Contractile Vacuole Complex—Its Expanding Protein Inventory - International Review of Cell and Molecular Biology - page 367

one cell type, in most cases, this is evidently due to stochastic data collection,
rather than to systematic differences.
2.1. Basic structure of CVC
Although the CVC is basically constructed according to the same principles
in different phyla, one can also recognize some differences. In its typical
form, for example, in ciliates, CV is connected via ampullae to several radial
arms (collecting canals) during diastole; they are transiently disconnected
during systole ( Tominaga and Allen, 1998 ). Each of the radial arms is in open
connection with a branched tubular network (spongiome) whose proximal
part appears smooth in the electron microscope (EM). Branching is best vis-
ible in the EM image collection by R. D. Allen, accessible at http://www5.
pbrc.hawaii.edu/allen/ . This smooth spongiome is connected to the deco-
rated spongiome whose tubules are studded with pegs, first discussed as
potential H þ -ATPase molecules ( McKanna, 1976 ). Subsequently, they
have been identified as the catalytic V1 head parts of the H þ -ATPase in
Dictyostelium ( Fok et al., 1993; Heuser et al., 1993; Nolta et al., 1993 )
and in Paramecium ( Fok et al., 1995; Wassmer et al., 2005, 2006 ). The
smooth spongiome expands upon systole to accommodate the excess of
membrane area from collapsing radial arms ( Section 4.1 ). Thus, the smooth
spongiome is a flexible membrane reservoir allowing for swelling during
diastole in ciliates ( Allen, 2000; Frankel, 2000 ) and in Dictyostelium
( Gerisch et al., 2002 ).
In Dictyostelium ( De Chastellier et al., 1978; Heuser et al., 1993; Zanchi
et al., 2010 ) and in the flagellate green algae Chlamydomonas ( Luykx et al.,
1997 ), smaller vacuoles are seen to emerge, to swell, and finally to fuse to
a large CV whose contents are discharged at the cell membrane. These vac-
uoles are also surrounded by a spongiome, an anastomosing tubular net-
work. The rather distinct structure of the CVC in Paramecium observed at
the light ( Fig. 9.1 ) and EM level ( Fig. 9.2 ) can be largely attributed to its
support by regularly arranged microtubules extending from the pore out
to the tips of radial arms ( Schneider, 1960 ). No such lining is reported from
Dictyostelium , neither from ultrastructural analysis ( De Chastellier et al.,
1978 ) nor from antibody labeling ( Gr¨f, 2009 ) or expression of GFP
(green fluorescent protein)-tagged tubulin ( Samereier et al., 2011 ). Never-
theless, there may be some association of the Dictyostelium CVC with micro-
tubules because of the disorganizing effect of nocodazole treatment ( Jung
et al., 2009 ).