A general framework to characterize inhibitors of calmodulin: Use of calmodulin inhibitors to study the interaction between calmodulin and its calmodulin binding domains

The prominent role of Ca²⁺ in cell physiology is mediated by a whole set of proteins involved in Ca²⁺-signal generation, deciphering and arrest. Among these intracellular proteins, calmodulin (CaM) known as a prototypical calcium sensor, serves as a ubiquitous carrier of the intracellular calcium signal in all eukaryotic cell types. CaM is assumed to be involved in many diseases including Parkinson, Alzheimer, and rheumatoid arthritis. Defects in some of many reaction partners of CaM might be responsible for disease symptoms. Several classes of drugs bind to CaM with unwanted side effects rather than specific therapeutic use. Thus, it may be more promising to concentrate at searching for pharmacological interferences with the CaM target proteins, in order to find tools for dissecting and investigating CaM-regulatory and modulatory functions in cells. In the present study, we have established a screening assay based on fluorescence polarization (FP) to identify a diverse set of small molecules that disrupt the regulatory function of CaM. The FP-based CaM assay consists in the competition of two fluorescent probes and a library of chemical compounds for binding to CaM. Screening of about 5300 compounds (Strasbourg Academic Library) by displacement of the probe yielded 39 compounds in a first step, from which 6 were selected. Those 6 compounds were characterized by means of calorimetry studies and by competitive displacement of two fluorescent probes interacting with CaM. Moreover, those small molecules were tested for their capability to displace 8 different CaM binding domains from CaM. Our results show that these CaM/small molecules interactions are not functionally equivalent. The strategy that has been set up for CaM is a general model for the development and validation of other CaM interactors, to decipher their mode of action, or rationally design more specific CaM antagonists. Moreover, this strategy may be used for other protein binding assays intended to screen for molecules with preferred binding activity. This article is part of a Special Issue entitled: 12th European Symposium on Calcium.