Paramecium trichocysts are unusual secretory organelles in that: (a) their crystalline contents are built up from a family of low molecular mass acidic proteins; (b) they have a precise, genetically determined shape; and (c) the crystalline trichocyst contents expand rapidly upon exocytosis to give a second, extracellular form which is also an ordered array. We report here the first step of our study of trichocyst structure. We have used a combination of x-ray powder diffraction, freeze-etching, and freeze-fracture electron microscopy of isolated, untreated trichocysts, and density measurements to show that trichocyst contents are indeed protein crystals and to determine the elementary unit cell of both the compact intracellular and the extended extracellular form.

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