Dynamics and structural changes of calmodulin upon interaction with its potent antagonist calmidazolium

Calmodulin (CaM) is a eukaryotic multifunctional, calcium-modulated protein that regulates the activity of numerous effector proteins involved in a variety of physiological processes. Calmidazolium (CDZ) is a potent small molecule antagonist of CaM and one the most widely used inhibitors of CaM in cell biology. Here, we report the structural characterization of CaM:CDZ complexes using combined SAXS, X-ray crystallography, HDX-MS and NMR approaches. Our results provide molecular insights into the CDZ-induced dynamics and structural changes of CaM leading to its inhibition. CDZ-binding induces an open-to-closed conformational change of CaM and results in a strong stabilization of its structural elements associated with a reduction of protein dynamics over a large time range. These CDZ-triggered CaM changes mimic those induced by CaM-binding peptides derived from protein targets, despite their distant chemical nature. CaM residues in close contact with CDZ and involved in the stabilization of the CaM:CDZ complex have been identified. These results open the way to rationally design new CaM-selective drugs. Figure and text for the Table of Contents (ToC) Calmidazolium is a potent and widely used inhibitor of calmodulin, a major mediator of calcium-signaling in eukaryotic cells. Structural characterization of calmidazolium-binding to calmodulin reveals that it triggers open-to-closed conformational changes similar to those induced by calmodulin-binding peptides derived from enzyme targets. These results open the way to rationally design new and more selective inhibitors of calmodulin.