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⁺-concentration^2,3 ([Ca²⁺]_i), that is, by Ca²⁺-mediated stimulus-secretion-coupling^4,5; the trichocyst and the cell membrane fuse to form a transient exocytosis canal^6,7 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 particles^6,9 (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 actinopods^11,12. 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.