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No filamentous material, actin or centrin, is known to be associated
with the CV of
Paramecium
(
Allen, 2000
) and F-actin-disrupting agents
have no effect (
Tani et al., 2000, 2002
). Also for
Dictyostelium
, the absence
of actin is reported (
Heuser, 2006
). Considering the failure to trace actin by
affinity (
Kersken et al., 1986
) and immunolabeling in the CVC of
Parame-
cium
(
Sehring et al., 2007a
) and in
Dictyostelium
(
Heuser, 2006
), there is
general agreement that the periodic contraction of the CV is not due to
actin/myosin interaction (
Allen, 2000; Heuser, 2006
). May the CVC nev-
ertheless contain actin? Remarkably, proteins modulating Rac- and Rho-
type GTPases are reported to be relevant for CV activity in
Dictyostelium
(
Knetsch et al., 2001; Rivero et al., 2002
). Both modulate actin dynamics,
though this is not their exclusive function (
Bustelo et al., 2007
); for
GTPase and modulators, see
Section 2.3
. Another intriguing detail is the
finding of actin-binding proteins in the CVC of
Tetrahymena
(
Watanabe
et al., 1998
).
More stringent hints come from
P. tetraurelia
. Silencing of two, out of
many isoforms of actin, had considerable effects. Silencing of
Pt
Act4 caused
formation of multiple CVCs (
Sehring et al., 2010
). Silencing of
Pt
Act9
slowed down the CV pumping cycle (
Sehring et al., 2007a
) although in
immunofluorescence this isoform was restricted to food vacuoles. In
Paramecium
, one might theoretically envisage very short polymers as they
are found in the cortex of the related parasitic phylum, Apicomplexa
(
Gould et al., 2011
). Also theoretically, such forms could escape detection
and still participate in organelle-specific functions. Remarkably, this actin
isoform does not possess the typical binding sites for drugs causing polymer-
ization or depolymerization (
Sehring et al., 2007b
). Thus, they would nei-
ther bind standard affinity stains (phalloidin) nor react to filament-degrading
drugs (cytochalasins). As discussed in
Section 7
, the presence of actin, even in
“cryptic” form, could be important for some specific functions. In fact, in
Dictyostelium
, where null mutants of the myosin-V-related MyoJ reveal its
crucial role for CV docking, cytochalasin A produces phenocopies thereof
(
Jung et al., 2009
).
Several myosin isoforms are associated with the CVC of
Dictyostelium
,
notably myoV (
Jung et al., 2009
). Type V myosin, called MyoJ in
Dictyostelium
, is a bidirectional motor associated with the CVCmicrotubules
in
Dictyostelium
(
Jung et al., 2009
). Therefore, microtubule-associated motor
proteins may serve for the maintenance of the extended shape of the CVC.
Too little information is available from ciliates. Another aspect of myosin-V
in mammals is its binding to secretory organelles in coordination with Rab
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