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Opalski et al.
2005
; Panstruga
2005
). MLO protein resides in the plasma membrane
and has seven transmembrane domains. MLO interacts constitutively with the
cytoplasmic calcium sensor calmodulin (Panstruga
2005
). A 20-amino acid CaM-
binding domain has been located in the rice OsMLO C-terminal cytoplasmic tail
(Kim et al.
2002a
). Loss of calmodulin binding halves the ability of MLO to nega-
tively regulate defense against powdery mildew (Kim et al.
2002b
). The result sug-
gests that MLO is a CAM-binding protein involved in the modulation of defense
reactions.
4.14.18
CaM-Binding Protein Involved in HR-Associated
Cell Death
A calmodulin-binding protein, AtBAG2, has been isolated from
Arabidopsis
(Kang
et al.
2006a
,
b
). The CaM-binding protein contains a central BCL-2-associated
athanogene (BAC) shown to be involved in programmed cell death involved in host
defense response. Agents generating ROS induced the
AtBAG6
transcript, indicating
relationship between CaM and ROS signaling (Kang et al.
2006a
).
4.14.19
CBP60 Family of Calmodulin-Binding Proteins
A plant-specifi c family of CaM binding proteins called CaM60s has been
detected in several plant species. CBP60s have been identifi ed in
Arabidopsis
(Reddy et al.
2002b
; Wang et al.
2009
; Zhang et al.
2010
), maize (Reddy et al.
1993
), bean (Ali et al.
2003
) and tobacco (Lu and Harrington
1994
). Some of
these CBPs have their CaM binding domain (CBD) at very close to the C-terminal
ends (Reddy et al.
1993
,
2002a
; Lu and Harrington
1994
) and others at the N
terminus (Wang et al.
2009
; Zhang et al.
2010
). Other than the presence of CBD,
CBP60s do not show sequence similarity to any other known domains (Ali et al.
2003
).
The two CaM-binding proteins detected in bean (
Phaseolus vulgaris
),
PvCBP60-C and PvCBP60-D, were found to be associated with host defense
responses (Ali et al.
2003
). A calmodulin binding protein, CBP60g, has been shown
to be involved in activating SA biosynthesis (Wang et al.
2009
; Zhang et al.
2010
).
Arabidopsis
CBP60g positively affects the expression of
SID2
, which encodes an
isochorismate synthase (ICS) that is involved in biosynthesis of SA (Wang et al.
2011
). CBP60g shows DNA binding activity, and it preferentially binds to a DNA
sequence that contains AATTTT, which is present in the promoter of ICS1. Mutants
that abolish CaM binding activity of CBP60g did not complement the mutant phe-
notype, suggesting that binding of CaM to CBP60g is essential for its function
(Wang et al.
2009
).
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