Decoding calcium signals by multifunctional CaM kinase

Abstract

Multifunctional Ca 2+ calmodulin -dependent protein kinase (CaM kinase) is one of the three major protein kinases coordinating cellular responses to hormones and neurotransmitters. It mediates the action of Ca 2+ on neurotransmitter synthesis and release, on carbohydrate metabolism and on the cytoskeleton. CaM kinase has structural/functional properties that facilitate its response to distinctive attributes of Ca 2+ signals which often involve transient increases that span a narrow concentration range and increases that are pulsatile rather than persistent. The kinase responds to the narrow working range of Ca 2+ signals by the use of calmodulin as the Ca 2+ sensor. It is activated by the binding of calmodulin to an autoinhibitory domain that keeps the kinase inactive in the basal state. The transient nature of the signal is accommodated by autophosphorylation of this autoinhibitory domain which allows the kinase to remain partially active after calmodulin dissociates and thereby switches it to a Ca 2+-independent species. The pulsatile nature of Ca 2+ signals may also be decoded by CaM kinase. Autophosphorylation traps calmodulin on autophosphorylated subunits by greatly reducing its off-rate. At high frequency of stimulation, calmodulin would remain trapped during the brief interval between Ca 2+ oscillations and each successive rise in Ca 2+ would recrult more calmodulin. This may enable a stimulus frequency dependent activation of CaM kinase.