Biphasic Ca2+-dependent switching in a calmodulin-IQ domain complex

The relationship between the free Ca2+ concentration and the apparent dissociation constant for the complex between calmodulin (CaM) and the neuromodulin IQ domain consists of two phases. In the first phase, Ca2+ bound to the C-ter EF hand pair in CaM increases the K-d for the complex from the Ca2+-free value of 2.3 +/- 0.1 mu M to a value of 14.4 +/- 1.3 mu M. In the second phase, Ca2+ bound to the N-ter EF hand pair reduces the K-d for the complex to a value of 2.5 +/- 0.1 mu M, reversing the effect of the first phase. Due to energy coupling effects associated with these phases, the mean dissociation constant for binding of Ca2+ to the C-ter EF hand pair is increased similar to 3-fold, from 1.8 +/- 0.1 to 5.1 +/- 0.7 mu M, and the mean dissociation constant for binding of Ca2+ to the N-ter EF hand pair is decreased by the same factor, from 11.2 +/- 1.0 to 3.5 +/- 0.6 mu M. These characteristics produce a bell-shaped relationship between the apparent dissociation constant for the complex and the free Ca2+ concentration, with a distance of 5-6 mu M between the midpoints of the rising and falling phases. Release of CaM from the neuromodulin IQ domain therefore appears to be promoted over a relatively narrow range of free Ca2+ concentrations. Our results demonstrate that CaM-IQ domain complexes can function as biphasic Ca2+ switches through opposing effects of Ca2+ bound sequentially to the two EF hand pairs in CaM.