Contractile Vacuole Complex—Its Expanding Protein Inventory - International Review of Cell and Molecular Biology - page 395

partially in some species. In Paramecium , different CRCs of the type RyR
and InsP 3 R are localized to the CVC, where they serve for partial Ca 2 þ
reflux. No CRCs of comparable type are reported from other systems,
but there is evidence of some other Ca 2 þ channels including TRP- and
P2X-type channels. The occurrence of mechanosensitive Ca 2 þ channels
is inferred from the presence of Stomatin in Paramecium and from the peri-
odic activity depending on internal pressure. Ca 2 þ /H þ and possibly other
antiporters have to be postulated for organelle function. The same holds
for aquaporins which have been ascertained only rarely for a CVC. Based
on silencing experiments, the importance of a variety of components for
CVC function in Paramecium is presented in Table 9.3 .
In Dictyostelium , also one of the most intensely analyzed systems, a variety
of unexpected membrane proteins were detected in the CVC ( Section 4.2 ),
such as MEGAP, Lvs, and PAT1, a PMCA-type Ca 2 þ pump. Also in
Dictyostelium , in agreement with vesicle trafficking, different Rab-type pro-
teins and Rab effectors and modulators, such as Drainin and Disgorgin, have
been found. Lineage-specific Rabs are localized to the CVC in Tetrahymena
and in Paramecium . In a proteomics analysis, evidence of SNAREs and Rabs
has also been found in the CVC of T. cruzi .
A microtubule system containing posttranslationally modified tubulin
maintains the shape of the CVC in amoebocytes and ciliates. Considerable
molecular information about microtubules and components of the pore
come from Tetrahymena . Different types of myosin, but no actin, have occa-
sionally been reported. How silencing of specific actin isoforms in Parame-
cium affects biogenesis ( Pt act4) and function ( Pt act9) remains unsettled. The
bidirectionally active myosinJ in Dictyostelium ( Jung et al., 2009 ) may serve
for in situ expansion of the CVC arms along the supporting microtubules. In
principle, this finding may be of relevance also for other systems where
dynein has been found ( Paramecium ).
7.2. Generalized scheme of CVC function
One may imagine the following scenario. The central role of a D H þ is para-
lleled by water and ion extrusion, supported by aquaporin and cation
antiporters. This leads to CV swelling which is sensed by mechanosensitive
Ca 2 þ channels (though still hypothetic for any of the CVCs) positioned at
strategic sites. By subsequent signaling steps, membrane fusion between the
CV and the plasma membrane (in ciliates also between CV and radial arms) is
induced. This scenario is supported by two observations in Paramecium.