Biology Reference
In-Depth Information
extracellular calmodulin is the subject of ongoing debates. Nevertheless, its
occurrence is difficult to negate as it is released by and exerts specific effects
on plant (
Chen et al., 2004; Cui et al., 2005
) and mammalian cells
(
Crockeretal.,1988;Houstonetal.,1997
). Extracellular calmodulin is also
a regulator of chemotaxis in
Dictyostelium
(
O'Day et al., 2012
).
In the CV lumen, calmodulin might provide some regulatory feedback
to ion channels although this has not been ascertained as yet. More recent
EM studies with
Paramecium
revealed calmodulin along the microtubular
cytoskeleton of the CVC (
Fok et al., 2008
). In
Tetrahymena
, calmodulin
has been found in association with CV pore by immunofluorescence
(
Numata and Gonda, 2001; Suzuki et al., 1982
). In summary, several details
concerning calmodulin in the CVC remain unsettled.
4. UNIQUE STRUCTURAL ASPECTS AND
MOLECULAR COMPONENTS
A membrane-bounded organelle that periodically swells in diastole
and shrinks in systole is quite unusual, particularly since biomembranes
are not expandable. And what does “shrink” mean in the present context?
How are CVC membranes transformed? In this section, we can also have a
glimpse on molecules whose presence in the CVC is poorly understood.
4.1. What enables reversible organelle expansion
and collapse?
In the LM, during diastole, the CV and the radial arm canals seem to
reappear from nothing as they swell. However, these membranes cannot
expand by stretching because, as a general rule, biomembranes cannot be
subjected to expansion or compression by more than
1% (
Andersen and
Koeppe, 2007; Mohandas and Evans, 1994
). In reality, the swelling of
CV and of radial arms (and ampullae, in
Paramecium
) during diastole takes
place by reversing de-tubularization of the membrane from a network into
a smooth form (with some spongiome still remaining around the canals).
This holds not only for
Paramecium
(
Tominaga and Allen, 1998
) but also
for
Dictyostelium
(
Heuser, 2006
) and probably also for other species.
This is an intriguing problem of membrane biology; particularly the revers-
ible branching of this membrane labyrinth is difficult to comprehend
in terms of classical lipid bilayer structure. Alone tubularization requires
special proteins
(
Shen et al., 2012
). Only one candidate, F-BAR
Search WWH ::
Custom Search