Agriculture Reference
In-Depth Information
Ca
2
+
signatures are recognized by different Ca
2
+
sensors to transduce specific
Ca
2
+
-mediating signals into downstream events (Sanders et al.
1999
; Harmon
et al.
2000
; Rudd and Franklin-Tong
2001
), though the molecular, cellular, and
genetic links between Ca
2
+
signatures and the multiple downstream signaling
events are largely obscure. Plants have several classes of Ca
2
+
sensor proteins,
including calmodulin (CaM) and CaM-related proteins (CMLs) (Zielinski
1998
;
Snedden and Fromm
2001
; Luan et al.
2002
; McCormack et al.
2005
; Perochon
et al.
2011
), calcineurin B-like (CBL) proteins (Luan et al.
2002
; Luan
2009
;
Weinl and Kudla
2009
), and Ca
2
+
-dependent protein kinases (CDPKs) (Harmon
et al.
2001
; Cheng et al.
2002b
; Hrabak et al.
2003
; Boudsocq and Sheen
2013
;
Schulz et al.
2013
; Hamel et al.
2014
).
CDPKs are Ser/Thr protein kinases that are found in vascular and nonvascular
plants, in green algae, and also in certain protozoa, but not found in animals or yeast;
CMLs and CBLs are also restricted to plants and some protists, while CaM exists
ubiquitously in all eukaryotic cells (Hrabak et al.
2003
; Defalco et al.
2010
). Unlike
CaM and CBL that must relay the Ca
2
+
-induced conformational change to their pro-
tein partners, CDPKs are a novel class of Ca
2
+
sensor having the unique feature of
both Ca
2
+
sensing and kinase activities within a single protein to directly translate
Ca
2
+
signals into phosphorylation events (Cheng et al.
2002b
; Ludwig et al.
2004
;
Harper and Harmon,
2005
; Boudsocq and Sheen
2010
). CDPKs are the best char-
acterized Ca
2
+
sensor involved in most of Ca
2
+
-stimulated protein kinase activities
identified in plants (Harmon et al.
2001
; Cheng et al.
2002b
; Hrabak et al.
2003
;
Boudsocq and Sheen
2013
; Schulz et al.
2013
; Hamel et al.
2014
).
CDPKs have a conserved molecular structure: an N-terminal variable domain,
followed by a Ser/Thr kinase domain joined to a C-terminal CaM-like domain
via a junction region that serves to stabilize and maintain kinase in an auto-inhib-
ited state (Harper et al.
1991
,
1994
; Harmon et al.
2001
; Cheng et al.
2002b
. See
Fig.
8.1
). The junction region is sometimes referred to as “auto-inhibitory junc-
tion domain.” The CaM-like domain harbors four EF-hand Ca
2
+
-binding motifs,
which are organized into two lobes that have distinct Ca
2
+
affinities, resulting
in different roles in CDPK regulation (Harper et al.
2004
; Harper and Harmon
2005
; Boudsocq and Sheen
2013
; Liese and Romeis
2013
). A model for CDPK
activation was proposed in which the auto-inhibitory junction domain interacts
with and keeps the kinase domain in an inactive state by acting as a pseudosub-
strate that restricts access to the kinase catalytic center in the absence of or at low
level of Ca
2
+
(Harper et al.
2004
; Harper and Harmon
2005
). Ca
2
+
binding to the
Fig. 8.1
A schema of the CDPK structure. The N-terminal variable domain is followed by a
Ser/Thr kinase domain (PK) and the auto-inhibition domain of this kinase; a C-terminal CaM-
like domain (CaM) serves as a Ca
2
+
sensor. See detailed explanation in the text
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