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into artifi cial vesicles (Hofmann et al.
2000
). A maize annexin preparation was
found to promote Ca
2+
infl ux into root epidermal protoplasts (Laohavisit et al.
2009
). It also formed a Ca
2+
-permeable conductance in planar lipid bilayers that
resembled plant plasma membrane Ca
2+
-permeable nonselective cation channel
(Laohavisit et al.
2009
).
Laohavisit et al. (
2010
) reported that annexins form a ROS-stimulated passive
Ca
2+
transport pathway in maize.
Arabidopsis
loss-of-function mutant for annexin1
(
Atann1
) was found to lack epidermal ROS-activated Ca
2+
conductance (Laohavisit
et al.
2012
). An ROS-activated Ca
2+
conductance was reconstituted by recombinant
annexin1 (ANN1) in planar lipid bilayers (Laohavisit et al.
2012
). The results
suggest that annexin1 presents a novel Ca
2+
-permeable transporter providing a
molecular link between ROS and cytosolic Ca
2+
in plants.
4.4
Ca
2+
Release Channels Involved in Releasing Stored Ca
2+
in Vacuole and Endoplasmic Reticulum into Cytosol
4.4.1
Inositol 1,4,5-Trisphosphate(InsP3)-Activated Ca
2+
Channel
Besides the Ca
2+
infl ux channels, Ca
2+
effl ux channels located in the membrane of
intracellular organelles have also been shown to be involved in signal transduction
(Schroeder and Thuleau
1991
). The plasma membrane Ca
2+
channels allow Ca
2+
infl ux from the cell wall space into the cytosol, while Ca
2+
release channels allow
release of stored Ca
2+
in vacuole and endoplasmic reticulum into the cytosol
(Sanders et al.
1999
). A number of Ca
2+
release channels have been found in the
vacuolar membranes. Inositol 1,4,5-trisphosphate (InsP3)-activated Ca
2+
channel is
the important Ca
2+
release channel (Alexandre et al.
1990
). InsP3-gated channels
release Ca
2+
from the vacuole (Alexandre and Lassales
1992
; Berridge
1993
) and
endoplasmic reticulum (ER) (Martinec et al.
2000
). InsP3 is a second messenger
that controls many cellular processes by generating internal calcium signals
(Berridge
1984
,
1993
; Berridge and Irvine
1989
). InsP3 releases calcium through an
intact intracellular plant membrane by activating a Ca
2+
channel (Alexandre et al.
1990
). It operates through receptors which resemble ryanodine receptors of human
muscle (Berridge
1993
). This calcium channel is voltage-dependent and opened
only on depolarization of the vacuoles (Alexandre et al.
1990
). The calcium released
through this channel induces calcium waves and oscillations (calcium signature) in
the cytosol (Berridge
1993
).
RACK1 (for Receptor for Activated C Kinase1) is an interactor which binds
with phosphorylated active forms of protein kinase C (PKC). It acts as a scaffold
protein, bringing activated PKC into contact with its various substrates (Shirasu
and Schulze-Lefert
2003
; Patterson et al.
2004
; Nakashima et al.
2008
). RACK1
binds InsP3 receptors and regulates Ca
2+
release by enhancing InsP3 receptor
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