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also outside the CVC, structural localization is adviced. Aquaporin can also
be assumed a general constituent of CVC membranes, although this postu-
late has been sufficiently verified only in
Amoeba proteus
(
Nishihara et al.,
2008
)
and the human-pathogenic
flagellate
T.
cruzi
(
Montalvetti
et al., 2004
).
3.2. Ca
2þ
release from the CVC by Ca
2þ
-channels
Several types of Ca
2
þ
-release channels (CRCs) are known from metazoan
cells, but only quite recently the most important types have been identified
in protozoa. Some of them are unambiguously, though unexpectedly, com-
ponents of the CVC.
3.2.1 CRCs type inositol 1,4,5-trisphosphate and Ryanodine receptors
Sequestration of Ca
2
þ
into the CVCmakes the organelle also a pool of Ca
2
þ
for backflow into the cytosol. In
Paramecium
, we registered spontaneous,
transient, local Ca
2
þ
signals (puffs) (
Ladenburger et al., 2006
). Concomi-
tantly, CRCs typical of
P. tetraurelia
(
Pt
CRC) have been found in the
CVC. One, designated
Pt
CRC-II-1, has been identified as a genuine ino-
sitol 1,4,5-trisphosphate (InsP
3
) receptor (InsP
3
R) (
Ladenburger et al.,
2006
). These InsP
3
Rs are distributed over the entire CVC except the dec-
orated spongiome. Reflux of Ca
2
þ
from the CVC via this constitutively
active InsP
3
Rs into the cytosol can serve for fine-tuning of cytosolic
Ca
2
þ
concentration. The CV also contains some additional CRCs
(
Ladenburger and Plattner, 2011
).
Pt
CRC-V-4 contains a rather short
domain homologous to an InsP
3
-binding domain (though this has not been
tested experimentally) and is also rather broadly distributed over other
organelles. Two other, closely related CRCs,
Pt
CRC-VI-2 and
Pt
CRC-
VI-3 are devoid of an InsP
3
-binding domain and show up near the CV pore
(
Ladenburger and Plattner, 2011
). The occurrence of different
Pt
CRC para-
logs in the CVC suggests functions in addition to [Ca
2
þ
]
i
fine-tuning, such
as restructuring of the spongiome (see below).
3.2.2 Other types of Ca
2þ
-release channels
The
Paramecium
genomic database contains still other candidates for CRCs
(
Plattner et al., 2012
). This includes two-pore channels (TPC) and transient
receptor potential channels. The activation of Ca
2
þ
by microinjection of the
two recently described channel activators (
Lee, 2004
), NAADP
þ
(nicotinic
acid adeninedinucleotide phosphate;
Galione et al., 2010
) and cADPR
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