Biology Reference
In-Depth Information
4.19
Calcium-Dependent Protein Kinases as Ca
2+
Sensors
4.19.1
Structure of CDPKs
Calcium-dependent protein kinases (CDPKs) are involved in Ca
2+
signaling
(Boudsocq et al.
2010
; Bush et al.
2010
). They belong to the CDPK/SnRK family
of protein kinases (Hrabak et al.
2003
). The protein kinase activity of CDPK is
stimulated by binding of calcium or its intrinsic CaM-like domain. The CDPKs
bind up to four moles of Ca
2+
per mole of enzyme (Lee et al.
1998
). CDPKs contain
fi ve domains: amino-terminal variable domain, catalytic domain, autoinhibitory
domain, CaM-like domain, and a short domain. The amino-terminal variable
domain varies in length from 40 to 180 amino acids. The catalytic domain is typi-
cal of serine/threonine protein kinases. Adjacent to the catalytic domain is an auto-
inhibitory domain that contains a pseudosubstrate sequence that can interact with
the active site and inhibit activity. Also in the inhibitory domain is a site that can
bind to the calmodulin-like domain in the presence of calcium. Binding of the
CaM-like domain to this site is proposed to contribute to the stimulation of kinase
activity in the presence of calcium (Yoo and Harmon
1996
; Harmon
2003
). The
calmodulin-like domain is adjacent to the autoinhibitory domain, and contains four
Ca
2+
binding helix-turn-helix structures known as EF-hands (Lee et al.
1998
;
Reddy et al.
2011a
,
b
). Following the calmodulin-like domain is a short domain of
variable length (Harmon
2003
; Fig .
4.7
).
4.19.2
PAMP/Elicitor Triggers Activation of CDPK
The PAMP/elicitor treatment induces Ca
2+
infl ux by activating the ion channels in
the plasma membrane (Kwaaitaal et al.
2011
; Ranf et al.
2011
). The increases in
Ca
2+
infl ux result in elevation in cytoplasmic Ca
2+
concentration (Luan et al.
2002
).
Several biotic stimuli trigger an increase in the concentration of cytoplasmic free
Ca
2+
, which then acts as a second messenger mediating a variety of cellular
responses. The cytoplasmic free Ca
2+
concentration under resting conditions is
maintained at very low levels (10-200 nM), ensuing low CDPK activity. An increase
in the cytoplasmic calcium results in CDPK activation. CDPKs function as sensors
of fl uctuations in cytosolic Ca
2+
and initiate downstream signaling events (Harmon
et al.
2000
; Hrabak
2000
; Kobayashi et al.
2007
; Ito et al.
2010
). CDPKs require
Ca
2+
for their activation. CDPKs contain a kinase domain, an autoinhibitory domain
and a CaM-like domain. The inhibitory domain contains a pseudosubstrate sequence.
CDPK is autoinhibited by an interaction of the pseudosubstrate site within its junc-
tion domain that blocks the active site of the kinase domain. Binding of Ca
2+
to the
CaM-like domain of the CDPK causes conformational change that extends to the
adjacent junction domain and fi nally disengages the autoinhibitor of the active site
(Huang et al.
1996
). The release of the pseudosubstrate domain from the active site
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