The entry of the calcium ion into the cytoplasm of cells from the extracellular medium or from intracellular stores plays an important signaling role. Cytoplasmic calcium acts as a signal by binding to high-affinity calcium-binding proteins. These proteins in turn act as transducers of the signal by activating other proteins or may be activated directly to carry out enzymatic or structural changes. In this way, many extracellular signals are converted into intracellular activities.
An important subset of these intracellular calcium-binding proteins are proteins that also interact with membranes in a calcium-regulated fashion. In the resting cell, many of these proteins are freely soluble in the cytoplasm (or nucleoplasm). However, when calcium enters the cell, these proteins move onto membrane surfaces. In this way, they make fundamental changes in the character of the membrane surface. Some, such as the annexins (Gerke et al. 2005), are so abundant that up to...