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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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