Calmodulin: a highly conserved and ubiquitous Ca2+ sensor

Abstract

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)