Calcium negatively modulates calmodulin interaction with IQGAP1

IQGAP1 regulates cytoskeletal dynamics through interactions with the Rho family GTPases Rac1 and Cdc42, F-actin, and β-catenin. Calmodulin interaction with IQ motifs of IQGAP1 negatively influences these IQGAP1 interactions. Although, calmodulin interacts with IQGAP1 in the absence of Ca²⁺ and was suggested to exhibit reduced binding when Ca ²⁺ bound, recent reports show substantially greater binding when Ca²⁺ is present. Binding evaluations have primarily relied on IQGAP1 interaction with calmodulin conjugated to Sepharose 4B. In this study we evaluated the Ca²⁺-dependence of calmodulin interaction with native IQGAP1 using a series of independent biochemical approaches. We found the apparent binding of calmodulin to IQGAP1 was Ca²⁺-independent, being between 5- and 20-fold greater in the absence than in the presence of Ca ²⁺. In addition, calmodulin interaction with IQGAP1 was negatively regulated by buffer [Ca²⁺] (IC₅₀=3.4×10 ^-7M). Regulation was specific to Ca²⁺, as Ba²⁺ was ∼400-fold less effective than Ca²⁺ at modulating the interaction. Moreover, testing of calmodulin mutants demonstrated that apocalmodulin tightly binds IQGAP1 and that the N- and C-terminal pair of EF hands are important for Ca²⁺ sensitivity. These data indicate that calmodulin may disassemble from IQGAP1 to facilitate IQGAP1 interaction with effectors of cytoskeletal reorganization during conditions of cell activation that promote increased cytosolic [Ca²⁺].