kalmodulin

Kalmodulin, commonly spelled calmodulin, is a highly conserved calcium-binding messenger protein found in all eukaryotic cells. It functions as a versatile intracellular sensor that translates changes in intracellular calcium levels into diverse cellular responses.

Calmodulin binds calcium ions via four EF-hand motifs, arranged as two in each of its N- and C-terminal lobes. The protein is about 149 amino acids long and has a mass of roughly 16.7 kDa. In the absence of calcium, calmodulin adopts a conformation that hides its hydrophobic surfaces; binding Ca2+ induces a conformational change that exposes these surfaces and enables interactions with target proteins.

Calmodulin regulates a wide array of enzymes and processes. Notable targets include myosin light-chain kinase, Ca2+/calmodulin-dependent protein kinases (CaMKs), and the phosphatase calcineurin, as well as various ion channels and enzymes involved in metabolism and gene expression. Through these interactions, calmodulin participates in muscle contraction, neurotransmitter release, synaptic plasticity, and broader calcium signaling networks.

In humans, three genes—CALM1, CALM2, and CALM3—encode highly similar calmodulin proteins, ensuring widespread and abundant expression. The protein localizes to the cytosol and can associate with membranes and other cellular structures as needed.

Pathogenic variants in CALM genes have been linked to severe inherited cardiac arrhythmias, such as catecholaminergic polymorphic ventricular tachycardia and long QT phenotypes, underscoring calmodulin’s critical role in calcium signaling and cardiac excitation-contraction coupling.