As we explain in Chapter 13, small increases in the concentration of free Ca2+ ions in the cytosol trigger a variety of cellular responses. In order for Ca2+ to function in intracellular signaling, the concentration of Ca2+ ions free in the cytosol usually must be kept below 0.1 - 0.2 ^M. Animal, yeast, and probably plant cells express plasma-membrane Ca2+ ATPases that transport Ca2+ out of the cell against its electrochemical gradient. The catalytic a subunit of these Pclass pumps is similar in structure and sequence to the a subunit of the muscle SR Ca2+ pump.
The activity of plasma-membrane Ca2+ ATPases is regulated by calmodulin, a cytosolic Ca2 +-binding protein (see Figure 3-28). A rise in cytosolic Ca2+ induces the binding of Ca2+ ions to calmodulin, which triggers allosteric activation of the Ca2+ ATPase. As a result, the export of Ca + ions from the cell accelerates, quickly restoring the low concentration of free cytosolic Ca2+ characteristic of the resting cell.
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