Calcium–Calmodulin-Dependent Myosin Phosphorylation by Pancreatic Islets

Pancreatic islets contain enzyme activity which catalyzes the phosphorylation by MgATP of cardiac, skeletal, or smooth muscle myosin light chains. The enzyme is activated by calcium (Ka = 10 μM) and calmodulin (Ka = 2 μM) and inhibited by trifluoperazine (Ki = 10 μM), a known inhibitor of calmodulin and of insulin secretion. The enzyme binds to a calmodulin affinity column when Ca²⁺ is present and is eluted when Ca²⁺ is omitted. These are the properties of myosin light chain kinase. Since phosphorylation of smooth muscle myosin is necessary for its activation by actin, the kinase may have a key role in coupling stimuli that increase intracellular calcium to the contractile processes involved in insulin secretion.