Intramembraneous changes on cationophore-triggered exocytosis in Paramecium

Abstract

SECRETORY vesicles (‘trichocysts’) of Paramecium contain proteins1 which are discharged by exocytosis. In P. aurelia (mating type VIII), trichocysts are firmly attached to the cell membrane before the onset of exocytosis, and membrane-intercalated granules occur at these contact sites (Fig. 1); these granules are arranged in a regular pattern and have been described previously as ‘b-’ and ‘c-type granules’2. They are surrounded by a ring of ‘a-type granules’ which connect the cell membrane to the membrane of the alveolar sacs in the region where the latter are penetrated by the trichocysts2,3. When the trichocysts are expelled, the cell membrane is penetrated within the ring of a-type granules and the trichocyst membrane fuses temporarily with the cell membrane2. The present study takes advantage of several circumstances: (i) cells could be frozen without prefixation or cryoprotectants; (ii) exocytosis triggering involves only the final step of membrane fusion; (iii) the regular ‘landmarks’ in the plasmalemma serve to pinpoint intramembraneous changes occurring during exocytosis.