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A general method for understanding the mechanisms of ligand recognition and activation of G protein-coupled receptors has been developed. A study of ligand-receptor interactions in the prototypic seven-transmembrane neurokinin-2 receptor (NK2) using this fluorescence-based approach is presented. A fluorescent unnatural amino acid was introduced at known sites into NK2 by suppression of UAG nonsense codons with the aid of a chemically misacylated synthetic tRNA specifically designed for the incorporation of unnatural amino acids during heterologous expression in Xenopus oocytes. Fluorescence-labeled NK2 mutants containing an unique 3-N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-2,3-diaminopropionic acid (NBD-Dap) residue at either site 103, in the first extracellular loop, or 248, in the third cytoplasmic loop, were functionally active. The fluorescent NK2 mutants were investigated by …