Handbook of Plant and Crop Physiology

mussen and Means [272,273] manipulated the levels of calmodulin by stably transforming mouse cell
lines with vectors that constitutively or inducibly express either calmodulin sense or antisense RNA. A
transient increase in calmodulin resulted in acceleration of proliferation, whereas a decrease in calmod-
ulin caused a transient cell cycle arrest. Constitutive elevation of intracellular calmodulin levels in these
cells shortened the cell cycle due to the reduction in the length of G 1. Calcium and calmodulin level de-
terminations during different stages of the cell cycle and the data on the effect of an elevated or reduced
level of calmodulin on the cell cycle indicate that three specific points in the cell cycle (G 1 /S, G 2 /M, and
metaphase/anaphase) are sensitive to calcium and calmodulin (Figure 3). Overexpression of calmodulin
inAspergillus nidulansincreased growth rate by decreasing cell cycle time, whereas a reduced level of
calmodulin prevented entry into mitosis [249].
Calcium and calmodulin have multiple functions and regulate a variety of processes including some
housekeeping functions [245,266,274]. Hence, it has been argued that the observed effects of calcium and
calmodulin manipulations on the cell cycle may not affect specific control points but could be due to the
requirement of calcium and calmodulin for many housekeeping functions. Studies with unicellular fungi
(yeast and A. nidulans), which are amenable to genetic manipulations, indicate that calcium and calmod-
ulin regulate specific decision points during the cell cycle [249]. However, the mechanisms by which cal-
cium and calmodulin control of cell cycle are beginning to be elucidated.

A. Mode of Calcium and Calmodulin Action in Regulating G 2 /M

Transition

Repression of calmodulin synthesis, thereby calmodulin levels, or reduced extracellular calcium in As-
pergilluscells blocked entry into mitosis [275,276]. Under these conditions tyrosine dephosphorylation
of p34 protein kinase that is needed for its activation is blocked and the activity of NIMA (never in mito-
sis mutant) protein kinase, a protein kinase required for the G 2 /M transition in Aspergillus, is also reduced.
Effects of reduced calmodulin and calcium could be reversed by elevating their levels. These studies with
Aspergillusindicate that calcium and calmodulin are required for activation of p34 kinase and another
protein kinase called NIMA that are associated with the G 2 /M transition [249]. The activation of p34 ki-
nase and NIMA protein kinase by calcium and calmodulin could be due to direct interaction of NIMA
protein kinase and the enzyme responsible for tyrosine dephosphorylation of p34 kinase with
calcium/calmodulin complex or indirect interaction through proteins that bind to the calcium/calmodulin
complex. The NIMA protein kinase and tyrosine phosphatase involved in p34 activation did not bind to
calcium/calmodulin and the activity of immunoprecipitated NIMA kinase was not affected by calcium
and calmodulin. These results indicate that the activation of p34 and NIMA kinases could be mediated by
the proteins that bind to the calcium/calmodulin complex. Over two dozen calmodulin-binding proteins

246 REDDY AND DAY

Figure 3 The phases of the cell cycle that require calcium and calmodulin. Arrows indicate the control points
that are regulated by calcium/calmodulin.