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MEMBRANE-TO-MEMBRANE CROSS-BRIDGES : A Means to Orientation and Interaction of Membrane Faces

The Journal of Cell Biology
The Rockefeller University Press
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MEMBRANE-TO-MEMBRANE CROSS-BRIDGES A Means to Orientation and Interaction of Membrane Faces WERNER W. FRANKE, JtÏRGEN KARTENBECK, HANSWALTER ZENTGRAF, ULRICH SCHEER, and HEINZ FALK. From the Department of Cell Biology, Institute of Biology II, University of Freiburg i . Br., Germany INTRODUCTION Electron microscopy has shown various examples of membrane faces which occur in parallel, being separated by a distance of 70-350 A . This distance is constant for a defined cellular situation, although it varies from one type of association to another . Prominent instances of this include the parallel associations (stacks) of cisternae, such as the Golgi apparatus and the so-called "impaired" endoplas- mic reticulum (ER) cisternae (for literature see 20, 24), as well as cases in which a cisternal (or vesicular) membrane parallels a single membrane ; a particularly clear example of this being the re- lationship of the plasma membrane with the "subsurface cisternae" of various cells (e .g ., 4, 32, 48) and with the alveolar sacs of the pellicle of many ciliates (e .g ., 27, 40, 45) . In a detailed elec- tron microscope screening of such parallel mem- brane-membrane associations in diverse cell types, intermembranous cross-bridges were consistently observed . This suggests that the cross-bridges function as "rungs" for the stabilization of such characteristic membrane arrangements . MATERIAL AND METHODS Materials used in the present study were : (a), cultures of the ciliate Tetrahymena pyriformis, amicronucleate strain GL, grown under exponential conditions as in- dicated elsewhere (19) ; (b), secondary nuclei-contain- ing caps of cyst-producing Acetabularia mediterranea cells (culture conditions as described in reference 23 ; (c), cotyledons of mustard seedlings, Sinapis albs L . grown in the dark for 36 hr after germination on moist filter paper, then transferred to a far-red il- lumination (compare 38), followed by an approxi-

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