A review with 6 refs. of the authors' work in developing a biophys. technique to investigate the structure, function and dynamics of membrane receptors. A fluorescence-based approach has been developed and applied to the prototypic G protein-coupled receptor NK2. A fluorescent unnatural amino acid was introduced at known sites into NK2 by suppression of nonsense codons with the aid of a chem. misacylated synthetic tRNA specifically designed for the incorporation of unnatural amino acids during expression in Xenopus oocytes. Fluorescence-labeled NK2 mutants contg. a unique fluorescent residue at different sites were investigated by spectrofluorometry in a native membrane environment. Intermol. distances were detd. by measuring the fluorescence resonance energy transfer (FRET) between the fluorescent unnatural amino acid and a fluorescently labeled NK2 ligand. These distances permit one to fix the ligand in space and define the structure of the receptor in a mol. model for NK2 ligand-receptor interactions. [on SciFinder (R)]