RC3/Neurogranin and Ca super(2)+/Calmodulin-dependent Protein Kinase II Produce Opposing Effects on the Affinity of Calmodulin for Calcium

The interaction of calmodulin with its target proteins is known to affect the kinetics and affinity of Ca super(2+) binding to calmodulin. Based on thermodynamic principles, proteins that bind to Ca super(2+)-calmodulin should increase the affinity of calmodulin for Ca super(2+), while proteins that bind to apo-calmodulin should decrease its affinity for Ca super(2+). We quantified the effects on Ca super(2+)-calmodulin interaction of two neuronal calmodulin targets: RC3, which binds both Ca super(2+)- and apo-calmodulin, and alphaCaM kinase II, which binds selectively to Ca super(2+)-calmodulin. RC3 was found to decrease the affinity of calmodulin for Ca super(2+), whereas CaM kinase II increases the calmodulin affinity for Ca super(2+). Specifically, RC3 increases the rate of Ca super(2+) dissociation from the C-terminal sites of calmodulin up to 60-fold while having little effect on the rate of Ca super(2+) association. Conversely, CaM kinase II decreases the rates of dissociation of Ca super(2+) from both lobes of calmodulin and autophosphorylation of CaM kinase II at Thr super(286) induces a further decrease in the rates of Ca super(2+) dissociation. RC3 dampens the effects of CaM kinase II on Ca super(2+) dissociation by increasing the rate of dissociation from the C-terminal lobe of calmodulin when in the presence of CaM kinase II. This effect is not seen with phosphorylated CaM kinase II. The results are interpreted according to a kinetic scheme in which there are competing pathways for dissociation of the Ca super(2+)-calmodulin target complex. This work indicates that the Ca super(2+) binding properties of calmodulin are highly regulated and reveals a role for RC3 in accelerating the dissociation of Ca super(2+)-calmodulin target complexes at the end of a Ca super(2+) signal.