Biology Reference
In-Depth Information
Similarly, silencing of Syb2-1 or Syb2-2 isoforms causes formation of addi-
tional CVCs in some cells which also display a grossly deformed morphology
(
Schilde et al., 2006
). Therefore, the overall cell-surface pattern appears
important.
De novo
formation of supernumerary CVCs, combined with
malpositioning, has been most frequently observed after Syx6 silencing—
the SNARE with a most dramatic effect on organelle performance
(
Sch
¨
nemann et al., 2013
). All this suggests that the CVC is an intriguingly
cooperative system, with multiple feedback phenomena and with multiple
cues controlling epigenetic positioning of CVCs in
Paramecium
.
In
Tetrahymena
, a precise geometrical arrangement of extended cortical
structures determines the actual position of the CVC (
Nanney, 1966
). The
molecules or stimuli (or local inhibitors) enabling proper placement of
de
novo
-forming CVCs are not known. Also in
Paramecium
, CVC docking sites
are stereotypically arranged, the microtubules of one of the radial arms being
in contact with the analogous structure of the old organelle (
Allen et al.,
1990
). Then, all arms reach about the same length. May the steady-state
equilibrium between ongoing polymerization at the new CVC, with
g
-tubulin as a crucial nucleator at the pore (
Klotz et al., 2003; Shang
et al., 2002
), and depolymerization at the periphery of the old CVC play
a role? If so, what regulates the positioning of
g
-tubulin?
Do phosphorylation/dephosphorylation processes contribute to proper
positioning? The relevance of phosphorylation processes has been assumed
for general surface pattern formation in
Paramecium
where distinct phospho-
proteins are localized to microtubule-organizing centers (
Sperling et al.,
1991
), centrin being a candidate (
Klotz et al., 1997
). After expression as a
GFP-fusion protein, centrin has been observed at the CVC pore in
Tetrahy-
mena
(
Stemm-Wolf et al., 2005
). Since calmodulin occurs at the pore
(
Numata and Gonda, 2001; Suzuki et al., 1982
), calmodulin and
calmodulin-dependent enzymes may act as regulators (possibly also
CDPK-type kinases with integrated calmodulin motifs which may be rec-
ognized by anticalmodulin antibodies). Some additional questions arise, for
example, whether the CRCs found at the CVC pore of
Paramecium
,
Pt
CRCVI-2 and
Pt
CRC-VI-3 (
Ladenburger and Plattner, 2011
), may play
any role in signaling for biogenesis. In summary, many details remain
unexplained at this time.
Another line of search may be suggested in consequence of the finding of
NIMA-related kinases, after overexpression as a GFP-fusion protein, at the
pore of
Tetrahymena
(
Wloga et al., 2006
). These kinases associate with
microtubule-organizing centers not only of the mitotic spindle (
O'Regan
Search WWH ::
Custom Search