PROGRESS IN PROTOZOOLOGY

MEMBRANE FUSIONS IN PROTOZOA 269
It is difficult to derive a time sequence or — even more — any possible
functional implications of static snap shots one gets from membranes
by cryofixation and freeze-fracturing. This holds even more if any such
attempt is done after previous chemical pretreatments (aldehyde fixation,
antifreeze impregnation). In retrospect it appears, as summarized
by Plattner (1981), that much of the controversies (see below)
between different groups, working with protozoa, and also other groups,
working with metazoan systems, came from such preparative difficulties.
Both Tetrahymena and Paramecium display "rosettes" already in
the resting stage, i.e., without triggering. After exocytosis triggering
by a Ca 2+ -ionophore, "rosettes" are no longer present, whereas the
"rings" persist and collapse (Plattner 1974). This indicates that only
"rosette" but not "ring" structures are directly involved in exocytosis
performance; the "rings" merely delineate the potential fusion zones
without participating in the fusion process. Even stronger evidence for
this comes from freeze-fracture work with mutant strains of Paramecium
tetraurelia (Beisson et al. 1976). A brief survey is given in Figs.
2 and 3. Some mutations make no (tl) or grossly defective {ftA, tam 38)
trichocysts which cannot be attached to the cell membrane. Other
mutations (nd) allow the attachment but not the subsequent exocytotic
discharge. Beisson et al. (1976) established that the discharge capacity
is coupled with the presence of "rosettes". This has later been
extended to further mutations (Beisson et al. 1980, Lefort-Tran
et al. 1981).
synthesis —
packing
transport
attachment
trigger }
membrane fusion )
discharge
membrane resealing
membrane detachment
trichless
tam 8. tam 38, ftb A
nd 9 (27 °C)
7S, K 401, kin 241
( nd 6, nd 7 (all temperatures) I
nd 9 {18 °C) = 'permissive'
non-permissive
Fig. 2. Sequence of events leading to exocytotic membrane fusion (left) and genetic
dsturbances at different levels in various mutant strains of Paramecium
tetraurelia. 7S is the wild type strain (K401, kin 241 are non-exocytotic mutations);
for nd 9 see text. From Plattner et al. (1980)
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