1% sodium desoxycholate (DOC) severs many of the interplaque regions, releasing individual plaques in which the particles are more clearly visible than before exposure to desoxycholate. Presumably, DOC acts by disrupting the hydrophobic bonds within the membrane; therefore, this type of cohesive force probably is a major factor maintaining the structural integrity of interplaque regions. This conclusion is consistent with the observation that interplaque regions undergo freeze-cleaving like simple bilayers with a plane of hydrophobic bonding.

Animals↗
PubMed1971

Structural differentiation of stacked and unstacked chloroplast membranes. Freeze-etch electron microscopy of wild-type and mutant strains of Chlamydomonas.

Wild-type chloroplast membranes from Chlamydomonas reinhardi exhibit four faces in freeze-etchreplicas: the complementary Bs and Cs faces are found where the membranes are stacked together; the complementary Bu and Cu faces are found in unstacked membranes. The Bs face carries a dense population of regularly spaced particles containing the large, 160 +/- 10 A particles that appear to be unique to chloroplast membranes. Under certain growth conditions, membrane stacking does not occur in the ac-5 strain. When isolated, these membranes remain unstacked, exhibit only Bu and Cu faces, and retain the ability to carry out normal photosynthesis. Membrane stacking is also absent in the ac-31 strain, and, when isolated in a low-salt medium, these membranes remain unstacked and exhibit only Bu and Cu faces. When isolated in a high-salt medium, however, they stack normally, and Bs and Cs faces are produced by this in vitro stacking process. We conclude that certain particle distributions in the chloroplast membrane are created as a consequence of the stacking process, and that the ability of membranes to stack can be modified both by gene mutation and by the ionic environment in which the membranes are found.

Buffers↗
PubMedDate not supplied

Effects of ions and gravity forces on the supramolecular organization and excitation energy distribution in chloroplast membranes.

This study was designed to explore the possible relationship between chloroplast membrane stacking or particle aggregation in stacked membrane regions (or both) and excitation energy distribution between photosystems I and II. To this end we have quantitatively examined the effects of different concentrations of univalent ions on the above-mentioned parameters, using a combination of freeze-fracture and thin-section electron microscopy for structural analysis and chlorophyll fluorescence measurements to assay energy-transfer processes. Membrane stacking was found to saturate at about 150mM-NaCl. Maximal EFs-face particle density and chlorophyll fluorescence occurred at about 100mM-NaCl, although only 50% of the potential EFs-face particles were located in stacked membrane regions at this salt concentration. Centrifugation (30,000 g, 1 h) could significantly increase the amount of stacked membranes at salt concentrations between 20- and 60-mM-NaCl; in contrast, centrifugation had little effect on cation-regulation of chlorophyll fluorescence properties. These and other findings suggest that neither chloroplast membrane stacking nor the aggregation of EF-face particles into stacked regions is directly related to the mechanism of excitation energy distribution between the two photosystems (as measured by chlorophyll fluorescence changes) although both structural and functional changes may be mediated by the same membrane component. It is proposed that the salt-induced stacking of chloroplast membranes and the concomitant aggregation of EF-face particles is mediated by the screening of negative surface charges on the membrane pigment-protein subunits, by the establishment of specific interactions between light-harvesting pigment-protein complexes and by 'entropic ordering' forces.

Chloroplasts↗
PubMedDate not supplied

Plasma membrane "rosettes" in carrot and sycamore suspension culture cells.

Suspension culture cells of carrot, Daucus carota L., and sycamore, Acer pseudoplatanus L., were freeze-fractured after ultrarapid freezing without fixation or cryoprotection in a propane-jet freezer. Infrequently, rosettes (ca. 24 nm diameter) of six (occasionally five) subunits (ca. 8 nm diameter) were observed in P-face views of the plasma membrane of both taxa. When present, rosette density was approximately 1/micron 2. Generally, rosettes were less frequently seen on plasma membranes exhibiting numerous vesicle fusion figures. Due to the high quality of the freezing, cellulose microfibril impressions were rarely seen on either PF or EF views of the plasma membrane, thus precluding correlations between microfibrils on the one hand and rosettes (and terminal globules) on the other. The presence of rosettes in suspension culture cells of these two species supports the putative role of rosettes in cellulose biosynthesis in higher plants.

Cell Membrane↗