Development of the Fluorescent Biosensor hCalmodulin (hCaM)L39C-monobromobimane(mBBr)/V91C-mBBr, a Novel Tool for Discovering New Calmodulin Inhibitors and Detecting Calcium

A novel, sensible, and specific fluorescent biosensor of human calmodulin (hCaM), namely hCaM L39C-mBBr/V91C-mBBr, was constructed. The biosensor was useful for detecting ligands with opposing fluorescent signals, calcium ions (Ca(2+)) and CaM inhibitors in solution. Thus, the device was successfully applied to analyze the allosteric effect of Ca(2+) on trifluoroperazine (TFP) binding to CaM (Ca(2+) K(d) = 0.24 mu M +/- 0.03 with a stoichiometry 4.10 +/- 0.15; TFP K(d) similar to 5.74-0.53 mu M depending on the degree of saturation of Ca(2+), with a stoichiometry of 2:1). In addition, it was suitable for discovering additional xanthones (5, 6, and 8) with anti-CaM properties from the fungus Emericella 25379. The affinity of 1-5, 7, and 8 for the complex (Ca(2+))(4)-CaM was excellent because their experimental K(d)s were in the nM range (4-498 nM). Docking analysis predicted that 1-8 bind to CaM at sites I, III, and IV as does TFP.