PROGRESS IN PROTOZOOLOGY

270 H. PLATTNER ET AL»
Some time ago we have documenlted the occurrence of some ill
defined, electron dense "connecting material" between trichocyst and
cell membrane (Plattner et al. 1975, 1977). This material has now
been further characterized by group- and charge-specific electron stains
in conjunction with enzymatic extraction experiments (W e s t p h a 1
and Plattner 1981 a, b; see Fig 4); it represents protein(s) with
a surplus of negative charges. Similar analyses with nd 9 cells grown
under permissive or non-permissive conditions, revealed that the simultaneous
presence (or absence) of "connecting material" (and of "rosettes";
see above) is correlated with the (un-) capability for exocytosis performance
(Beisson et al. 1980, Matt et al. 1980, Plattner et al.
1980). We have chosen the term "connecting material" because of our
ignorance on its precise nature and function (for possible speculations
see below) and to indicate that it connects physically the trichocyst and
cell membrane (selectively in those strains which are capable of membrane
fusion: Plattner et al. 1980); this holds for a variety of
exocytosis trigger procedures (Matt et al. 1980). Furthermore, microinjection
studies using exocytotically normal and defective Paramecium
tetraurelia strains as donors and/or receptors gave independent support
for the functional importance of a kind of "connecting material"
(Beisson et al. 1980).
Table 1 summarizes some of the features relevant for exocvtotic
membrane fusion in Paramecium tetraurelia.
Not only the "connecting material" between trichocyst and cell membrane
is sensitive selectively to proteolytic enzymes (W e s t p h a 1 and
Plattner, 1981 a, b; Fig. 4) but also the "rosette" (and the "ring")
MIP when analyzed by freeze-fracturing (V i 1 m a r t and P la 11 n e r
1983).
In conclusion, the potential exocytotic fusion sites in paramecia contain
both membrane-integrated and membrane-associated proteins. This
is quite opposite to what was almost generally believed to be essential
for membrane fusion in metazoan cells (for MIP c.f. Orci and Perrelet
1978; for membrane associated material c.f. Laws on and
Raff 1979). Studies with model systems (liposomes) had also come
to the conclusion of a primary role of membrane lipids for the determination
of fusion capacity (c.f. Papahadjopoulos 1978). How
can these discrepancies be reconciled? Some points were recently discussed
within a broader frame (Plattner 1981) and ithe most crucial
aspects pertinent to membrane fusions in protozoa will be discussed
in more detail in the following.
What might be the functional role of (this assembly of membraneintegrated
and membrane-attached proteins for membrane fusion? With
certain expectations in mind we localized a Ca 2 +-stimulated ATPase
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