PROGRESS IN PROTOZOOLOGY

272 H. PLATTNER ET AL»
activity (Fig. 5) at the preformed exocytotic fusion sites of paramecia
(P 1 a 11 n e r et al. 1978). This activity was absent from exocytotically
inactive mutations devoid of "rosettes" and simultaneously of "connecting
material" (P 1 a 11 n e r et al. 1980); see Table 1. Some differences
were also found biochemically, when pellicles were isolated from exocytosis-capable
and -incapable strains and assayed by spectral photometric
tests for their Ca 2+ -ATPase activity (B i 1 i n s k i et al. 1981 b).
All this opens up several possibilities: The ATPase could represent
a Ca 2 +-pump (keeping the local Ca 2+ concentration low and thus avoiding
membrane fusion in the untriggered state; see also B e i s s o n
et al. 1980), a site of protein phosphorylation (as in other exocytotic
systems, like synaptosomes), a Ca 2+ -channel (if combined with a pump,
as in other systems, like sarcoplasmic reticulum) or it could indicate
the presence of contractile elements of the actomyosin type. Since exocytosis
occurs in response to ATPase inhibitors (Matt et al. 1980),
the pump function appears possible, although we have no definite proof
for this assumption. Protein phosphorylation would also be compatible
with our cytochemical findings (Plattner et al. 1977, 1980) but this
interpretation would still require biochemical confirmation. A Ca 2+ channel
function was inferred by Satir and Oberg (1978) but it was
shown later, that an alternative interpretation of their results would
be possible (Matt et al. 1980), although their concept could be correct.
As to contractile elements, it was shown by immunofluorescence (rabbitanti
(Paramecium) actin IgG) and affinity fluorescence (DNase I, heavy
meromyosin) labeling (Tiggemann and Plattner 1981) as well
as by immuno-electron microscopic methods (Tiggemann et al.
1981) that the potential fusion sites proper, i.e., the "connecting material"
regions, are devoid of actin (Plattner et al. 1982; see Fig. 6).
This questions any direct interference of actin (or microfilaments,
respectively) in the membrane fusion process — in opposition to earlier
assumptions along these lines (Poste and Allisson 1973).
In conclusion we assume that our cytochemical findings could indicate
most likely the local presence of a Ca 2+ -dependent ATP-splitting
activity, possibly combined with a Ca 2 +-transport function (pump and
channel?). "Rosettes" and "connecting material" occur always together
and only in exocytosis-capable Paramecium strains. This could be seen
as being relevant for the structural assembly of the "rosette" MIP (which
would be held together by the apposed "connecting material") and probably
as being also functionally relevant, e.g., as a combined enzymeactivator
type system (see discussion in Plattner et al. 1980). As
to the latter possibility it would be rewarding to test for the presence
of calmodulin or related proteins within the structural component with
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