title: Biphasic Ca2+-dependent switching in a calmodulin-IQ domain complex.
abstract: 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 Kd for the complex from the Ca2+-free value of 2.3 +/- 0.1 microM to a value of 14.4 +/- 1.3 microM. In the second phase, Ca2+ bound to the N-ter EF hand pair reduces the Kd for the complex to a value of 2.5 +/- 0.1 microM, 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 approximately 3-fold, from 1.8 +/- 0.1 to 5.1 +/- 0.7 microM, 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 microM. 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 microM 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.
authors.affiliation: Division of Molecular Biology and Biochemistry, University of Missouri, Kansas City, Missouri 64110-2499, USA.
authors.forename: D J
authors.initials: DJ
authors.lastname: Black
authors.forename: Jared
authors.initials: J
authors.lastname: Leonard
authors.forename: Anthony
authors.initials: A
authors.lastname: Persechini
mesh_terms: D002118:Calcium / Q000737:chemistry*; D002147:Calmodulin / Q000737:chemistry* / Q000235:genetics; D002148:Calmodulin-Binding Proteins / Q000737:chemistry*; D019922:GAP-43 Protein / Q000737:chemistry*; D009154:Mutation; D011487:Protein Conformation; D017434:Protein Structure, Tertiary
chemical_list: C512165:BSCaMIQ peptide; D002147:Calmodulin; D002148:Calmodulin-Binding Proteins; D019922:GAP-43 Protein; D002118:Calcium