Contractile Vacuole Complex—Its Expanding Protein Inventory - International Review of Cell and Molecular Biology - page 377

ribosomes, occasionally pegs (possibly H þ /V1 pegs) are recognized on the
same ER cisternae (unpublished observation). Theoretically, a third path-
way for targeted delivery of H þ -ATPase SUs to the CVC could be via endo-
cytosis ( Brown et al., 2009 ). In fact, the 100 kDa-SU vat Min Dictyostelium is
shared by both, the CVC and the endolysosomal membranes ( Clarke et al.,
2002 ). This pathway to the CVC is discussed in Section 5.2 . Altogether, the
pathway of H þ -ATPase constituents to the CVC probably occurs either
from the ER (directly or via the Golgi apparatus) or via endocytosis, or
by both mechanisms. In summary, the involvement of the Golgi in CVC
biogenesis is poorly understood.
3. HANDLING OF CALCIUM BY CVC
The CVC of Paramecium has been shown to sequester and extrude
Ca 2 þ and thus to contribute to intracellular calcium homeostasis. On the
one hand, these cells are highly permeable to Ca 2 þ ; on the other hand, as
in any cell, intracellular concentration of free (dissolved) Ca 2 þ , [Ca 2 þ ] i ,
has to be kept low for the following reasons. Not only is too high a [Ca 2 þ ] i
level toxic, but low basic levels are also required for the use of Ca 2 þ as a
second messenger. Later on, reflux of Ca 2 þ into the cytosol has also been
documented, but nothing is known about any messenger effect in the CVC.
3.1. Calcium uptake by CVC
CVCs dispose of a primary active Ca 2 þ -uptake mechanism only exception-
ally, for example, in Dictyostelium . Only a secondary active process based on
primary H þ uptake (H þ pump) is universally distributed ( Section 3.1.2 ).
3.1.1 Ca 2þ sequestration by CVC
The CV of Paramecium releases substantial amounts of Ca 2 þ ( Stock et al.,
2002 ). This is based on chemiosmosis as it strictly depends on the D H þ gen-
erated by the organellar H þ -ATPase. This is derived from the fact that con-
canamycin B, an efficient H þ -ATPase inhibitor in Paramecium ( Grønlien
et al., 2002 ), retards by about 10-fold the reestablishment of [Ca 2 þ ] i homeo-
stasis after a significant Ca 2 þ load ( Plattner et al., 2012 ).
No Ca 2 þ -ATPase of the type sarcoplasmic/endoplasmic reticulum
Ca 2 þ -ATPase has been detected in the CVC of Paramecium ( Hauser
et al., 1998, 2000 ) and the same is true for the PMCA (plasma membrane
Ca 2 þ -ATPase) ( Elwess and Van Houten, 1997 ). This differs from
Dictyostelium where PAT1, a PMCA-type Ca 2 þ pump, occurs also in the