Contractile Vacuole Complex—Its Expanding Protein Inventory - International Review of Cell and Molecular Biology - page 396

(i) Stomatin proteins, potential scaffolds for positioning mechanosensitive
channels, are found in the CVC. (ii) The extrusion site (pore) contains par-
ticular isoforms of SNAREs and CRCs.
Internal restructuring from a branched tubular to a planar configuration
and reverse, as occurring during systole/diastole cycles, can be produced by
F-BAR proteins (known so far only from Dictyostelium ). More difficult to
understand is the mechanism of branching and debranching of spongiome
tubules. The process of branching/debranching may adjust the spongiome
membrane area for regulatory processes to the actual functional require-
ments. It will require an extra set of CVC-resident proteins which may
include SNAREs and CRCs for reversible fusion/fission processes. This
hypothetic function would be beyond their indisputable role for the delivery
of membrane proteins by vesicles. In fact, different SNAREs are all scattered
over the entire smooth spongiome in Paramecium ( Kissmehl et al., 2007;
Schilde et al., 2006; Sch¨nemann et al., 2013 ). Reversibly branched tubules
may alternatively be formed by proteins similar to those known from the ER
which also possesses a tubulovesiclular organization. Among them are, in the
ER, members of the Reticulon superfamily ( Friedman and Voeltz, 2011 )in
conjunction with a GTPase termed atlastin, as summarized by ( Daumke and
Praefcke, 2011 ). Atlastin proteins are related to large GTPases of the type
dynamin ( Byrnes et al., 2013 ) which in turn could cause fission—again
hypothetically, if present. It has to be emphasized that, with the exception
of SNAREs and CRCs, none of these proteins are established components
of the smooth spongiome, but for F-BAR proteins this is rather likely.
Clearly, the reversible formation of branched tubules in the smooth
spongiome is currently difficult to understand and requires analysis. The
same holds for the possible stabilization of the densely packed tubule system
by trans-SNARE complexes.
7.3. Steady-state biogenesis by vesicle trafficking
and protein turnover
Steady-state turnover of organelle components requires protein delivery by
vesicle trafficking although no vesicle trafficking is directly seen in the EM.
This cryptic biogenesis serves for replacement of molecular components for
which SNAREs, exocyst complexes, and Rab proteins are required. For
vesicle delivery, local [Ca 2 þ ] provided by some of the organelle-resident
CRCs would play a role. Some components may be jointly delivered and
some may reach the CVC after synthesis on free ribosomes. The involve-
ment of the Golgi apparatus is very likely, but not analyzed in detail. Many