four helix-loop-helix motifs (also known as EF hand domains) that bind to four Ca^2 ions with high affin-
ity (Figure 2). Plant CaM is structurally and functionally very similar to animal CaM [200]. Calmodulin
has no enzyme activity, but when bound to Ca^2 it can modulate the activity and function of numerous
unrelated target proteins such as enzymes and certain structural proteins [19,106,167]. The crystal struc-
ture of CaM revealed that it has two globular domains, each with a pair of EF hands, connected by a cen-
tral helix [207] and also provided the structural basis for its interaction with target proteins [208]. The
CALCIUM IN STRESS SIGNAL TRANSDUCTION 707
Figure 2 Schematic structural diagrams showing the features of calmodulin and other EF hand proteins from
plants. (A) Calmodulin and calmodulin-related proteins. AtCaM2 (148 aa), Arabidopsiscalmodulin 2 [201];
AtCBP22 (191 aa), Arabidopsiscalcium-binding protein 22 [202]; AtTCH3 (324 aa), Arabidopsistouch 3 [41];
PhCaM53 (184 aa), Petunia hybridacalmodulin 53 [203]. (B) Other EF hand proteins involved in various stress
responsive process in plants (see the text for details). AtCBL1 to 3 (213, 226, and 226 aa, respectively), Ara-
bidopsiscalcineurin B–like-1, -2, and -3 proteins [204]; AtCBL4 (191 aa), ArabidopsisCBL4 or salt-overly
sensitive3 (SOS3) protein [63]; AtCP1 (155 aa), salt-induced Arabidopsiscalcium-binding protein [119]; Os-
EFA27 (244 aa), rice EF hand protein responsive to abscisic acid [205]; PvHRA32 (161 aa), bean hypersensi-
tive reaction associated [206]; and AtRBOHA (944 aa), Arabidopsisrespiratory burst oxidase homologue A
[150]. The myristoylation motif in CBLs is indicated by an asterisk (*). The AtRBOHA protein (944 aa) did
not permit the depiction of its entire length. Therefore, interruptions in the protein are denoted by (//) in the N-
and C-termini.