The Calcium-Sensing Receptor (CaSR) is a G-protein-coupled receptor that regulates calcium homeostasis by altering parathyroid hormone release, and which binds divalent and trivalent cations, amino acids, polyamines, and polycationic ligands. To obtain information about the structural properties of the CaSR, we expressed milligram quantities of a pure, homogeneous, and functional fragment of the human CaSR extracellular domain (residues 20-535). The expressed and purified protein is folded and binds both neomycin and Calcium. It forms dimers in the absence of reducing agents such as beta-mercaptoethanol. Thermal denaturation studies show it has enthalpy and entropy values of unfolding equal to Delta H = -178 +/- 4 kJ/mol and Delta S = -535 +/-13 J/mol/K. The protein has significant secondary structure with alpha-helical, beta-sheet, beta-turns, and disordered content of 36.6 +/- 6.7%, 13.3 +/- 5.3%, 20.2 +/- 3.3%, and 29.4 +/- 4.0%, respectively. The described method for the expression and purification of CaSR should prove useful for further structural studies of this physiologically important protein. (c) 2006 Elsevier Inc. All rights reserved.