Calmodulin: a highly conserved and ubiquitous Ca2+ sensor.

Calcium ions (Ca²⁺) play critical roles in various biological phenomena. The free Ca²⁺ concentration in the cytoplasm of a resting cell is at the 10^-7 M level, whereas that outside the cell is 10^-3 M, creating a 10,000-fold gradient of Ca²⁺ concentrations across the cell membrane, separating the intracellular and extracellular solutions.^1),2) When a cell is activated by external stimuli, the intracellular Ca²⁺ concentration increases to levels of 10^-6-10^-5 M through Ca²⁺ entry from the extracellular solution via plasma membrane Ca²⁺ channels and/or Ca²⁺ release from intracellular stores. This transient increase in Ca²⁺ functions as an important signal mediated by Ca²⁺ sensors. Thus, Ca²⁺ signals are transmitted to intracellular loci such as distinct, localized targets of Ca²⁺ sensors. Among numerous Ca²⁺ sensors present in cells, calmodulin is a highly conserved and ubiquitous Ca²⁺ sensor.³⁾.