Biology Reference
In-Depth Information
binding affi nity for InsP3. Overexpression of RACK1 markedly augments Ca
2+
release, while depletion of RACK1 by interference RNA diminishes Ca
2+
release
(Patterson et al.
2004
).
4.4.2
Cyclic Adenosine 5
′
-Diphospho-Ribose (cADPR)
Gated Channels
Cyclic adenosine 5
-diphosphoribose (cADPR) gated channels are another family
of channels, which release Ca
2+
from the vacuole and endoplasmic reticulum
(Navazio et al.
2001
). The cADPR-gated pathway has been shown to be voltage-
dependent and it does not spontaneously desensitize. It is colocalized with an InsP3-
gated calcium release pathway in individual vacuoles (Allen et al.
1995
). cADPR
elevates cytosolic free calcium in plants (Navazio et al.
2001
). cADPR-mediated
induction of abscisic acid-responsive gene expression has been shown to be exerted
by means of mobilization of internal Ca
2+
stores (Wu et al.
1997
).
′
4.4.3
Slowly Acting Vacuolar (SV) Channel
Another class of Ca
2+
release channel type residing in the vacuolar membrane is
gated by voltage and acts by membrane depolarization (Sanders et al.
2002
). This
channel is known as the Slow Vacuolar (SV) channel in reference to its voltage-
activation kinetics (Hedrich and Neher
1987
; Hedrich and Marten
2011
). The SV
channel is activated by rises in [Ca
2+
]
cyt
and this response potentially endows the
channel with the capacity to catalyze Ca
2+
-induced Ca
2+
release (Bewell et al.
1999
). The channel is regulated by [Ca
2+
]
cyt
and by phosphorylation (Allen and
Sanders
1995
). This channel is downregulated by 14-3-3 proteins (van den
Wijngaard et al.
2001
).
4.4.4
NAADP-Activated Ca
2+
Effl ux Channel
A distinct Ca
2+
release channel activated by the nicotinamide adenosine dinucleo-
tide phosphate (NADP) metabolite nicotinic acid adenine dinucleotide phosphate
(NAADP) has been detected in ER of higher plants (Navazio et al.
2000
; Patel
2004
). The NAADP potently mobilizes Ca
2+
from ER in red beet (
Beta vulgaris
)
and caulifl ower (Navazio et al.
2000
). The exclusively endoplasmic reticulum loca-
tion of the NAADP-sensitive Ca
2+
pathway distinguishes it from the InsP3- and
cADPR-gated pathways. The NAADP-gated Ca
2+
release pathway was found to be
independent of cytosolic free Ca
2+
and therefore incapable of operating Ca
2+
-
induced Ca
2+
release (Navazio et al.
2000
).
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