Calmodulin is a ubiquitous intracellular calcium-sensing protein with four EF-hand motifs, each capable of binding one Ca²⁺ ion — giving it four calcium-binding sites in total. When all four sites are occupied, calmodulin undergoes a conformational change that allows it to activate target enzymes such as myosin light-chain kinase (MLCK), calcineurin, and various protein kinases.