PubMed Health⌕ Search

Biomedical subjects

D L Caspar

Publications and source records attributed to D L Caspar.

8 recordsLinked to original sources
PubMed1979

Divalent cations cooperatively stabilize close membrane contacts in myelin.

Intact nerve myelin compacts to a dehydrated structure of closely apposed membranes when exposed to isotonic solutions at least 10 mM in calcium or tetracaine. The repeat period of the membrane pair in the compacted structure measured by X-ray diffraction is about 126 A in both central and peripheral mammalian nerve myelins whereas the normal periods are about 158 and 178 A, respectively. The electron density profile of compacted myelin shows an asymmetric membrane unit with thickness similar to that of the symmetric bilayer of flocculated myelin lipids. The centrosymmetrically averaged myelin membrane profile is similar to that of the lipid bilayer except at the surface where residual protein is concentrated. Dispersions of extracted total myelin lipids flocculate under similar conditions to those causing myelin compaction, indicating that similar forces act in both processes. Compaction is always accompanied by lateral segregation of intramembrane particles out of the close-packed domains. Lateral displacement of intramembrane proteins form compacted domains can be driven by the attraction of the lipid surfaces for each other. Rates of compaction vary with compacting reagent, concentration, tissue, and temperature, and probably reflect the permeability of the tissue. Extensive compaction by calcium or tetracaine leads to disruption and vesiculation of the spirally wrapped myelin membranes.

Animals↗
PubMed1979

Polyoma virion and capsid crystal structures.

X-ray diffraction shows that complete virus particles and empty capsids crystallize isomorphously. The surface morphology of the protein coat, as revealed by electron microscopy, is the dominant structural feature determining the intensity of x-ray reflections to a resolution of approximately 30 angstroms. The structure and variability of the viral chromatin core can now be analyzed by comparison of electron density maps.

Crystallization↗
PubMed1979

Structural states of myelin observed by x-ray diffraction and freeze-fracture electron microscopy.

Coordinated freeze-fracture electron microscopy and x-ray diffraction were used to visualize the morphological relation between compacted and native period membrane arrays in myelinated nerves treated with dimethylsulfoxide (DMSO). Comparison of x-ray diffraction at room temperature and at low temperature was used as a critical measure of the extent of structural preservation. Our x-ray diffraction patterns show that in the presence of cryoprotective agents, it is possible to preserve with only small changes the myelin structure which exists at room temperature. These changes include a slight increase in packing disorder of the membrane, a small, negative thermal expansion of the membrane unit, and some reorganization in the cytoplasmic half of the bilayer. The freeze-fracture electron microscopy clearly demonstrates continuity of compact and native period phases in DMSO-treated myelin. Finally, the use of freezing to trap the transient, intermediate structure during a structural transition in glycerol is demonstrated.

Animals↗
PubMed1977

Gap junction structures. I. Correlated electron microscopy and x-ray diffraction.

X-ray crystallographic methods and electron microscope image analysis have been used to correlate the structure and the chemical composition of gap junction plaques isolated intact from mouse liver. The requirement that the interpretations of X-ray, electron microscope, and chemical measurements be consistent reduces the uncertainties inherent in the separate observations and leads to a unified picture of the gap junction structures. Gap junctions are built up of units called connexons that are hexagonally arrayed in the pair of connected cell membranes. X-ray diffraction and electron microscope measurements show that the lattice constant of this array varies from about 80 to 90 A. Analysis of electron micrographs of negatively stained gap junctions shows that there is significant short range disorder in the junction lattice. even though the long range order of the array is remarkably regular. Analysis of the disorder provides information about the nature of the intermolecular forces that hold the array together.

Animals↗
PubMed1977

Gap junction structures. II. Analysis of the x-ray diffraction data.

Models for the spatial distribution of protein, lipid and water in gap junction structures have been constructed from the results of the analysis of X-ray diffraction data described here and the electron microscope and chemical data presented in the preceding paper (Caspar, D. L. D., D. A. Goodenough, L. Makowski, and W.C. Phillips. 1977. 74:605-628). The continuous intensity distribution on the meridian of the X-ray diffraction pattern was measured, and corrected for the effects of the partially ordered stacking and partial orientation of the junctions in the X-ray specimens. The electron density distribution in the direction perpendicular to the plane of the junction was calculated from the meridional intensity data. Determination of the interference function for the stacking of the junctions improved the accuracy of the electron density profile. The pair-correlation function, which provides information about the packing of junctions in the specimen, was calculated from the interference function. The intensities of the hexagonal lattice reflections on the equator of the X-ray pattern were used in coordination with the electron microscope data to calculate to the two-dimensional electron density projection onto the plane of the membrane. Differences in the structure of the connexons as seen in the meridional profile and equatorial projections were shown to be correlated to changes in lattice constant. The parts of the junction structure which are variable have been distinguished from the invariant parts by comparison of the X-ray data from different specimens. The combination of these results with electron microscope and chemical data provides low resolution three- dimensional representations of the structures of gap junctions.

Animals↗
PubMed1975

Myelin structure transformed by dimethylsulfoxide.

X-ray diffraction patterns from nerves bathed for about one-half hour in Ringer's solution containing dimethylsulfoxide at concentrations of 10% or more show reflections from a new, highly ordered structure with a repeat period about two-thirds that of native myelin. The proportion of myelin transformed is greater at higher concentrations, and above 40% the native pattern is no longer observed. Replacing the dimethylsulfoxide with Ringer's solution leads to the rapid reappearance of the native diffraction pattern. The effect of dimethylsulfoxide can be accounted for by the loss of water from the spaces between the membrane units without significant modification of the bilayer structure.

Animals↗