Brief reviewsCalmodulin Is a Limiting Factor in the Cell☆
Section snippets
CaM Biosensors
Persechini and coworkers Persechini and Cronk 1999, Romoser et al. 1997 have developed biosensors for Ca2+–CaM to investigate CaM-dependent signaling in cells (Figure 1). CaM biosensors exhibit fluorescence resonance energy transfer (FRET) that is attenuated when they bind Ca2+–CaM. Ca2+ biosensors, in which CaM has been fused to the C-terminus of a CaM biosensor, enable the free Ca2+ concentration to be monitored Persechini and Cronk 1999, Persechini et al. 1997.
CaM has four Ca2+-binding
Thermodynamic Coupling
Before entering into a discussion of the implications of a limiting amount of CaM, we should briefly review a critical feature of CaM–target complexes, namely that their affinities for Ca2+ can differ considerably from the affinity of CaM alone. Thermodynamic coupling requires that Ca2+-dependent changes in the affinity of CaM for its targets are balanced by proportionate changes in the Ca2+-binding affinity of CaM when bound to them. This is nicely demonstrated by the Ca2+-binding properties
The Magnitude and Spatial Range of a Ca2+ Signal
The magnitude of any increase in the intracellular free Ca2+ concentration is determined by the total amount of Ca2+ entering the cell and/or released from internal stores, the activities of Ca2+ pumps and exchangers, and buffering by Ca2+-binding ligands. With a total intracellular concentration of 5 μM (20 μM in Ca2+-binding sites) or more Kakiuchi et al. 1982, Tansey et al. 1994, CaM is a potentially important Ca2+ buffer. However, by itself CaM has a relatively low affinity for Ca2+, with
Diffusional Transport
Given a limiting amount of CaM, inhomogeneities in the relative distributions of targets and CaM would be expected to result in significant spatial variations in the free Ca2+–CaM concentrations produced at a given free Ca2+ concentration. In the face of a sustained global increase in the concentration of free Ca2+, however, we would expect diffusional transport to equalize such differences. These predictions have recently been borne out in the work of Teruel et al. (2000). These investigators
Mass Action
In addition to influencing the equilibrium distribution of CaM by favoring binding to more abundant targets, mass action would also be expected to produce transients in the concentrations of CaM–target complexes due to redistribution effects. To illustrate this we have used a simple mathematical model to simulate two simple cases. In both cases the total CaM concentration is 20 μM, with an equimolar mixture of two CaM targets that bind (Ca2+)4–CaM with dissociation constants of 2 nM and 50 nM
Changes in Total CaM
Given a limiting CaM concentration, significant changes in the total amount of CaM would be expected to alter the levels of target activity produced in the cell. Indeed, the total concentration of CaM has been observed to change in response to a variety of physiological stimuli, and in synchrony with the cell cycle (Takuwa et al. 1995). As might be expected, deliberate manipulations of the total CaM concentration have significant effects on the properties of the cell. In general, increases in
Conclusions
Simply by virtue of its many target proteins CaM is clearly a very important signaling protein. Hopefully, this brief review has also made it clear that it is more than just a passive conduit for changes in the intracellular concentration of free Ca2+. The combined effects of mass action, diffusional transport, and energy coupling enable a limiting pool of CaM to play an active role in coordinating complex and subtle patterns of target activity. In particular, it is evident that competition for
Acknowledgements
This work was supported by Public Health Service Grant No. DK53863 to A. Persechini.
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