Many central biol. signal transduction processes such as synaptic transmission occur at the level of cell membranes. It is therefore not surprising that the biol. function of membrane proteins in general and neuroreceptors in particular can be modulated by pharmacol. active substances. Here we describe a general method for the investigation of ligand recognition and activation of membrane-bound receptors. A fluorescent unnatural amino acid was introduced at known sites within the prototypic seven-transmembrane neurokinin NK2 receptor, by the suppressor tRNA approach. Intermol. distances were detd. by measuring the fluorescence resonance energy transfer between the fluorescent receptor donor and a fluorescence labeled NK2 heptapeptide acceptor antagonist mol. The method allowed us to det. the three-dimensional structure of integral membrane proteins by non-crystallog. procedures even in living cells, and to apply ultrasensitive fluorescence detection methods for ligand-receptor screening studies in natural environment. [on SciFinder (R)]