Calmodulin: a highly conserved and ubiquitous Ca sensor.

Calcium ions (Ca) play critical roles in various biological phenomena. The free Ca concentration in the cytoplasm of a resting cell is at the 10 M level, whereas that outside the cell is 10 M, creating a 10,000-fold gradient of Ca concentrations across the cell membrane, separating the intracellular and extracellular solutions. When a cell is activated by external stimuli, the intracellular Ca concentration increases to levels of 10-10 M through Ca entry from the extracellular solution via plasma membrane Ca channels and/or Ca release from intracellular stores. This transient increase in Ca functions as an important signal mediated by Ca sensors. Thus, Ca signals are transmitted to intracellular loci such as distinct, localized targets of Ca sensors. Among numerous Ca sensors present in cells, calmodulin is a highly conserved and ubiquitous Ca sensor..