Probing Site-Specific Calmodulin Calcium and Lanthanide Affinity by Grafting

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Abstract
Ca2+ binding is essential for the biological functions of calmodulin (CaM) as a trigger/sensor protein to regulate many biological processes in the Ca2+-signaling cascade. A challenge in understanding the mechanism of Ca2+ signaling is to obtain site-specific information about the Ca2+ binding properties of individual Ca2+-binding sites of EF-hand proteins, especially for CaM. In this paper, we report the first estimation of the intrinsic Ca2+ affinities of the four EF-hand loops of calmoduin (I−IV) by individually grafting into the domain 1 of CD2. Taking advantage of the Trp residues in the host protein, we first determined metal-binding affinities for Tb3+, Ca2+, and La3+ for all four grafted EF-loops using Tb3+ aromatic resonance energy transfer. EF-loop I exhibits the strongest binding affinity for Ca2+, La3+, and Tb3+, while EF-loop IV has the weakest metal-binding affinity. EF-loops I−IV of CaM have dissociation constants for Ca2+ of 34, 245, 185, and 814 μM, respectively, with the order I > III ≈ II > IV. These findings support a charge-ligand-balanced model in which both the number of negatively charged ligand residues and the balanced electrostatic dentate−dentate repulsion by the adjacent charged residues are two major determinants for the relative Ca2+-binding affinities of EF-loops in CaM. Our grafting method provides a new strategy to obtain site-specific Ca2+ binding properties and a better estimation of the cooperativity and conformational change contributions of coupled EF-hand proteins.
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