Calcium Ion Signaling System: Calcium Signatures and Sensors - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 240

CBLs in Arabidopsis are encoded by a multigene family of at least 10 members
that have similar structural domains with small variations in the length of the cod-
ing regions (Kim et al. 2000 ; Albrecht et al. 2001 ; Luan et al. 2002 ). CBLs have a
helix-loop-helix structural motif (the EF hands) that acts as the Ca 2+ binding site.
CBLs contain three EF hands, whereas CaMs contain four EF hands. The EF-hand
sequence consists of a 12-amino acid loop that uses amino acids at positions 1, 3,
5, 7, and 12 for interaction with Ca 2+ . The Asp at position 1, Gly at position 6, and
Glu at position 12 are the most highly conserved amino acids in the loop. Both
CaMs and CBLs have similar amino acid sequences in their EF-hand motifs.
However, they do not show signifi cant similarity in their primary amino acid
sequences (Luan et al. 2002 ).
Several CBLs have a conserved myristoylation site in their N-terminal regions.
The myristoylation site is required for its location to membrane (Batisti et al.

2008 ). In the Ca 2+ -free state, the myristoyl moiety in recoverin is inaccessible to
membranes. The Ca 2+ -induced conformational change exposes the myristoyl group
and facilitates the association of recoverin with the membrane (Ishitani et al. 2000 ;
Kim et al. 2000 ; Batisti

ć

et al. 2008 ).
Unlike CaMs, which interact with a large number of target proteins, CBLs are
known to interact only with the family of SNF1-like protein kinases. These kinases
are called as CBL-interacting kinases (CIPKs) and CBL interacts with CIPKs
through the C-terminal nonkinase domain that contains a conserved region among
different CIPK members (Batisti

ć

et al. 2008 ; Luan 2009 ). Micromolar levels of
Ca 2+ are required for the interaction of CBL and CIPK (Halfter et al. 2000 ). With at
least 10 CBLs and 25 CIPKs reported in Arabidopsis , many functional CBL-CIPK
pairs can be formed that potentially function in a large array of signaling processes
involving Ca 2+ signaling (Luan et al. 2002 ; Lecourieux et al. 2006 ; Batisti

ć

ć et al.

2008 ).

4.17 NADPH Oxidase as Calcium-Binding Protein

NADPH oxidase, resembling the human neutrophil respiratory burst NADPH
oxidase has been detected in plasma membrane fractions in plant cells (Torres et al.
2002 ; Yoshioka et al. 2003 ). The respiratory burst oxidase homolog (Rboh) in plants
has EF-hand calcium-binding motifs in its N-terminal extension. Rboh is an intrin-
sic plasma membrane protein and the extended N-terminal domain of Rboh projects
into the cytosol (Keller et al. 1998 ). All plant rboh genes carry EF-hands that bind
Ca 2+ and plant Rboh proteins were shown to be stimulated directly by Ca 2+ (Sagi and
Fluhr 2001 ). Ca 2+ -stimulated Rboh enzymes may be positioned close to Ca 2+ chan-
nels localized on the plasma membrane (Lecourieux et al. 2006 ). The activated
NADPH oxidase generates superoxide radicals in plant cells, which are converted
to H 2 O 2 (Sagi and Fluhr 2006 ). These results suggest that Ca 2+ infl ux may activate
ROS signaling system.