Calcium Ion Signaling System: Calcium Signatures and Sensors - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 230

binding domain in one molecule has been identifi ed in lily ( Lilium longifl orum )
plants (Patil et al. 1995 ). The predicted structure of CCaMK contains a catalytic
domain followed by two regulatory domains, a calmodulin-binding domain and a
visinin- like Ca 2+ -binding domain. The calmodulin-binding region contains three
Ca 2+ -binding EF-hand motifs and a biotin-binding site (Patil et al. 1995 ). Although
Ca 2+ can regulate the kinase activity via a visinin-like domain, Ca 2+ -CaM enhances
the kinase activity toward a substrate and inhibits its autophosphorylation activity,
suggesting that Ca 2+ -CaM may regulate substrate specifi city in vivo (Takezawa
et al. 1996 ). CCaMK homologs have been detected in tobacco, apple, maize, and
Arabidopsis (Patil et al. 1995 ; Watillon et al. 1995 ; Lu and Feldman 1997 ).

4.14.5 Receptor-Like Kinases as CaM-Binding Proteins

Some receptor-like kinases (RLKs) have been identifi ed as CaM-binding proteins
(Charpenteau et al. 2004 ). A CaM-binding protein, AtCaMRLK, has been identifi ed
as a RLK in A. thaliana . AtCaMRLK polypeptide shows a sequence characteristic
of receptor kinases: an amino terminal signal sequence, a domain containing seven
leucine-rich repeats, a single putative membrane-spanning segment and a protein
kinase domain. A region of 23 amino acids, located near the kinase domain binds
CaM in a calcium-dependent manner. The CaM-binding motif of AtCaMRLK was
found to be conserved in several other members of the plant RLK family, suggesting
a role for Ca 2+ /CaM in the regulation of RLK-mediated pathways (Charpenteau
et al. 2004 ). Several RLKs are known to be involved in plant defense responses
(Navarro et al. 2004 ; Zipfel et al. 2004 ; Benschop et al. 2007 ).

4.14.6 NAD Kinase as CaM-Binding Protein

NAD kinase is the enzyme involved in elevation of NADPH levels in plant cells
and it has been found to be a calmodulin-binding protein (Harding et al. 1997 ).
NADPH oxidase triggers ROS production. The calcium-binding protein calmodu-
lin is involved in generation of ROS. Transgenic tobacco plants expressing a for-
eign calmodulin gene showed enhanced NADPH oxidase - dependent production
of ROS (Harding et al. 1997 ; Harding and Roberts 1998 ). NADPH levels were
elevated rapidly through the activation of NAD kinase in the stimulated tobacco
cells. Elicitor treatment also induced burst of ROS in transgenic tobacco cell cul-
tures. Higher levels of NADPH in transgenic calmodulin cells led to a more rapid
and intense burst of ROS, suggesting the involvement of an NADPH oxidase in
the CaM-induced ROS production (Harding et al. 1997 ). These studies suggest
that the calmodulin-binding protein NAD kinase is involved in triggering ROS
generation which plays an important role in triggering plant immune responses
(Fig. 4.6 ).