Abstract: An isolated mineralocorticoid receptor (MR) polypeptide, or functional portion thereof, having one or more mutations that alter the solubility or crystal-forming properties and confer the ability to generate soluble protein complexes with the MR and ligands that only weakly bind the native polypeptide, and a polynucleotide encoding it are disclosed. Representative mutations are C808S and S810L substitutions. Expression of the MR polypeptide in E. coli is also provided. A solved three-dimensional crystal structure of an MR ligand binding domain polypeptide is also disclosed, along with a crystalline form of the MR ligand binding domain polypeptide.