Biology Reference
In-Depth Information
4.5
Ca
2+
Effl ux from Cytosol to Vacuole and Endoplasmic
Reticulum (ER)
4.5.1
H
+
/Ca
2+
Antiport System
Ca
2+
effl ux from cytosol to vacuole and ER through H
+
/Ca
2+
antiporters (carriers)
and calcium ATPases (pumps) may also be involved in signal transduction (Hirschi
et al.
1996
; Sanders et al.
1999
,
2002
; Zhu et al.
2010
). Ca
2+
is sequestered into
vacuoles through the H
+
/Ca
2+
antiport system that is driven by the proton-motive
force of the tonoplast H
+
-translocating ATPase (Schumaker and Sze
1986
). Ca
2+
uptake is tightly coupled to H
+
loss. At least one Ca
2+
is exchanged for each H
+
(Schumaker and Sze
1986
). The H
+
/Ca
2+
exchange in tonoplast vesicles is electro-
genic, generating a membrane potential, interior positive. This exchange depends
on the pH gradient between the vacuole and cytoplasm. The transmembrane move-
ment of H
+
is catalyzed by H
+
-conducting proteins (Schumaker and Sze
1986
).
A H
+
/Ca
2+
antiporter has been cloned from
Arabidopsis
, and the protein has been
designated as CAX1 (calcium exchanger 1; Hirschi
2001
). CAX1 appears to trans-
port cytosol Ca
2+
into vacuole and ER and the Ca
2+
entering into these organelles
may regulate calcium release channels in them (Hirschi
2001
). Cell-specifi c vacuolar
calcium storage mediated by CAX1 has been shown to regulate apoplastic calcium
concentration (Conn et al.
2011
). Ca
2+
effl ux systems may also participate in signal
transduction (Sanders et al.
2002
).
4.5.2
Calcium Ion Pumps
Calcium ion pumps may also exist in plant cell membrane. These include P-type
ATPases, which are ATP-fuelled pumps sharing a common enzymatic mechanism
involving a phosphorylated reaction cycle intermediate, hence P type (Palmgren and
Harper
1999
). The pumps become phosphorylated at an aspartate residue in the
sequence DKTGT. The P-type ATPases directly use ATP to drive ion translocation
(Sanders et al.
2002
). More than 40 P-type ATPases have been reported in
A. thali-
ana
. Two P-type ATPases, Ca
2+
-ATPases and H
+
-ATPases, are involved in Ca
2+
signaling.
Plant cells contain two distinct types of Ca
2+
pumps, types IIA and IIB, based on
protein sequence identities (Axelsen and Palmgren
1998
). The Ca
2+
-ATPases are
regulated by binding with calmodulin (Sze et al.
2000
). ACA2, a type IIB pump
detected in ER of
Arabidopsis
cells is normally kept in an autoinhibited conforma-
tion and is activated when Ca
2+
induces calmodulin to bind to the N-terminal domain
of the pump (Harper et al.
1998
; Hwang et al.
2000a
). This binding of calmodulin
might have disrupted an inhibitory interaction within the pump, thereby resulting in
a “release of inhibition” (Hwang et al.
2000a
). It has been demonstrated that a
CDPK can inhibit the basal and calmodulin-stimulated activities of the Ca
2+
pump
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