A. Protein Kinases and Phosphatases
Signals either directly or through messengers such as Ca^2 regulate the activity of protein kinases and
protein phosphatases that in turn regulate phosphorylation and dephosphorylation, respectively, of many
proteins in the cell. The enzyme activity or biological property of many proteins is strongly influenced by
(de)phosphorylation status. It has been well established that Ca^2 -regulated protein phosphorylation
plays a pivotal role in signal transduction in animal cells. Several Ca^2 /CaM-dependent protein kinases
[168,169], protein kinase C, and a Ca^2 - and phospholipid-dependent protein kinase [170] mediate the
Ca^2 effects on protein phosphorylation in animal cells. Ca^2 -regulated protein phosphorylation is be-
lieved to be involved in signal amplification, in obtaining sustained responses, as well as in producing di-
verse responses to transient rises in [Ca^2 ]cyt[168,169]. Demonstration of a central role for Ca^2 -regu-
lated protein kinases in Ca^2 signaling in the animal system led plant scientists to investigate the presence
of Ca^2 -regulated protein kinases. Current evidence indicates that there are different types of Ca^2 -reg-
ulated protein kinases: a Ca^2 -dependent and CaM-independent protein kinase (CDPK) and Ca^2 /CaM-
dependent protein kinases CCaMK and CaM K II (Figure 1). The CDPK type protein kinase is unique to
plants and has been well characterized as compared with CaM kinases [18,171].
CDPK is ubiquitous in plants. It has been purified to homogeneity from soybean [176] and partially
purified from a number of other plant systems [18]. Calcium directly binds to the CDPK and stimulates
the kinase activity by about 100-fold, whereas CaM did not have any significant effect on the kinase ac-
tivity [177]. Using primers that correspond to amino acid sequences obtained from proteolytic fragments
of a purified CDPK, complementary DNA (cDNA) encoding CDPK has been isolated from soybean
[172]. The predicted amino acid sequence of soybean CDPK has revealed a unique structural organiza-
tion different from that of all the known protein kinases, indicating that it represents a new family of pro-
tein kinases that are unique to plants. The deduced primary structure of the CDPK contains a protein ki-
nase catalytic domain followed by a CaM-like region with four Ca^2 binding motifs (Figure 1). The
kinase domain of CDPK shows significant homology with the mammalian Ca^2 /CaM-dependent protein
704 REDDY AND REDDYFigure 1 Diagrammatic representation of the catalytic and regulatory domains of representative examples of
calcium and calcium/calmodulin-dependent protein kinases. CDPK (508 amino acids), calcium-dependent and
calmodulin-independent protein kinase [172]; Ca^2 /CaM K II (415 aa), calcium/calmodulin-dependent protein
kinase II from plants [173]; CCaMK (520 aa), calcium and calcium/calmodulin-dependent protein kinase [174];
and CaM K II (565 aa), calcium/calmodulin-dependent protein kinase II from animals [175]. The hinge region
that separates the kinase domain from the autoinhibitory domain in animal CaM K II is denoted by H. The
myristoylation motif in CDPK is indicated by an asterisk (*).