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These include calmodulin (CaM) and CaM-related proteins, which typically
contain four elongation factor (EF)-hand domains for Ca
2+
binding (Snedden and
Fromm
2001
). Calcineurin B-like (CBL) proteins detected in
Arabidopsis
are
another family of Ca
2+
sensors (Luan et al.
2002
). Another class of calcium binding
proteins is Ca
2+
-dependent and calmodulin-independent protein kinases (CDPKs).
Ca
2+
-dependent protein kinases have protein kinase and calmodulin-like Ca
2+
-
binding domains in a single protein, which allows their direct activation by Ca
2+
(Hrabak et al.
2003
). The CDPK proteins function both as Ca
2+
sensors and as
effectors of their Ca
2+
-sensing activity (Luan et al.
2002
; Geiger et al.
2010
). CDPKs
contain a C-terminal calmodulin-like regulatory domain that functions to couple
the calcium sensor (calmodulin-like domain) directly to its responder (kinase)
(Luan et al.
2002
).
4.13
Calmodulins as Ca
2+
Sensors
Calmodulin (CaM) is one of the best characterized Ca
2+
sensors. CaM has no
catalytic activity of its own, but upon binding Ca
2+
, it activates numerous target
proteins involved in a variety of cellular processes (Snedden and Fromm
2001
;
Reddy et al.
2011a
). CaMs contain an autoinhibitory domain that occludes the
active site in the resting state. Ca
2+
binds to a site near or overlapping the autoin-
hibitory domain, thereby releasing it from the active site and activating the protein
(Luan et al.
2002
).
Calmodulin genes are activated during pathogenesis and transcription of these
genes occurs within a few minutes of pathogen invasion. Transcription of calmodu-
lin isoform 4 (
GmCaM4
) is rapidly induced within 30 min after pathogen
(
Pseudomonas syringae
pv.
glycinea
) stimulation in soybean (Park et al.
2007
). Two
zing fi nger homeodomain transcription factors, GmZF-HD1 and GmZF-HD2
proteins have been shown to activate the
GmCaM4
gene expression in response
to the bacterial pathogen. The pathogen induced binding of GmZF-HD1 and
GmZF-HD2 to repeats of ATTA homeodomain binding site in the
GMCaM4
promoter (Park et al.
2007
).
CaM contains two structurally similar domains connected by a fl exible central
linker. Each domain of the protein binds two calcium ions with positive cooperativ-
ity. The binding of Ca
2+
transforms the protein into its active form through a reori-
entation of the existing helices of the protein (Zhang and Yuan
1998
). CaM typically
contains four elongation factor (EF)-hand domains for Ca
2+
binding (Snedden and
Fromm
2001
). The 'EF hand' is a helix-loop-helix structure. CaM is an acidic
EF-hand protein and is composed of 148 amino acids arranged in two globular
domains connected with a long fl exible helix. Each globular domain contains a pair
of intimately linked EF hand (Snedden and Fromm
2001
; Rainaldi et al.
2007
).
Ca
2+
-free CaM exhibits a fl at, hydrophilic molecular surface, while the Ca
2+
-
saturated form of the protein contains a Met-rich cavity containing hydrophobic
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