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B Satir

Publications and source records attributed to B Satir.

15 recordsLinked to original sources
PubMed1977

Dibucaine-induced synchronous mucocyst secretion in Tetrahymena.

Synchronous secretion of all available mature mucocysts was induced in late log phase cultures of Tetrahymena thermophilia (B III) by the local anaesthetic dibucaine. No assembled fusion rosettes were seen within the plasma membrane after release until 2-3 hrs of regrowth, thus proving that the rosettes are not permanent sites within the plasma membrane but have to be reassembled each time for a new fusion event to occur. Concomitant with the reappearance of assembled fusion rosettes, the cell cytoplasm fills up with precursors of new mucocysts thus linking the two events together.

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PubMed1976

Genetic control of membrane mosaicism.

The mosaic and dynamic character of cellular membranes is illustrated by the specific intramembrane particle array, the fusion rosette, found to be essential for membrane fusion and secretion in the ciliated protozoa, Tetrahymena and Paramecium. The rosette is not a permanent site within the membrane. When secretion of mucocysts is synchronized by treating cells with the local anesthetic dibucaine, all rosettes disappear, only to reassemble as new mucocysts mature. Assembly of the functional rosette is under genetic control. A series of secretory mutants of Paramecium, blocked in various stages of the secretory cycle, has been studied (11). Mutants that do not secrete lack the fusion rosette, although other intramembrane particle components of the fusion site are present. Certain properties of the rosette, in particular its particle partition coefficient, are temperature-dependent, which may affect the ability of the rosette particles to assemble. A temperature-sensitive mutant, nd9, secretes normally, and has rosettes at 18 degrees C, but fails to secrete attached trichocysts at 27 degrees C.

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PubMed1976

Genetic analysis of membrane differentiation in Paramecium. Freeze-fracture study of the trichocyst cycle in wild-type and mutant strains.

Using a series of mutants of Paramecium tetraurelia, we demonstrate, for the first time, changes in the internal structure of the cell membrane, as revealed by freeze-fracture, that correspond to specific single gene mutations. On the plasma membrane of Paramecium circular arrays of particles mark the sites of attachment of the tips of the intracellular secretory organelles-trichocysts. In wild-type paramecia, where attached trichocysts can be expelled by exocytosis under various stimuli, the plasma membrane array is composed of a double outer ring of particles (300 nm in diameter) and inside the ring a central rosette (fusion rosette) of particles (76 nm in diameter). Mutant nd9, characterized by a thermosensitive ability to discharge trichocysts, shows the same organization in cells grown at the permissive temperature (18 degrees C), while in cells grown at the nonpermissive temperature (27 degrees C) the rosette is missing. In mutant tam 8, characterized by normal but unattached trichocysts, and in mutant tl, completely devoid of trichocysts, no rosette is formed and the outer rings always show a modified configuration called "parentheses", also found in wild-type and in nd9 (18 degrees C) cells. From this comparison between wild type and mutants, we conclude: (a) that the formation of parentheses is a primary differentiation of the plasma membrane, independent of the presence of trichocysts, while the secondary transformation of parentheses into circular arrays and the formation of the rosette are triggered by interaction between trichocysts and plasma membranes; and (b) that the formation of the rosette is a prerequisite for trichocyst exocytosis.

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PubMed1973

Membrane fusion in a model system. Mucocyst secretion in Tetrahymena.

The freeze-fracture, freeze-etch technique can be employed to reveal new details of the process of fusion of two unit membranes For this study, mucocyst discharge in Tetrahymena pyriformis provides a model system with certain general implications The undischarged mature mucocyst is a saclike, membrane-bound, secretory vesicle containing crystalline material The organelle tip finds its way toward a special site, a rosette of 150 A diameter particles within the plasma membrane. To match this site, the mucocyst membrane forms an annulus of 110 A diameter particles, above whose inner edge the rosette particles sit. Discharge of some mucocysts is triggered by fixation. As discharge proceeds, the organelle becomes spherical and its content changes from crystalline to amorphous. The cytoplasm between the two matching membrane sites is squeezed away and the membranes fuse Steps in membrane reorganization can be reconstructed from changes in rosette appearance in the fracture faces. First, a depression in the rosette-the fusion pocket-forms. The rosette particles spread at the lip as the pocket deepens and enlarges from 60 to 200 nm. The annulus particles then become visible at the lip, indicating completed fusion of the A fracture faces of mucocyst and plasma membranes The remaining B faces of the two membranes have opposite polarities When the content of the mucocyst is released, the edges of these faces join so that the unit membrane runs uninterruptedly around the lip and into the pocket.

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PubMed1971

Nucleolar aging in Tetrahymena during the cultural growth cycle.

Nucelolar morphology was studied by electron microscopy in control and actinomycin D-treated populations of Tetrahymena pyriformis (W) during the cultural growth cycle. Nucleoli exhibit an "aging" cycle concomitant with the cultural growth cycle, but independent of the individual cell cycle. Four different stages in the course of this aging process have been defined. Stage 1 occurs upon inoculation (low number of cells per milliliter) and lasts through lag and accelerating growth phases. In this stage, many small nucleoli are found at the nuclear periphery. In stages 2 and 3, nucleolar fusion begins. Stage 2 dominates the first half of logarithmic growth, and stage 3 dominates the second half. In late decelerating growth phase, the nucleoli enter stage 4. In this stage, only a few large nucleoli are present and these are apparently inactive in ribosome production. In stationary phase, where total RNA remains constant, only stage 4 nucleoli are present. The relative preponderance of granular vs. fibrous components in the nucleoli changes during this cycle, the granular component dominating stage 1 nucleoli and the fibrillar, stage 4 nucleoli. There is a shortening of the intermediate nucleolar stages in the treated cultures; fusion occurs early and is now pronounced. Not enough ribosomes accumulate to carry the treated cultures through the number of generations equivalent to those of the control, which produces a premature stationary phase.

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