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From another point of view, it may be feasible to compare the
de novo
biogenesis of the CVC with that of cilia and flagella. Both organelles are
separated by a membrane from the outside medium (considering that the
lumen CVC periodically opens to the outside), the outside/luminal medium
is rich in Ca
2
þ
(CV;
Stock et al., 2002
) and not remarkably acidic (CV
according to stains undergoing protonation;
Wassmer et al., 2009
), and both
organelles are supported by microtubules with antero- and retrograde motor
proteins attached (for cilia and flagella, see
Ishikawa and Marshall, 2011
).
This suggests the potential relevance of similar factors for the regulation
of organelle biogenesis. Considering that
de novo
genesis of the CVC starts
before cell replication, phosphorylation processes may be crucial, just as
in cilia. Thereby, cyclin-dependent kinases (
Tam et al., 2007
) and MAP
(mitogen-activated protein) kinase (
Berman et al., 2003
) act as regulators,
as do other phosphorylation processes and kinases (
Besschetnova et al.,
2010
). Probably, the level of local [Ca
2
þ
]
i
also plays a role in the biogenesis
of cilia and flagella (
Besschetnova et al., 2010; Wemmer andMarshall, 2007
).
The latter may be controlled by any of the CVC-resident CRCs (beyond
the fine-tuning effect described for
Pt
CRC-II/InsP
3
Rin
Section 3.2.1
).
Depending on local [Ca
2
þ
]
i
, the insertion of new membrane could thus
be regulated. Cytosolic levels of soluble tubulin are also a factor regulating
ciliary length (
Sharma et al., 2011
). At this point, a depolymerizing Kif18
activity may act as a switch between the old and the new CVC during
de
novo
formation.
The expansion of scaffolding microtubules from the nucleation site of a
newly forming CVCwould be paralleled by formation of radial canals and of
the spongiome by vesicle delivery. EM images obtained after silencing dif-
ferent CVC membrane proteins show unilateral association of membranes
(
Fig. 9.4
, inset), thus suggesting this sequence for
de novo
biogenesis.
In summary, both soluble factors and/or insoluble structural components
may be relevant for the regulation of
de novo
CVC biogenesis. One can
now consider several lines of thoughts that are all amenable to experimental
analysis.
7.5. Complexity of protein pattern to be expected
in future research
Together, from all systems analyzed, one may currently sum up the number
of proteins identified in the CVC to nearly about 100, some in multimeric
complexes. Considering the complexity of the organelle several times, more
proteins may reasonably be expected.
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