Multiple C‐terminal tail Ca 2+ /CaMs regulate Ca V 1.2 function but do not mediate channel dimerization

Interactions between voltage‐gated calcium channels (Ca V s) and calmodulin (CaM) modulate Ca V function. In this study, we report the structure of a Ca 2+ /CaM Ca V 1.2 C‐terminal tail complex that contains two PreIQ helices bridged by two Ca 2+ /CaMs and two Ca 2+ /CaM–IQ domain complexes. Sedimentation equilibrium experiments establish that the complex has a 2:1 Ca 2+ /CaM:C‐terminal tail stoichiometry and does not form higher order assemblies. Moreover, subunit‐counting experiments demonstrate that in live cell membranes Ca V 1.2s are monomers. Thus, contrary to previous proposals, the crystallographic dimer lacks physiological relevance. Isothermal titration calorimetry and biochemical experiments show that the two Ca 2+ /CaMs in the complex have different properties. Ca 2+ /CaM bound to the PreIQ C‐region is labile, whereas Ca 2+ /CaM bound to the IQ domain is not. Furthermore, neither of lobes of apo‐CaM interacts strongly with the PreIQ domain. Electrophysiological studies indicate that the PreIQ C‐region has a role in calcium‐dependent facilitation. Together, the data show that two Ca 2+ /CaMs can bind the Ca V 1.2 tail simultaneously and indicate a functional role for Ca 2+ /CaM at the C‐region site.