have been identified in animal systems [277–279]. Some preliminary results suggest that a calmodulin-
dependent protein kinase, a multifunctional enzyme that requires calcium and calmodulin for its activa-
tion, could be a likely candidate in mediating the calcium/calmodulin effect on NIMA protein kinase and
NIMT (a cdc25homologue) of Aspergillus[122]. The purified calmodulin-dependent protein kinase has
been shown to phosphorylate NIMA kinase and NIMT in vitro in a calcium/calmodulin-dependent man-
ner. Furthermore, B-type cyclins that are known to associate with CDC25 proteins and regulate their ac-
tivity [280] have been found to act as substrates for calcium/calmodulin-dependent protein kinase in vitro
[122]. However, the effect of this phosphorylation on the activity of these enzymes is not known.
Human p54(cdc25-c) dephosphorylates cyclinB/Cdk1 and triggers mitosis. A study of the activation
of p54(cdc25-c) by phosphorylation indicates that a calcium/calmodulin-dependent step may be in-
volved in its initial activation [281]. The calcium/calmodulin-dependent protein kinase (CaM kinase) II
could phosphorylate p54(cdc25-c) in vitro and increase its phosphatase activity. An inhibitor of the CaM
kinase II resulted in a cell cycle block at G 2 phase. The Cdk1 remained tyrosine phosphorylated in the
blocked cells.
Studies with plants indicate that there are a number of calmodulin-binding proteins in plants
[246,282]. The identity and function of some of these proteins are being elucidated [279,283]. A cDNA
that encodes a calcium/calmodulin-dependent protein kinase has been isolated from plants [284]. In ad-
dition to calcium/calmodulin-dependent protein kinase, plants contain a unique calcium-regulated protein
kinase that requires calcium but not calmodulin [calcium-dependent and calmodulin-independent protein
kinase, also called calcium-dependent protein kinase (CDPK)] [285,286] and appears to be present in all
plants. A kinesin-like calmodulin-binding protein (KCBP) was isolated from Arabidopsisand other flow-
ering plants [282,287,288]. KCBP has two unique domains that are not present in known kinesin-like pro-
teins (molecular motors that move along microtubules): a calmodulin-binding domain at the C-terminus
following the motor domain and a myosin tail homology domain in the tail [282,289,290]. KCBP binds
calmodulin in a calcium-dependent manner at physiological calcium concentration [282] and the binding
of calmodulin inhibits KCBP from binding microtubules or dissociates the preformed KCBP/MT com-
plex [290–292]. KCBP has been immunolocalized in association with the preprophase band, spindle ap-
paratus, and phragmoplast [293]. A non–calcium/calmodulin-regulated kinesin-like protein in humans
(HsEg5) is phosphorylated by a cyclin/Cdk complex [294]. Whether any of the calcium, calcium/calmod-
ulin-regulated protein kinases and calmodulin-binding proteins other than KCBP are involved in plant
cell cycle regulation is not known.
B. Calcium /Calmodulin in Metaphase /Anaphase Transition
Several lines of evidence indicate that calcium and calmodulin are required for the metaphase-anaphase
transition [260–263]. A transient increase in cytosolic free calcium at the onset of anaphase has been
demonstrated. As indicated earlier, one of the critical events that take place during the metaphase-
anaphase transition is inactivation of p34 kinase due to degradation of cyclins. Studies indicate that cal-
cium and calmodulin could be involved in degradation of cyclins [122]. It has been demonstrated that mi-
cromolar concentrations of calcium induce cyclin B degradation in metaphase-arrested Xenopusegg
extracts [295]. The addition of a synthetic peptide that binds to the calcium/calmodulin complex, prior to
raising the calcium level in the extract, blocked cyclin degradation and inactivation of p34 kinase [295].
The inhibition of cyclin degradation by micromolar concentration of calcium with calcium/calmodulin-
binding peptide could be reversed by adding calmodulin, suggesting that the calcium action is mediated
by calmodulin. Furthermore, by using appropriate inhibitors the involvement of calpain, a calcium-de-
pendent protease, and protein kinase C was eliminated. These results indicate that calcium and calmod-
ulin are involved in cyclin degradation in Xenopuseggs. It is known that cyclins are degraded by ubiqui-
tin-dependent proteolysis [47]. Proteasome activity was shown to be influenced by calcium specifically
during the metaphase-anaphase transition in ascidian meiotic cycle [296] and a subunit of the proteasome
was shown to bind calcium [297].
C. Calcium in the G 1 /S Transition
D-type cyclins do not have the ubiquitin/proteasome destruction box motif of the mitotic cyclins. Instead
they have PEST sequences that are typical of short-lived proteins. Loss of cyclin D1 induced by serum
CELL CYCLE REGULATION IN PLANTS 247