The ability of G protein-coupled receptors to transduce signaling typically is induced by the binding of an appropriate ligand (agonist), resulting in a conformational change of the receptor and the subsequent interaction with the G protein heterotrimer. Some mutants of G protein-coupled receptors, known as constitutively active mutants, have the capac-ity to activate the G protein-signaling cascade even in the absence of ligand. In this chapter, we describe three methods that most directly allow characterization of constitu-tively active mutants and discriminate them from the wild-type receptors. All methods are based on the spontaneous signaling function in the absence of ligand and its conse-quences on the receptor.
