Calmodulin and Its Role in the Second-Messenger System

This chapter provides an overview of calmodulin and its role in the second-messenger system. Calmodulin is a ubiquitous regulatory protein of the second-messenger system. It exhibits Ca²⁺-dependent regulatory activities toward several enzymes and proteins. The protein is implicated in a variety of cellular processes that are established as Ca²⁺-modulated. These include cyclic nucleotide and Ca²⁺ metabolisms, muscle contraction, secretory processes, microtubule and mitotic apparatus assembly, and glycogen metabolism. Thus, calmodulin plays a central role in the second-messenger system as a general mediator of the Ca²⁺ signal. The regulatory mechanism of calmodulin represents a novel type of enzyme regulation, where a monomeric protein is triggered by Ca²⁺ to associate with and then modulate the activity of a number of enzymes. Such a mechanism appears to lend flexibility to the coordination of various cellular processes. Calmodulin is purified to homogeneity from various animal sources and extensively characterized. All calmodulins exhibit similar physical properties. Chemical data suggest that calmodulins from various vertebrates are essentially identical.