Protein secretion in Tetrahymena thermophila: characterization of the secretory mutant strain SB281.
The ciliated protozoon Tetrahymena thermophila contains membrane-bounded secretory organelles termed mucocysts, the release of which has previously been characterized ultrastructurally as a model system for the events occurring during membrane fusion and protein secretion. Recently, a series of secretory mutant strains of Tetrahymena has been isolated following mutagenesis of a parental wild-type strain designated SB210. In this study, the correlates of non-release in one unique mutant strain of this series, designated SB281, are described. SB281 appears to express a diminished (undetectable) level of the major 34000 Mr proteinaceous secretory product of Tetrahymena, as determined by Western immunoblot analysis and indirect immunofluorescence labelling. Thin-section electron-microscopic studies of these cells reveal that they possess no docked or free mature mucocysts. In addition, freeze-fracture electron microscopy demonstrates that an intramembrane particle array termed the rosette, present in the plasma membrane of wild-type cells above sites of docked mucocysts, is absent in the plasma membrane of mutant SB281 cells. A morphometric analysis of intramembrane particles in the plasma membrane of both wild-type and mutant cells indicates that both strains have a similar intramembrane particle density in both leaflets of the the plasma membrane. Although assembled rosettes are missing in the plasma membrane of mutant cells, a 15 nm intramembrane particle size class does exist in the plasma membrane of the mutant, but this size class is significantly reduced in number relative to wild-type.
D000818:Animals; D002462:Cell Membrane / Q000648:ultrastructure; D005455:Fluorescent Antibody Technique; D005614:Freeze Fracturing; D008561:Membrane Fusion; D008854:Microscopy, Electron; D009154:Mutation; D010316:Particle Size; D011506:Proteins / Q000378:metabolism*; D013045:Species Specificity; D013768:Tetrahymena / Q000378:metabolism / Q000648:ultrastructure*
D011506:Proteins