trans

242 INTRACELLULAR SIGNALING
protein recruitment domains. Nonetheless, it
is clear that protozoan parasites contain the
basic biochemical components needed to
generate a variety of signals. Indeed, some signal
components that are conserved between
metazoans and protozoan parasites have been
identified by genome analysis. Future work is
expected to identify mechanisms that affect the
timing and spatial organization of signal production,
identify the variety of cell activities
subject to signal control, and demonstrate the
utility of signal processes as therapeutic targets
against protozoan parasites.
CALCIUM
Signal properties of Ca 2
Ca 2 has several features that make it suitable
to interact with proteins and serve as a signal.
In particular, Ca 2 can be readily distinguished
from other abundant cations and anions due
to its unique combination of ionic radius and
charge density. Ca 2 -binding proteins create
selective pockets that are large enough for
Ca 2 with an ionic radius of 1.14 Å but too
large for Mg 2 with an ionic radius of 0.86 Å.
Although Na has an ionic radius of 1.16 Å, it is
distinguished from Ca 2 by its different charge
density. The tightly fitting Ca 2 is then in position
to have its water shell displaced. Because
Ca 2 has a lower energy of hydration than
other divalent cations, it exhibits faster association
kinetics when it binds to proteins. The
Ca 2 signal is further aided by a steep concentration
gradient that is maintained across the
plasma membrane. The ability of Ca 2 and
phosphate to form a precipitate suggests that
early in the evolution of life, Ca 2 was expelled
from the cell. Intracellular free Ca 2 concentrations
([Ca 2 ] i ) are typically maintained around
100 nM while the extracellular environment is
maintained at values around 2 mM. Therefore,
small fluxes in Ca 2 are able to significantly raise
[Ca 2 ] i above the background value. The resultant
high signal-to-background ratio is a critical
feature of the Ca 2 signal.
A Ca 2 signal occurs when the free concentration
of Ca 2 is raised to an extent that specific
Ca 2 -binding proteins become activated.
However, this phenomenon of amplitude regulation
does not explain all of the effects of Ca 2
signal propagation. In particular, oscillations
in amplitude occur. Generally, oscillations
result when Ca 2 is released from an internal
compartment followed by re-filling of the
compartment as [Ca 2 ] i is returned to baseline
values. The oscillations can be duplicated
experimentally with pulsed release of caged
Ca 2 or with pulsed shifts in medium Ca 2
under conditions where an influx channel
remains constantly open. Both methods activate
specific cell responses including gene transcription
and protein kinase activity when the
oscillation frequency is appropriate. Therefore
the Ca 2 signal is subject to amplitude and
frequency modulation. Spatial confinement of
the Ca 2 signal is also important. Free diffusion
of Ca 2 within cells is limited, in part because
of the activity of specific organelles. When the
Ca 2 sensitive luminescent protein aequorin
is targeted to various cellular compartments,
a disproportionate accumulation of Ca 2 into
mitochondria is observed. Proximity of mitochondria
to the plasma membrane and to
Ca 2 -releasing organelles such as endoplasmic
reticulum (ER) allow them to sequester Ca 2
even when the average [Ca 2 ] i is below the dissociation
constant of the mitochondrial transport
systems. Although Ca 2 oscillations have
been detected in Entamoeba, further work is
needed to learn if other protozoan parasites
share this ability. Nonetheless, stored Ca 2 in
intracellular organelles has been detected.
Studies with targeted aequorin in procyclic
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA