Methods for identifying a compound that alters fluorescence resonance energy transfer (FRET) of a protein. The methods include use of a genetically engineered cell that includes a target protein. The target protein includes one or more heterologous domains. In one embodiment, a target protein includes two heterologous domains, and in another embodiment, the target protein includes a heterologous domain and the cell further includes a second protein that includes a heterologous domain. A heterologous domain may include a chromophore or an amino acid to which a fluorescent dye attaches. The fluorescence lifetime of one or more chromophore, one or more fluorescent dye, or the combination thereof, is measured after contacting the cell with a compound A difference between the fluorescence lifetime in the presence of the test compound and the fluorescence lifetime in the absence of the test compound indicates that the test compound alters the FRET of the target protein.