Various Ca2+-binding proteins help to relay the cytosolic Ca2+ signal. The most important is calmodulin, which is found in all eukaryotic cells and can constitute as much as 1% of a cell's total protein mass. Calmodulin functions as a multipurpose intracellular Ca2+ receptor, governing many Ca2+-regulated processes. It consists of a highly conserved, single polypeptide chain with four high-affinity Ca2+-binding sites (Figure 15–33A). When activated by Ca2+ binding, it undergoes a conformational change. Because two or more Ca2+ ions must bind before 0.4 nM 0.6 nM 0.9 nM
Figure 15–32 Vasopressin-induced Ca2+ oscillations in a liver cell. The cell was loaded with the Ca2+-sensitive protein aequorin and then exposed to increasing concentrations of the peptide signal molecule vasopressin, which activates a GPCR and thereby PLCβ (see Table 15–2). Note that the frequency of the Ca2+ spikes increases with an increasing concentration of vasopressin but that the amplitude of the spikes is not affected. Each spike lasts about 7 seconds. (Adapted from
N.M. Woods, K.S.R. Cuthbertson and
P.H. Cobbold, Nature 319:600–602, 1986. With permission from Macmillan Publishers Ltd.) peptide portion of target protein calmodulin adopts its active conformation, the protein displays a sigmoidal response to increasing concentrations of Ca2+ (see Figure 15–16).
The allosteric activation of calmodulin by Ca2+ is analogous to the activation of PKA by cyclic AMP, except that Ca2+/calmodulin has no enzymatic activity itself but instead acts by binding to and activating other proteins. In some cases, calmodulin serves as a permanent regulatory subunit of an enzyme complex, but usually the binding of Ca2+ instead enables calmodulin to bind to various target proteins in the cell to alter their activity.
When an activated molecule of Ca2+/calmodulin binds to its target protein, the calmodulin further changes its conformation, the nature of which depends on the specific target protein (Figure 15–33B). Among the many targets calmodulin regulates are enzymes and membrane transport proteins. As one example, Ca2+/ calmodulin binds to and activates the plasma membrane Ca2+-pump that uses ATP hydrolysis to pump Ca2+ out of cells. Thus, whenever the concentration of Ca2+ in the cytosol rises, the pump is activated, which helps to return the cytosolic Ca2+ level to resting levels.
Many effects of Ca2+, however, are more indirect and are mediated by protein phosphorylations catalyzed by a family of protein kinases called Ca2+/calmodulin-dependent kinases (CaM-kinases). Some CaM-kinases phosphorylate transcription regulators, such as the CREB protein (see Figure 15–27), and in this way activate or inhibit the transcription of specific genes.