Chemistry Reference
In-Depth Information
cellular second messengers, such as NAADP, IP3, IP6, Sphingosine-1-Phospate, and cADPR
(Allen and Sanders, 1995; Navazio et al., 2000; Lemtiri-Chlieh et al., 2003). Then a specific
cellular Ca
2+
signature is sensed by Ca
2+
-binding proteins, the Ca
2+
sensors (Dodd et al., 2010;
Reddy and Reddy, 2004). The sensors themselves may become active to transduce the signal
by themselves, or choose to bind to their interacting proteins and affect their partners'
activity to transduce the signal. In detail, there are three major classes of Ca
2+
sensors identified
in plants. The first one is calmodulins (CaMs) and calmodulin-related proteins (CMLs). CaMs
is a group of small acidic protein, highly conserved in eukaryotes (Snedden and Fromm, 2001),
and contains four EF hands (one major Ca
2+
binding motif) where bind Ca
2+
. Moreover this
binding action induces a conformational change of CaM, leading to exposure of hydrophobic
surfaces and further triggering electrostatic interactions with target proteins - CaM-binding
proteins (CBPs) (Hoeflich and Ikura, 2002). CBPs have been found to take part in regulating
transcription, metabolism, ion transport, protein folding, cytoskeleton-associated functions,
protein phosphorylation and dephosphorylation, as well as phospholipid metabolism (Yang
and Poovaiah, 2003; Reddy and Reddy, 2004). Furthermore, different CaM proteins exhibit
differential expression and are likely to show differential affinity to Ca
2+
and to their target
proteins (McCormack et al., 2005; Popescu et al., 2007), which makes CaM be equipped with
multiple capabilities in Ca
2+
signal transduction. With many similarities to CaMs, CMLs are
mostly composed of four EF-hands and lack other known functional domains. Like CaMs,
they relay the signal by binding to other proteins resulting in activation or inactivation of
interacting proteins. Over 300 proteins that interact with CaMs and CMLs have been identified
in plants (Popescu et al., 2007). The second class of Ca
2+
sensor is represented by the calcium-
dependent protein kinases (CDPKs/CPKs) who are serine/threonine protein kinases contain a
catalytic kinase domain and EF-hand motifs (Cheng et al., 2002). The third typical sensor type
is the EF-hand-containing Ca
2+
-modulated protein named SCaBP (SOS3 (Salt-Overly-Sensitive
3)-like Ca
2+
-biniding proteins)/Calcineurin B-like (CBL) proteins, which is plant-specific
(Luan et al., 2002). CBLs interact with a family of protein kinases called CBL-interacting
protein kinases (CIPKs) (Luan et al., 2009; Weinl and Kudla, 2009; Batistic et al., 2010). In
addition to EF-hand-containing Ca
2+
binding proteins, there are other proteins without that
motif acting as sensors who also can bind Ca
2+
, like PLD (introduced in 2.2.2), annexins and C2
domain-containing proteins (Clark and Roux, 1995; Reddy and Reddy, 2004; Laohavisit and
Davies, 2011), however their functions in abiotic stress responses haven't been deeply
explored, and only some reports suggest PLD and annexin be relevant to stress signal
transduction (White et al., 2002; reviewed by Laohavisit and Davies, 2011).
However here we will mainly stress on the EF-hand-containing sensors due to their
considerable significance in signal transduction pathways. Based on their functional styles,
these sensors are assigned into two camps termed as sensor relays and sensor responders
(Kudla et al., 2010). The sensor relays do not have any known enzymatic or other functional
domains except the EF hands. They interact with other proteins and regulate their activities,
just like CaMs/CMLs, and CBLs (with one exception, CaM7) (McCormack et al., 2005; Luan,
2009; DeFalco et al., 2010). The members of another camp are characterized by an additional
a catalytic or functional domain, except EF hands, whose activity is regulated by Ca
2+
binding to EF-hand motifs. So definitely CDPKs belong to this camp, and other members are
