PROGRESS IN PROTOZOOLOGY

274 H. PLATTNER ET AL»
some fusion concentrated entirely on the physical effects on lipids (c.f.
Papahadjopoulos 1978). It deserves also considerable interest
now that membrane associated proteins were found more and more
also on the contact sites of secretory organelles and plasmalemma in
metazoan systems (for review see Plattner 1981).
All this would imply that proto- and metazoan systems would not
be principally different with regard to membrane fusion processes, but
that protozoa would contain the same elements in an amplified form.
One could further speculate, that regular MIP arrays and apposed "connecting
material" would have to be present, wherever a membrane
fusion event would have to take place precisely at a certain site. This
assumption would hold not only for the above mentioned cases in
protozoa but again also for some metazoan systems (e.g., neuromuscular
junctions, where transmitter molecules are released rapidly and precisely
over underlying junctional folds; c.f. Meldolesi et al. 1978).
Further examples and arguments along these lines were presented
elsewhere (Plattner 1981). Another example for the simultaneous
occurrence of MIP arrays and membrane-attached materials in fusogenic
zones of protist cells would be the mating structures of Chlamydomonas
(see below).
Other Fusion Sites in Protists
The theory proposed above could also account for the occurrence of
MIP aggregates in mating structures (Chlamydomonas: Weiss et al.
1977 a; Euplotes: Luporini, personal communication). Regular MIP
arrays were reported also from the Paramecium cytoproct and contractile
vacuole outlet (Allen 1978 b), where they were thought, however,
to reflect the pattern of underlying microtubules rather than being involved
in membrane fusion events. MIP aggregates are also found
around the contractile vacuole outlet of Chlamydomonas (Weiss et.
al. 1977 b). For other membrane fusion events in protists see also the
review articles by R. D. Allen (1978 a and these proceedings, Part I).
Possible Changes During Fusion
The actual fusion process is now assumed to be a rather focal event
(Plattner 1981, see Fig. 7); quite a similar fusion model was recently
put forward also for artificial lipid membranes (Hui et al. 1981). Also
for this reason large intramembraneous MIP shifts before fusion would
not be required, although it appears likely that the actual fusion site
would be merely lipidic. It could be represented by a small focal pertur-
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