Calmodulin Regulates the Plasma-Membrane Pumps That Control Cytosolic Ca2+ Concentrations

As we explain in Chapter 15, in muscle cells and in many other types of cells, 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 µM. Animal, yeast, and probably plant cells express plasma-membrane Ca2+ ATPases that transport Ca2+ out of the cell against its electrochemical gradient. The catalytic α subunit of these P-class pumps is similar in structure and sequence to that of the muscle SR Ca2+ pump.

The activity of plasma-membrane Ca2+ ATPases is regulated by calmodulin, a cytosolic Ca2+-binding protein (see Figure 3-33). A rise in cytosolic Ca2+ induces the binding of Ca2+ ions to calmodulin, which triggers activation of the Ca2+ ATPase. As a result, the export of Ca2+ ions from the cell accelerates, quickly restoring the low concentration of free cytosolic Ca2+ characteristic of the resting cell.