Calcium Ion Signaling System: Calcium Signatures and Sensors - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 241

4.18 Ca 2+ -Binding Proteins Without EF-Hands

Some Ca 2+ -binding proteins do not contain EF-hand structural motifs. These proteins
contain other Ca 2+ -binding domains such as the C2 domain (Reddy 2001 ). The C2
domain is a Ca 2+ -phospholipid-binding site, and Ca 2+ binding is coordinated by four
to fi ve amino acid residues provided by bipartite loops (Rizo and Sudhof 1998 ).
These domains often mediate Ca 2+ -dependent phospholipid binding.
The copines are a family of Ca 2+ -dependent, phospholipids-binding proteins
(Tomsig and Cruetz 2002 ). Copine proteins (in French, copine means “friend”,
these proteins are named copine because of their tight association with lipid mem-
branes) contain two protein kinase C conserved 2 (C2) domains in the terminal
region and a von Willebrand A (VWA) domain in the C-terminal region (Jambunathan
et al. 2001 ; Jambunathan and McNellis 2003 ; Laxal and Munnik 2002 ). Copines
bind membrane phospholipids due to the presence of two C2 domains in the
N-terminal portion that are activated by calcium. C2 domain-containing proteins
include protein kinase C and phospholipase C. The C-terminal half of the copine
molecule, called the VWA domain, may be involved in targeted protein-protein
interactions (Lecourieux et al. 2006 ). Copines have a calcium-dependent phospho-
lipid-binding activity (Tomsig and Cruetz 2000 ).
In Arabidopsis CPN1 ( copine1 ) is a negative regulator of plant defense-related
hypersensitive response (HR) (Jambunathan et al. 2001 ; Jambunathan and McNellis
2003 ). Mutation of the CPN1 ( COPINE 1 ) gene in Arabidopsis results in increased
resistance to bacterial ( Pseudomonas syringae pv. tomato ) and oomyceteous
( Peronospora parasitica ) pathogens. The mutant showed constitutive expression of
PR genes (Jambunathan and McNellis 2003 ). The results suggest that CPN1 plays a
role in disease resistance responses, possibly as a suppressor of defense responses.
Copines may play a role in membrane traffi cking (Hua et al. 2001 ) and they may
represent a universal transduction pathway (Tomsig et al. 2003 ).
Copines have been suggested to represent a universal transduction pathway for
calcium signaling because the copines are capable of interacting with a wide vari-
ety of target proteins including a MAP kinase kinase (MEK1), a protein phospha-
tase, a Cdc-42-binding kinase, the transcription factor Myc binding protein,
ubiquitin conjugating enzyme, and an enzyme involved in exocytosis (homolog of
SNIP [SNAP-25 Interacting protein]) (Tomsig et al. 2003 ). The copine binds to a
domain of the target protein that is predicted to form a characteristic coiled coil.
The interaction with copines may result in recruitment of target proteins to mem-
brane surfaces and regulation of the enzymatic activities of target proteins (Tomsig
et al. 2003 ).
C2 domain has been detected in all characterized plant phospholipase D enzymes
(PLDs) (Wang 1999 , 2001 ). Ca 2+ may associate directly with PLD through the C2
domain. A positive correlation between increased [Ca 2+ ] cyt and PLD activity has
been reported (De Vrije and Munnik 1997 ). It suggests that PLDs may be involved
in increased [Ca 2+ ] cyt .