Receptor Mutant Missing Specific Active State

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The precise mechanism by which heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors translate a ligand-binding event that occurs in the extracellular part of the receptor in order to activate a G protein docked on the cytoplasmic portion of the receptor remains poorly understood. Lecat et al. used fluorescence resonance energy transfer (FRET) to monitor binding of neurokinin A tagged with the fluorophore Texas Red to the NK2 neurokinin receptors tagged with green fluorescent protein. They measured ligand binding and two biological effects of receptor activation--an increase in the concentration of intracellular free calcium and increased synthesis of cAMP (adenosine 3',5'-monophosphate). These measurements were made for both the wild-type receptor and a mutant receptor with two point mutations in the extracellular domain. These mutations were previously suggested to decrease the apparent affinity of neurokinin for the receptor, and it was thus suggested that the altered amino acids were critical for ligand binding. Lecat et al. report that although the wild-type receptor activates both calcium and cAMP signals, the mutant receptor can only enhance calcium signals. This result supports the hypothesis that the receptor has at least three states, an inactive one, an active one in which it stimulates calcium signals, and a second active state (which the mutant receptor cannot occupy) that enhances cAMP signals. The authors interpret their measurements and modeling to indicate that the mutations actually do not alter the affinity of the receptor for neurokinin, but rather inhibit the transitions of the receptor from the inactive to active states. The maximal FRET signal observed with the mutant receptor was less than that measured for the wild-type receptor, which may mean that the distance between the donor and acceptor fluorophores is greater when the receptor can no longer achieve the activated state in which it promotes synthesis of cAMP.S. Lecat, B. Bucher, Y. Mely, J.-L. Galzi, Mutations in the extracellular amino-terminal domain of the NK2 neurokinin receptor abolish cAMP signaling but preserve intracellular calcium responses. J. Biol. Chem. 277, 42034-42048 (2002). [Abstract] [Full Text]