Contractile Vacuole Complex—Its Expanding Protein Inventory - International Review of Cell and Molecular Biology - page 394

et al., 2007 ). As shown with neurons, it also associates with cytoplasmic
microtubules containing acetylated tubulin ( Chang et al., 2009 ), as it occurs
at the CVC pore ( Gaertig et al., 1995 ). All these hypothetic considerations
may be relevant for the epigenetically controlled positioning of the CVC.
7. CONCLUSIONS AND HYPOTHESES
Different types of CVC proteins have been analyzed in the different
systems in widely different detail. This also encompasses proteins of vital
importance for organelle function. Therefore, it appears justified in this final
section to ask for likely general features and components. The aim is not only
to establish a basic structural and functional feature of the organelle—even if
hypothetic—but also to suggest an outline for further scrutiny in systems
lagging behind.
7.1. Summary of a molecular anatomy of CVC
The CVC shows a similar principle of construction and activity in different
freshwater protists, but it displays a most regular form in ciliates. Typically,
the CVC is made up of a CV to which, in ciliates, radial arms (collecting
canals) are attached in a most regular form. From there a tubular network
emerges (spongiome). The spongiome consists of the tubular network of
the smooth spongiome which is in continuity with the more distal decorated
spongiome that is studded with H þ -ATPase molecules. For the decorated
spongiome, Allen (1995) has proposed a tubulation effect of H þ -ATPase/
pump molecules which are connected to each other by molecular links.
From the smooth spongiome, the sequestered fluid reaches the radial arms
(collecting canals), the ampullae, and the CV. Ampullae fuse and disconnect
in each activity cycle, as does the CV with the cell membrane. Although
there is some variation in the structure of the CVC in different species, they
all share these essential features.
The H þ -ATPase generates a D H þ , thus energizing the membranes for
chemiosmotic activity linked to the uptake of water and ions, including
Ca 2 þ . However, other H þ pumps can also occur in some systems. Also, pri-
mary active Ca 2 þ transport is no general feature of CVCs.
A variety of proteins (SNAREs, Rab-GTPases), serving for membrane-
to-membrane interactions including fusion, are known predominantly from
ciliates. In Paramecium , we currently know of seven types of CVC-resident
SNAREs, each three v-/R- and t-/Q-SNAREs and one Qab-SNARE.
SNAREs are complemented by exocyst components, though analyzed only