Contractile Vacuole Complex—Its Expanding Protein Inventory - International Review of Cell and Molecular Biology - page 378

CVC ( Moniakis et al., 1995 ). However, PAT1 possesses no conserved auto-
inhibitory calmodulin-binding domain ( Pittman, 2011 ). Considering on the
one hand that the potential calmodulin-binding domain in the carboxy-
terminal part of orthodox PMCA molecules differs in Dictyostelium PAT1
( Moniakis et al., 1995 ) and on the other hand the wide variability of
calmodulin-binding sites, in general ( Fraga et al., 2010 ), it remains open
whether PAT1 has to be classified as a genuine or as an atypical PMCA.
CaM-binding studies could give the answer. See also Section 3.2.5 for
the effect on anticalmodulin drugs on CV performance.
An alternative way of Ca 2 þ sequestration is reported from the CVC of
T. cruzi , that is, a H þ -pyrophosphatase (H þ -PPase, Montalvetti et al., 2004 ).
However, proteomics analysis of CVC-enriched fractions also revealed
H þ -ATPase SU B ( Ulrich et al., 2011 ). In this parasite, the situation may
be different insofar as its CVC is assumed to receive membrane components
by fusion with acidocalcisomes whose H þ -PPase activity is well established
( Docampo et al., 2005; Moreno and Docampo, 2009 ).
In summary, the H þ -ATPase can be considered the only primary active
transporter in the CVC of Paramecium . For the additional PMCA-type pump
in Dictyostelium , it has to be analyzed to what extent it might support the
organelle-resident H þ -ATPase. The same is true of the H þ -PPase in
Trypanosoma .
3.1.2 Calcium/proton exchangers and Aquaporin
In the absence of a Ca 2 þ pump in the CVC of ciliates, one must conclude
that a secondary active Ca 2 þ transport is available, operating on the basis of
the D H þ generated by the H þ -ATPase. Though not yet identified in any
CVC at a molecular level, such CAX (cation exchanger) molecules are
found by genomics analysis in Dictyostelium , Paramecium , and Apicomplexa,
the parasitic close relatives of ciliates ( Shigaki et al., 2006 ). Functionally, such
activity has been established, for example, in Dictyostelium , but assigned to
acidic vesicles, addressed as “acidosomes” ( Rooney and Gross, 1992 ). From
experience with Chlamydomonas , this activity may be localized to
acidocalcisome and/or to CV membranes, both endowed with H þ pumps
and Ca 2 þ -sequestration activity ( Ruiz et al., 2001 ). Since the CV of Para-
mecium also secretes Na þ , in addition to Ca 2 þ and Cl ( Stock et al.,
2002 ), several such transporters should be expected. In fact, in some systems
different antiporters and Ca 2 þ pumps can cooperate ( Orlowski and
Grinstein, 2007 ). Considering the very likely occurrence of CAXmolecules