Bivalent-cation-stimulated ATPase activity at preformed exocytosis sites in Paramecium coincides with membrane-intercalated particle aggregates

Abstract

TRICHOCYSTS of Paramecium cells, although representing morphologically specialised1 secretory vesicles, obey the general rules for exocytosis. Expulsion can be triggered by artificial increase of the intracellular Ca2+-concentration2,3 ([Ca2+]i), that is, by Ca2+-mediated stimulus-secretion-coupling4,5; the trichocyst and the cell membrane fuse to form a transient exocytosis canal6,7 through which the protein-contents8 are discharged. Unlike other secretory granules, however, trichocysts are, long before discharge, closely attached in a regular pattern to the plasmalemma which, at these attachment sites, contains regular arrays of membrane-intercalated particles6,9 (MIP) (Fig. 1a,b). Many MIP occur also within the tip region of the trichocyst membrane; Fig. 1c also shows some non-etcheable ‘membrane-connecting material’ between trichocyst and cell membrane which probably corresponds to electron-dense materials seen on ultrathin sections6. ‘Central granule patches’ would correspond to ‘fusion rosettes’ described also for other ciliates10 and actinopods11,12. The sporadic occurrence of similar MIP aggregates at presumable exoendocytosis sites of endothelial13 and neurohypophysis14 cells might indicate that Paramecium cells are capable of maintaining an otherwise rather ephemeral situation, that is, the stage of membrane-to-membrane attachment preceding membrane fusion. The requirement of energy15 and Ca2+ (refs 4, 5) and the presumable involvement of bivalent-cation-stimulated ATPase activity16 in the course of exocytosis led us to search for a functional correlate of the specialised structuries observed at exocytosis sites.