Contractile Vacuole Complex—Its Expanding Protein Inventory - International Review of Cell and Molecular Biology - page 383

between ampullae and the CV as well between the CV and the cell mem-
brane. (This would be different from the CRCs of the type InsP 3 R and RyR
or mixed types thereof, as outlined above).
One of the basic aspects of Stomatin function in other systems is the
implication of actin-mediated effects on mechanosensitive ion-channel
activity, as derived from the coincidence of these components together with
Mec-2 in C. elegans ( Mannsfeldt et al., 1999 ). In the absence of any indica-
tion of CVC-associated actin in Paramecium , it remains to be seen which
mechanosensor occurs in this organelle. (However, see Section 5.1 for lim-
itations in actin visualization.) Also, remarkable is the accelerated pumping
frequency after silencing of Pt Sto4 ( Reuter et al., 2013 ). So far it was possible
only in Chlamydomonas to show the association of mechanosensitive chan-
nels type TRP5 with the region containing the CV ( Fujiu et al., 2011 ),
whereas other types were detected in other cell regions. Aquaporin, another
component of the CVC—found in A. proteus ( Nishihara et al., 2008 ) and to
be expected in other systems ( Section 3.1 )—is also known to associate with
Stomatin in human erythrocytes ( Rungaldier et al., 2013 ). Again internal
pressure could modify its activity, since human AQP1is known to close
in response to membrane-tension increments ( Ozu et al., 2013 ).
3.2.5 Calmodulin
Calmodulin on the CV seems to be crucial for organelle function. The Ca 2 þ -
mobilizing effect of the anticalmodulin drug W7 on CV performance in
Dictyostelium can be explained by its effect on LvsA binding to the CV
( Malchow et al., 2006 ), a protein mandatory for CV function
( Section 4.2 ). Similarly W7, calmidazolium, and trifluoperazine reduce the
pulsation rate of the CV in Tetrahymena ( Bergquist, 1989; Suzuki et al., 1982 ).
However, there is some discussion on the precise localization of calmod-
ulin to the CVC in different species. The CV membrane in Dictyostelium dis-
plays bound calmodulin ( Moniakis et al., 1995 ), but in EM
immunolocalization studies, calmodulin is absent from the cytosolic side of
theCVmembranein Dictyostelium ( Zhu et al., 1993 ), as it is in Paramecium
( Momayezi et al., 1986 ). Rather, by immuno-EM labeling, calmodulin has
been localized to the lumenal side of the CV of Paramecium ( Momayezi
et al., 1986 )and Dictyostelium ( Zhu et al., 1993 ). This corresponds to an extra-
cellular localization. Whereas calmodulin is generally established as a cytosolic
high-affinity/low-capacity Ca 2 þ -binding protein ( Cohen and Klee, 1988 ),