Patent ID: 8211655

Claim:
A method for determining activation of a cell surface receptor that binds to an intracellular binding partner and endocytoses with the intracellular binding partner to associate with an endosome, comprising the steps of: (a) providing a cell expressing (i) a fusion protein of a fragment of β-galactosidase fused to the intracellular binding partner, (ii) a fusion protein of a fragment of β-galactosidase fused to an endosome-associated protein portion, wherein said fragments of β-galactosidase weakly bind each other so that complementation activity depends on interaction of the fusion proteins, and (iii) the cell surface receptor, wherein the cell surface receptor is unlabeled; (b) contacting said cell with a ligand for sufficient time for said ligand to bind to said receptor and any binding of said intracellular binding partner to said receptor to occur; then (c) lysing said cells with a lysing medium comprising a substrate for said β-galactosidase that produces a detectable product; and (d) determining said detectable product, wherein production of said detectable product indicates endosomal localization of said receptor.