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calmodulin A protein, consisting of 148 amino-acid residues, that is an important regulator of numerous cellular activities. The protein is capable of binding four calcium ions (Ca2+), which causes a conformational change in the molecule, enabling it to interact with various enzymes, including adenylate cyclase, guanylate cyclase, phosphorylase kinase, and phospholipases. During the contraction of smooth muscle calmodulin binds calcium ions and subsequently activates the enzyme myosin light chain kinase. This enzyme phosphorylates the head of the myosin molecule, enabling it to bind to actin. Calmodulin may also regulate the functioning of the spindle observed during mitosis.